TY - JOUR
AU - Klepal,, Waltraud
AB - Abstract Lattice organs on the dorsal part of the carapace were examined by scanning electron microscopy (SEM) in females, males, and/or cypridiform ascothoracid-larvae (in the ascothoracid-larva I stage, for the first time ever) of six species of Ascothoracida representing four genera and three families: Waginella sandersi (Newman, 1974), W. ?metacrinicola (Okada, 1926), and Gorgonolaureus muzikaeGrygier, 1981 (family Synagogidae); BaccalaureusBroch 1929, unidentified species (Lauridae); and Ascothorax gigasWagin, 1968 and A. synagogoides (Wagin, 1964) (Ascothoracidae). All were of the “keel in a trough” or “tube in a trough” type, but they varied even more than those of previously studied ascothoracidans in number, form, orientation, and terminal pore position. Such extensive variability, summarized graphically herein, limits the potential utility of Ascothoracida (parasites of anthozoans and echinoderms) as an out-group for polarizing lattice organ character-state variation in Cirripedia (free-living and parasitic barnacles). While the ground-pattern of lattice organs in Thecostraca (comprising Ascothoracida, Cirripedia, and Facetotecta, or “y-larvae”) includes two anterior and three posterior pairs, ascothoracid-larvae and males of AscothoraxDjakonov, 1914 and DendrogasterKnipovich, 1890 (family Dendrogastridae) have only two posterior pairs; evidence as to which pair is missing is discussed. The hypothesis that dorsal setae in thecostracan nauplii are the precursors of lattice organs in later developmental stages is reexamined; one-to-one positional matching of such setae to lattice organs is difficult in Ascothoracida. Newly characterized structures of unknown function, termed “reticulated pore-plates”, exist along the hinge line in a juvenile male of G. muzikae. The “pits” reported earlier along the anterior valve margin in ascothoracid-larva II of A. synagogoides are actually clusters of pores that may be homologous to these pore-plates. Potentially homologous pore-fields in other ascothoracidans are reviewed from the literature or described anew using SEM. SPECIAL NUMBER: CRUSTACEAN SYMBIOSES INTRODUCTION Lattice organs are chemosensory organs, purportedly highly modified setae, that are nearly ubiquitous on the outer dorsal carapace surface of the cypridiform larvae of thecostracan crustaceans (i.e., barnacles and allies), including the subclasses Cirripedia, Ascothoracida, and Facetotecta. We focused on ascothoracidans, which parasitize a wide array of anthozoans and echinoderms. They have nauplius larvae and bivalved larvae that are referred to as either “a-cyprids” (e.g., Høeg et al., 2004, 2014) or “ascothoracid-larvae” (e.g., Kolbasov et al., 2008). In deference to history (e.g., Wagin, 1947) we use the latter term and refer to the two bivalved instars of certain members of the order Dendrogastrida as ascothoracid-larva I and II; all other thecostracans have only one cypridiform instar. Lattice organs were first discovered by scanning electron microscopy (SEM) and named by Elfimov (1986, 1995a, 1995b), who found five pairs of well-delineated, elongate pore fields, each with a large terminal pore, near the dorsal midline of the cyprid larva of a great many species of barnacle. Itô & Grygier (1990) meanwhile used SEM to describe five pairs of “cardic organs,” each having the form of a tube lying in a trough, dorsally on the carapace of the ascothoracid-larva of the ascothoracidan Baccalaureus falsiramusItô & Grygier, 1990. Cardic organs had been observed earlier by light microscopy in other larval ascothoracidans with bivalved carapaces and in bivalved adult Waginella sandersi (Newman, 1974) (summarized by Itô & Grygier, 1990; see also Grygier, 1992a). Using SEM, Jensen et al. (1994b) surveyed the lattice organs of a number of cirripede cyprid larvae representing thoracican, acrothoracican, and parasitic rhizocephalan barnacles, as well as the cardic organs of ascothoracid-larva II of the ascothoracidan Ulophysema oeresundenseBrattström, 1936. Based on the generally common number (five pairs, herein “LO1” to “LO5”) and common disposition of these organs (two anterior and three posterior pairs), and the gradual morphological transitions they were able to demonstrate among these taxa, Jensen et al. (1994b) concluded that all were homologous, the ascothoracidan condition being plesiomorphic, and that all should be called lattice organs. Since then, lattice organs have also been found and described through the use of SEM on the bivalved carapaces of adults of one or both sexes of several supposedly basal synagogid ascothoracidans: three species of SynagogaNorman, 1888 (Grygier & Ohtsuka, 1995; Kolbasov & Newman, 2018; Kolbasov et al., 2019), Sessilogoga captiva Kolbasov & Grygier in Kolbasov, Petrunina, Olesen, Ho, Chan & Grygier, 2020, and Waginella metacrinicola (Okada, 1926) (Grygier & Itô, 1995). Only the anterior two pairs of lattice organs were described in this last species; the posterior lattice organs and structures near the lattice organs, mentioned in a note added by MJG and WK to Grygier & Itô (1995), are documented herein. Lattice organs arranged in four, not five, pairs have also been studied with SEM in ascothoracid-larvae of one species, and males of two, of the ascothoracidan genus DendrogasterKnipovich, 1890 (Kolbasov et al., 2008) and in male Ascothorax rybakoviKolbasov & Petrunina, 2018 (see Kolbasov & Petrunina, 2018). Almost all ascothoracidan lattice organs so far described are of a form called either “keel in a trough” sensuKolbasov et al. (1999) or “crest in a trough” sensuHøeg et al. (2004). The nipple-like lattice organs in rounded shallow pits found by Kolbasov et al. (2008) in ascothoracid-larva II of D. astericolaKnipovich, 1890 are the only exception. In some ascothoracidans examined herein, the so-called “keel” is not attached or is only partly attached to the floor of the trough. For these, and probably for ascothoracidans in general, “tube in a trough” would be a more accurate expression, leading us to revive the term “tube” or “tube-like” as originally used by Grygier & Itô (1995), Grygier & Ohtsuka (1995), and Itô & Grygier (1995). Since the pioneering work of Jensen et al. (1994b), many articles have appeared documenting lattice organs with SEM in the cyprid larvae of various groups of Cirripedia, of either the “keel in a trough” type, with or without minute pores along the tube, or the “pore field” type (with or without a distinct terminal pore), often accompanied by tree-diagrams of inferred changes in their character states. Among these are Jensen et al. (1994a) and Høeg & Rybakov (2007) for akentrogonidan rhizocephalans and Kolbasov et al. (1999) and Kolbasov & Høeg (2007) for acrothoracican burrowing barnacles. Studies of different groups of thoracican stalked barnacles include Yan et al. (2005), Buckeridge & Newman (2006), and Høeg et al. (2009) for Iblomorpha; Alvarez et al. (2003), Celis et al. (2008), and Kolbasov et al. (2013) for Lepadomorpha; and Celis et al. (2008) and Høeg et al. (2009) for Heteralepadomorpha. Cursory or detailed descriptions of lattice organs have to some degree become standard in descriptions of the larval development of barnacle species (e.g., Walossek et al., 1996; Yan & Chan, 2001; Rybakov et al., 2002; Chan, 2003; Yan, 2003; Chan et al., 2005; Ponomarenko et al., 2005; Tu et al., 2009; Rao & Lin, 2014). At least some SEM-based data on lattice organs have so far been published for the cyprid larvae of at least 12 species of acrothoracican burrowing barnacles, 27 stalked thoracican barnacles, 11 sessile thoracican barnacles, 14 kentrogonid rhizocephalans, and seven akentrogonid rhizocephalans. It has become increasingly clear, mostly on the basis of molecular systematic analysis, that the subclass Facetotecta, or “y-larvae,” is the sister group of Ascothoracida+Cirripedia within Thecostraca (Pérez-Losada et al., 2002, 2009; Høeg et al., 2009; Petrunina et al., 2014; Ewers-Saucedo et al., 2019). Lattice organs of several forms, both described and undescribed, of the “cypris y” genus HansenocarisItô, 1985 were described using SEM (Høeg & Kolbasov, 2002; Kolbasov et al., 2007), thus allowing Facetotecta to be included in the above-mentioned trees of character-state evolution (e.g., Høeg et al., 2004: fig. 6). Structures “reminiscent of possible lattice organs” have also been described from three species of fossil Thylacocephala (Lange & Schram, 2002: 161), an enigmatic group of predatory arthropods purported by some (e.g., Pinna et al., 1985) to be related to Cirripedia. The number and size of the structures in question are, however, greater than in extant thecostracan larvae. Walossek et al. (1996) proposed that the dorsal setae in rhizocephalan nauplii are the ontogenetic precursors of the lattice organs in this group’s cyprid larvae. By an SEM-based comparison of cyprids to a number of nauplii, Rybakov et al. (2003) provided strong support for this idea for the anterior two pairs of lattice organs, but much weaker, and non-universal, evidence for the posterior three pairs. Taking another approach, Høeg et al. (1998) examined the internal ultrastructure of the lattice organs of cyprid larvae of an acrothoracican barnacle and a rhizocephalan by means of transmission electron microscopy (TEM). The structure was similar in both and consistent with a chemosensory function, and these authors suggested a homology with the similarly structured “sensory dorsal organ” of other, mostly branchiopodan and malacostracan crustaceans. This suggestion was amplified by Lerosey-Aubril & Meyer (2013), who regarded thecostracan lattice organs as likely homologues of the “dorsal sensory pit organs” of both the anterior and posterior sensory dorsal organs of malacostracans in particular, based on many ultrastructural similarities. We present herein, as part of a comprehensive SEM-based survey of Ascothoracida (see also note added by MJG and WK to Grygier & Itô, 1995: 225–226; Kolbasov et al., 2008), additional information on the lattice organs of bivalved ascothoracid-larvae and/or adult males or females of species not or incompletely reported on until now. Some of its findings were mentioned as unpublished data by Kolbasov et al. (2008). Our aim was to confirm whether any feature of lattice organs, either within Ascothoracida as a whole or within life-history stages corresponding to cirriped cyprid larvae, can plausibly be adopted as an outgroup condition for cladistic studies within Cirripedia. The distribution of dorsal setae (potential lattice organ precursors) in late ascothoracidan nauplii is also reviewed. We also describe other structures found near the dorsal or anterior carapace margins of various ascothoracidans as a tentative new class of cuticular organs termed “submarginal pore-plates.” MATERIALS AND METHODS All the examined specimens came from the first author’s personal research collection and remain in his possession. Due to the general scarcity of material and the necessity of completing a comprehensive SEM study of as many species as possible in the two-month period of the fellowship under which this work was done, except as noted below, only one specimen of each species or stage was examined for lattice organs. The species, their life history stage(s), and their provenance were Waginella ?metacrinicola (possibly W. axotremataGrygier, 1983), brooding female, SW Kyushu, Japan, legacy from the late T. Itô (see note added by MJG and WK to Grygier & Itô, 1995: 225–226 for collection details); W. sandersi (Newman, 1974), isopod-parasitized female from same original sample as type series, off Patagonia, gift from G.D.F. Wilson (see Newman, 1974 for details, as Synagoga sandersi); Gorgonolaureus muzikaeGrygier, 1981, juvenile male 1 molt before adult from the Hawaiian Islands, collected by MJG (see Grygier, 1987); Baccalaureus sp. (B. maldivensisPyefinch, 1934 species group), male, Sesoko Island, Okinawa, 26 April 1990, collected by MJG (see Grygier, 1996); Ascothorax gigasWagin, 1968, ascothoracid-larva I, Argentine Islands, Antarctica, gift from D.B. Fratt (see Grygier & Fratt, 1984); Ascothorax synagogoides (Wagin, 1964), ascothoracid-larva II and male, off southern California, on loan from the Scripps Institution of Oceanography Benthic Invertebrate Collection (see Grygier, 1991a for details, as Parascothorax synagogoides); and Dendrogaster astropectinis (Yosii, 1931), male, Toyama Pref., Japan, legacy from the late T. Itô, gifted to him by M. Komatsu (no detailed collection data available). Efforts to find lattice organs in the male Dendrogaster, as well as in the following larval material on loan from the Zoological Institute, Russian Academy of Sciences, failed [but see Kolbasov et al. (2008) for a more successful subsequent try]: Dendrogaster astericola, two ascothoracid-larva II, from holotype female, White Sea; D. murmanensisWagin, 1950, three ascothoracid-larva I (one dissected to expose carapace of ascothoracid-larva II), Tatar Strait, Russian Far East; and D. leptasteriaeWagin, 1950, six ascothoracid-larvae I (several dissected to expose carapace of ascothoracid-larva II), from type lot, SE Sakhalin (see Grygier, 1991c; Kolbasov et al., 2008 for collection details). Formalin-preserved specimens were partly dissected, for example by the removal of a carapace valve, or they were examined whole. Some cleaning was done with needles, but no chemical or ultrasonic cleaning. The specimens were dehydrated through ethanol and acetone series and then dried from liquid CO2 in a Polaron critical point dryer (Polaron, Watford, UK). They were then mounted on brass stubs with double-sided carbon tape, coated with gold in a Hummer Jr. sputter coater (Technics EMS, Springfield, VA, USA), and examined and photographed on 35 mm film in a JEOL JSM-35CF scanning electron microscope (JEOL, Akishima, Japan) at an operating voltage of 15 kV. The negatives were kept by WK in Vienna, while MJG has kept a set of prints made from these negatives (multiple sets of those showing lattice organs). The photo plates presented here were prepared by scanning prints and manipulating the resulting digital files using Adobe Illustrator (Adobe, San Jose, CA, USA) and CorelDRAW X3 Graphics Suite (Corel Corporation, Ottawa, Canada). Manipulations included reduction of “charging” streaks (unavoidable when the stage was tipped at close to 90°), optimization of contrast and brightness, clean-up of backgrounds, and the trimming of photographs and scale bars and their arrangement into plates. In a search for candidate setae for lattice organ precursors, an unidentified, probably laurid, late-instar (NV or NVI) metanauplius from Red Sea plankton (RV Meteor cruise 5, stn. 641, haul 6/4, 100–120 m in 235 m, 12 July 1987, ~19:00, 12˚39.5′N, 43˚14.5′E), similar to “Red Sea metanauplius type I” of Boxshall & Böttger-Schnack (1988), was temporarily mounted in glycerine on a glass slide and drawn using a compound microscope equipped with a drawing tube. RESULTS Lattice organs Waginella ?metacrinicola female (anterior of left valve, posterior of right valve;Figs. 1A, C–E, 5G). Five lattice organs, two anterior, three posterior; LO1 and LO2 both nearly perpendicular to valve margin, but LO1 converging posteriorly about 20°, LO2 curved and converging anteriorly to a similar degree (Figs. 1A, 5G); LO3 and LO4 converging anteriorly (LO3 more than LO4), LO5 nearly parallel to valve margin (Fig. 1C). Troughs 15–19 µm long, 2–3 µm wide. One end of each trough with narrow, virgule-like tip (Fig. 1D, E), this end lateral in LO1 and LO2, posterior in LO3 and LO4, anterior in LO5. Distance between virgule ends of LO3 and LO4 40 µm (opposite ends closer than this), between virgule ends of LO4 and LO5 66 µm (Fig. 1C). Troughs deep, but exposed surface of tube approximately level with general cuticle; lips and inner walls of troughs highly wrinkled, but evidently not elevated or otherwise distinguished from general valve cuticle (Fig. 1D, E). Tubes evidently originating at virgule end and evidently formed as keel attached all along trough, with some wrinkles continuous between trough wall and tube; tubes tapering slightly from origin and then broadening very slightly at tip, with raised longitudinal striations (adhering bacterial debris?), terminal pores not visible but circular ridge about 1 µm across (possibly encircling terminal pore?) observed just subapically on tubes of LO4 and LO5 (Fig. 1D, E). Figure 1. Open in new tabDownload slide Lattice organs and associated pore-pits in Waginella spp. W. ?metacrinicola female (A–E): LO1, LO2, and pore-pit (in white square), anterior at upper left, valve margin at upper right (A); detail of pore-pit in A (B); LO3–5 and pore-pit between LO3 and LO4 (arrow), anterior at top, valve margin out of picture to left (C); LO4 (D); LO5 (E). Waginella sandersi female (F–I): LO1, LO2 (largely hidden by biofilm) and mound with apical pit (arrow) (F); LO3–5, anterior above, valve margin to right, with position of crater-like pit marked by arrow (G); LO4, anterior end above (H); LO5, anterior end left (I). LO1–LO5, lattice organs 1–5, respectively; pgt, pitted gland-tubes; vt, virgule-shaped tip of lattice organ trough. Scale bars in μm. Figure 1. Open in new tabDownload slide Lattice organs and associated pore-pits in Waginella spp. W. ?metacrinicola female (A–E): LO1, LO2, and pore-pit (in white square), anterior at upper left, valve margin at upper right (A); detail of pore-pit in A (B); LO3–5 and pore-pit between LO3 and LO4 (arrow), anterior at top, valve margin out of picture to left (C); LO4 (D); LO5 (E). Waginella sandersi female (F–I): LO1, LO2 (largely hidden by biofilm) and mound with apical pit (arrow) (F); LO3–5, anterior above, valve margin to right, with position of crater-like pit marked by arrow (G); LO4, anterior end above (H); LO5, anterior end left (I). LO1–LO5, lattice organs 1–5, respectively; pgt, pitted gland-tubes; vt, virgule-shaped tip of lattice organ trough. Scale bars in μm. Waginella sandersi female (left valve only) (Figs. 1F–I, 5F). Five lattice organs, two anterior (in accord with earlier light-microscope observations of male by Grygier,1983), three posterior. LO3 located at bend of dorsal margin at rear end of hinge line, LO4 just behind this bend, LO5 much farther posteriorly (Fig. 1G). LO1 and LO2 converging anteriorly, LO3 converging more strongly anteriorly, LO4 weakly converging anteriorly but front end curving away from valve margin, LO5 parallel to valve margin with slight lateral curve of front end (Fig. 1F, G). Due to curvature of valve surface, distance of lattice organs from margin of valve not measured, but all at approximately same distance, LO3 slightly closer than others, LO5 slightly farther away (Fig. 1F, G). Front ends of LO1 and LO2 23 µm apart, those of LO3 and LO4 41 µm apart, and those of LO4 and LO5 135 µm apart. Troughs 15–19 µm long, 2.2–3.7 µm wide, very deep and steep-sided, their lips neither thickened nor wrinkled, their inner walls smooth or exhibiting very delicate ribbing. Tubes up to about 1.5 µm wide, occupying entire lengths of troughs, that of LO2 mostly obscured by fouling biofilm (Fig. 1F). Because of fouling, tube orientation of all lattice organs unclear, with neither end of tube nor any terminal pore being visible (Fig. 1H, I); however, tube of LO5 clearly tapering toward rear (Fig. 1I) and that of LO3 spindle-shaped or tapering towards front (Fig. 1G). Gorgonolaureus muzikae juvenile male (right valve only) (Figs. 2, 5H). Five lattice organs present, two anterior, three posterior. LO3 located in front of break in valve outline, LO4 behind this break. LO1 diverging posteriorly, others more or less parallel to valve margin (Figs. 2A, 5H). Distance from valve margin about 75–80 µm for all lattice organs. LO1 and LO2 87 µm apart (gap between rims of troughs), LO3 and LO4 124 µm apart, LO4 and LO5 109 µm apart. Each trough expressed as wide, shallow, smooth-surfaced dish with distinctly raised, rounded rim, being deepest at origin of tube and roughly twice as long as wide (Fig. 2B–D); trough of LO1 16.65 µm long, those of LO3–5 21.1 × 11.6 µm, 28.6 × 12.6 µm, and 22 × 10.7 µm, respectively. Tubes carrot-shaped, about 13 µm long, 2 µm wide when present (Fig. 2B, C), but broken in LO2 and nearly completely ripped off in LO4 (Fig. 2D); tips clearly pointing posteriorly in most lattice organs, but anteriorly in LO3 (Figs. 2C, 5H). Tube free for whole length, as shown by manner of breakage in LO4 (Fig. 2D). Terminal pore not seen in any lattice organ; instead, possible slit-like opening (or shrinkage artifact?) present along proximal third of tube at least in LO1 and perhaps also LO3 (Fig. 2B, C). Figure 2. Open in new tabDownload slide Lattice organs and pore-plates of Gorgonolaureus muzikae juvenile male; charging streaks due to near-90˚ viewing angle. LO1, LO2 (tube broken), and neighboring array of pore-plates, anterior end right, mesial above (A); LO1 with slit-like proximal aperture or shrinkage artifact, anterior end right (B); LO3 with slit-like proximal aperture or shrinkage artifact, anterior end right (C); trough of LO4 (tube ripped away), anterior end right (D); tube of LO5, anterior end right (E); largest pore-plate, located anterior to LO1, anterior above, mesial left (F). LO1 and LO2, lattice organs 1 and 2, respectively. Scale bars in μm. Figure 2. Open in new tabDownload slide Lattice organs and pore-plates of Gorgonolaureus muzikae juvenile male; charging streaks due to near-90˚ viewing angle. LO1, LO2 (tube broken), and neighboring array of pore-plates, anterior end right, mesial above (A); LO1 with slit-like proximal aperture or shrinkage artifact, anterior end right (B); LO3 with slit-like proximal aperture or shrinkage artifact, anterior end right (C); trough of LO4 (tube ripped away), anterior end right (D); tube of LO5, anterior end right (E); largest pore-plate, located anterior to LO1, anterior above, mesial left (F). LO1 and LO2, lattice organs 1 and 2, respectively. Scale bars in μm. Baccalaureus sp. male (both valves) (Figs. 3H, 5J). Lattice organs absent except for single, unpaired one anteriorly on left valve, oriented perpendicular to valve margin. Trough 9.9 µm long, 5.1 µm wide, well defined and bounded by low ridge, its floor smooth, unlike granular surface of general cuticle (Fig. 3H). Tube spindle-shaped, apparently free for whole length, 8.9 × 1.8 µm with tiny terminal pore at mesial end. Figure 3. Open in new tabDownload slide Lattice organs and pore-pits in various ascothoracidans. Lattice organs of Ascothorax gigas ascothoracid-larva I (which lacks LO3), left and right LO1, LO2, LO4, and LO5 (left LO2 omitted) from top to bottom, anterior end at top of each (A–G). Baccalaureus sp. male (H, I): lattice organ, mesial to lower right, anterior to upper right (H); two of at least five polygonal meshes in submarginal row with pore in center (white stars), among meshes lacking such a pore, mesial to lower right, anterior to lower left (I). Dendrogaster astropectinis male (J, K): inner view of right valve (posterior process extending far to right), with arrow pointing out location of pore-field on inner posterior margin (J); pore-field (K). hl, hinge line; su, so-called “sucker”. Scale bars in μm. Figure 3. Open in new tabDownload slide Lattice organs and pore-pits in various ascothoracidans. Lattice organs of Ascothorax gigas ascothoracid-larva I (which lacks LO3), left and right LO1, LO2, LO4, and LO5 (left LO2 omitted) from top to bottom, anterior end at top of each (A–G). Baccalaureus sp. male (H, I): lattice organ, mesial to lower right, anterior to upper right (H); two of at least five polygonal meshes in submarginal row with pore in center (white stars), among meshes lacking such a pore, mesial to lower right, anterior to lower left (I). Dendrogaster astropectinis male (J, K): inner view of right valve (posterior process extending far to right), with arrow pointing out location of pore-field on inner posterior margin (J); pore-field (K). hl, hinge line; su, so-called “sucker”. Scale bars in μm. Note on other cuticular structures: The dorsal half of the carapace has a reticulated surface similar to that of a “Tessmann’s larva” (a type of ascothoracid-larva characteristic of Lauridae; e.g., Grygier, 1988; Itô & Grygier, 1990). As in those larvae, there are ridge-bounded pores along the ridges and at their points of intersection. Additionally, some meshes near the posterior part of the hinge have, in the middle, a lipped but not ridge-bounded pore surrounded by a smooth zone lacking the granulation that is otherwise typical of the mesh cuticle (Fig. 3I). It does not seem likely that these represent another form of lattice organ because there are too many of them. At least five lie in a row parallel to the valve margin, each separated from the next by one or two ordinary meshes. Farther ventrally, setae are seen to arise within some meshes close to their surrounding ridges, so it is possible the pores under discussion are actually the sockets of lost setae. Ascothorax gigas ascothoracid-larva I (both valves) (Figs. 3A–G, 5K). Four pairs of latttice organs, two anterior, two posterior (in accord with earlier light-microscope observations on ascothoracid-larva II by Grygier & Fratt, 1984). LO1 diverging posteriorly, left LO2 perpendicular and right one parallel to valve margin, LO3 apparently absent (see Discussion), LO4 parallel to margin, LO5 slightly converging posteriorly (Fig. 5K); right LO4 farther anterior than left one, but other pairs opposite each other. LO1 about 34 µm from margin, LO2 about 60 µm from margin, and these two pairs 120 µm apart as measured between tube bases; right LO2 and LO4 175 µm apart. LO4 about 32 µm from margin, left LO5 about 40 µm from margin (right side distorted), distance between these pairs uncertain because of foreshortening in photo. Tip of tube directed posteriorly in LO1, LO4, and LO5, anteriorly in LO2 (Fig. 3A–G). Troughs ill-defined with wavy contours (radially wrinkled in one case; Fig. 3C) and hints of incomplete bounding ridge, so only tubes measured: 6.3–10.8 µm long and 1.2–2.4 µm wide. Anterior two pairs of tubes wider than posterior ones (1.9–2.4 µm versus 1.4–1.9 µm), but lengths generally quite mixed (Fig. 3A–G). Tubes free, at least in distal half, tapering to blunt tip with no visible terminal pore, and usually ornamented with 6–14 incomplete and sometimes intersecting raised annulations about 0.25 µm wide and high with narrower spaces between. Annulations faint in some longer tubes (Fig. 4E, F), thus possibly representing shrinkage artifacts. Figure 4. Open in new tabDownload slide Lattice organs and pore arrays of Ascothorax synagogoides, interpreted as lacking LO3. Ascothoracid-larva II (A–D): LO1 and LO2, anterior at top (A); LO2 (B); posterior lattice organs (LO4 and LO5 of both valves) (C); row of pore arrays (“pits” of Grygier, 1991a) along anterodorsal outer margin of valve (D). Male (E–I): LO1 and LO2, respectively, both with anterior at top (E, F); posterior lattice organs (LO4 and LO5) of both valves, anterior at top (G); left LO5, anterior at top (H); right LO4 and LO5, with pore at midlength of latter, anterior at top (I). LO1–2 and LO4–5, lattice organs 1, 2, 4, and 5, respectively; adm, anterodorsal margin; hl, hinge line. Scale bars in μm. Figure 4. Open in new tabDownload slide Lattice organs and pore arrays of Ascothorax synagogoides, interpreted as lacking LO3. Ascothoracid-larva II (A–D): LO1 and LO2, anterior at top (A); LO2 (B); posterior lattice organs (LO4 and LO5 of both valves) (C); row of pore arrays (“pits” of Grygier, 1991a) along anterodorsal outer margin of valve (D). Male (E–I): LO1 and LO2, respectively, both with anterior at top (E, F); posterior lattice organs (LO4 and LO5) of both valves, anterior at top (G); left LO5, anterior at top (H); right LO4 and LO5, with pore at midlength of latter, anterior at top (I). LO1–2 and LO4–5, lattice organs 1, 2, 4, and 5, respectively; adm, anterodorsal margin; hl, hinge line. Scale bars in μm. Figure 5. Open in new tabDownload slide Diagram (with key to structures at lower left) of approximate locations, trough and/or tube orientations, and terminal pore positions when known (tube shown only when terminal pore not seen, or when proximal slit (shrinkage artifact?) present instead) of lattice organs representing four families of Ascothoracida so far examined using SEM; when only one valve examined, contralateral lattice organ(s) not shown. Synagoga millipalus, male (see Grygier & Ohtsuka, 1995) (A); Synagoga grygieri, male and female (see Kolbasov & Newman, 2018) (B); Synagoga arabesque, male and female (see Kolbasov et al., 2019) (C); Sessilogoga captiva, male (see Kolbasov et al., 2020) (D); Sessilogoga captiva, female (see Kolbasov et al., 2020) (E); Waginella sandersi, female, also showing approximate position of anterior volcano-like pore-pit, presence of posterior one suspected but not confirmed (present study) (F); Waginella ?metacrinicola, female, also showing approximate positions of anterior and posterior pore-pits (see Grygier & Itô, 1995; also herein) (G); Gorgonolaureus muzikae, male, with missing or damaged tubes reconstructed as dotted outlines (present study) (H); Baccalaureus falsiramus, ascothoracid-larva (see Itô & Grygier, 1990) (I); Baccalaureus sp., male (present study) (J); Ascothorax gigas, ascothoracid-larvae I (present study) (K); Ascothorax synagogoides, ascothoracid-larva II (present study) (L); Ascothorax synagogoides, male, lacking detailed SEM photographs of right LO1 and LO2 (present study) (M); Ascothorax rybakovi, male (see Kolbasov & Petrunina, 2018) (N); Ulophysema oeresundense, ascothoracid-larva II (see Jensen et al., 1994b) (O); Dendrogaster astericola, ascothoracid-larva II (see Kolbasov et al., 2008) (P); Dendrogaster dichotoma (right, and left anterior) and D. astropectinis (left posterior, dotted), males (see Kolbasov et al., 2008) (Q). Figure 5. Open in new tabDownload slide Diagram (with key to structures at lower left) of approximate locations, trough and/or tube orientations, and terminal pore positions when known (tube shown only when terminal pore not seen, or when proximal slit (shrinkage artifact?) present instead) of lattice organs representing four families of Ascothoracida so far examined using SEM; when only one valve examined, contralateral lattice organ(s) not shown. Synagoga millipalus, male (see Grygier & Ohtsuka, 1995) (A); Synagoga grygieri, male and female (see Kolbasov & Newman, 2018) (B); Synagoga arabesque, male and female (see Kolbasov et al., 2019) (C); Sessilogoga captiva, male (see Kolbasov et al., 2020) (D); Sessilogoga captiva, female (see Kolbasov et al., 2020) (E); Waginella sandersi, female, also showing approximate position of anterior volcano-like pore-pit, presence of posterior one suspected but not confirmed (present study) (F); Waginella ?metacrinicola, female, also showing approximate positions of anterior and posterior pore-pits (see Grygier & Itô, 1995; also herein) (G); Gorgonolaureus muzikae, male, with missing or damaged tubes reconstructed as dotted outlines (present study) (H); Baccalaureus falsiramus, ascothoracid-larva (see Itô & Grygier, 1990) (I); Baccalaureus sp., male (present study) (J); Ascothorax gigas, ascothoracid-larvae I (present study) (K); Ascothorax synagogoides, ascothoracid-larva II (present study) (L); Ascothorax synagogoides, male, lacking detailed SEM photographs of right LO1 and LO2 (present study) (M); Ascothorax rybakovi, male (see Kolbasov & Petrunina, 2018) (N); Ulophysema oeresundense, ascothoracid-larva II (see Jensen et al., 1994b) (O); Dendrogaster astericola, ascothoracid-larva II (see Kolbasov et al., 2008) (P); Dendrogaster dichotoma (right, and left anterior) and D. astropectinis (left posterior, dotted), males (see Kolbasov et al., 2008) (Q). Ascothorax synagogoides ascothoracid-larva II (both valves) (Figs. 4A–C, 5L). Four pairs of lattice organs, two anterior, two posterior (in accord with earlier light-microscope observations by Grygier, 1991a). LO1 and LO2 parallel to valve margin (Fig. 4A), LO3 apparently absent (see Discussion), LO4 and LO5 converging posteriorly and well separated on right but adjacent and connected by transverse groove on left (Fig. 4C), presumably an abnormality. LO1 and LO2 25–30 µm from valve margin, gap between them 16–18 µm long; LO4 and LO5 10–12 µm from margin, right ones 21 µm apart. Troughs of anterior two pairs longer than those of posterior two pairs (10.9–14.1 µm versus 6.5–9.0 µm); width of an anterior one 1.2 µm. Troughs deep and narrow, with minute dots covering general cuticle up to edge of each trough (Fig. 4B); tubes not clearly observed and no terminal pores visible, but wrinkles seen in bottom of one trough. Tube of right LO2 slender (0.6 µm thick) and directed posteriorly, distal 60% appearing free (Fig. 4B). Ascothorax synagogoides male (left valve and posterior of right) (Figs. 4E–I, 5M). Four pairs of lattice organs, two anterior and two posterior, both of latter located posterior to end of hinge (LO3 apparently absent; see Discussion). Orientations various: left LO1 perpendicular to margin; left LO2 converging posteriorly (Fig. 5M); LO4 both converging posteriorly, left LO5 converging anteriorly, right LO5 parallel to valve margin (Fig. 4G). Left LO1 and LO2 24 µm apart; right LO4 and LO5 much closer together (5 µm) than left ones (25 µm) (Fig. 4G). On left side, LO2 twice as far (43 µm) from valve margin as LO1 (22 µm). On both sides, LO4 and LO5 at same distance from valve margin (Fig 4G), but this distance not measurable. Troughs 4.9–7.5 µm long and 1.6–2.3 (usually 2.1–2.3) µm wide, quite deep with overhanging, not completely even lips, and with tubes well inside (Fig. 4E, F, H, I). General valve surface and exposed surface of tubes with regular array of minute dots (about 40 µm−2), but these absent from zone about 1 µm wide around troughs as well as within troughs. Tubes just under 1.0 µm thick. On left valve, free tip of tube at outer end in LO1, posterior in LO2, anterior in LO4 and LO5; on right valve free tip anterior in LO4 but unclear in LO5 (Fig. 5M). Terminal pore with protruding lip usually present at tip of tube (Fig. 4E, H, I), but at midlength of tube in right LO5 (Fig. 4I). Pore-plates and pore-fields Waginella ?metacrinicola female (Figs. 1A–C, 5G). General surface of valve set with numerous “pitted gland tubes” as described by Grygier & Itô (1995) (Fig. 1A, C). In addition, volcano-like pit, considered a possible artifact by Grygier & Itô (1995), observed between free ends of LO1 and LO2, slightly closer to valve margin than to line connecting lattice organs (Fig. 1A); its crater 5.5 µm wide with 4 or 5 pores inside (Fig. 1B), clearest one 1.2 µm across (alternative interpretation: inner cuticle of pit shrunken and folded so as to mimic pores). Similar pit with diameter of 7.5 µm present posteriorly between free ends of LO3 and LO4 (Fig. 1C); its interior not examined. Waginella sandersi female (Figs. 1F, 5F). Volcano-shaped elevation 26 µm across at base and 9 µm high found between and mesial to LO1 and LO2. Crater (or pit) at apex about 6 µm across, but inner surface not visible due to orientation of specimen on stub. No similar elevation associated with LO3 and LO4, although poorly observed (due to orientation) structure close to hinge possibly interpretable as similar pit flush with valve surface (Fig. 1G). Gorgonolaureus muzikae juvenile male (Fig. 2A, F). Numerous single and clustered pores found in band extending from in front of LO1 to behind LO2 and from same level as these lattice organs up to valve margin. Clustered pores forming many clearly bounded pore-plates with 2–31 pores each (70% with 2–7 pores); a pore-plate with 25 pores 17.6 × 11.1 µm, one with 31 pores 32.8 × 21.1 µm. Pores separated by reticulate ridges forming polygons 3.5–5.4 µm across with usually curved sides. Pores in center of each mesh with raised oval lips with axes of 1.7 × 1.0 µm. Ascothorax synagogoides ascothoracid-larvae II (Fig. 4D). Anterodorsal margin of valve of one specimen with 105 µm long band of about 45 oval pores clumped in groups of five or six, each clump corresponding to a “pit” in the description by Grygier (1991); such groups of pores also present along anterior margin of another specimen. Pore apertures 2.3–3.5 µm long and 0.7 µm across, with lips slightly protruding from larger (due to greater width) oval structures, these lying adjacent to each other with outer edges clearly marking edges of clusters. Dendrogaster astropectinis male (Fig. 3J, K). Valve with round indentation with eight pores just behind end of hinge, and facing mesially, a little beyond origin of posterior process of valve. Whole array 7.1 µm across; individual oval pores 1.0 µm across long axis. Note on putative sucker: Wagin’s (1950) description of male D. astropectinis as having the carapace valves developed into a sucker is an exaggeration, although the anteriormost part of the posterior process of each valve is produced into a curved ventral lobe (Fig. 3J), the two of which together define a sort of cup. DISCUSSION Lattice organs Previous studies (see Introduction) and our findings herein indicate that lattice organs in Ascothoracida are small structures (troughs 4.9–30 µm long) that may be either quite narrow (trough width 1.2–3.8 µm in most) or, in a male Gorgonolaureus muzikae (herein) and larvae and the sole examined male so far of Baccalaureus spp. (Itô & Grygier, 1990; herein), rather open and up to 15 µm wide. The shortest lattice organs so far reported in the literature are those of adult Sessilogoga captiva (as short as 4.7 µm; Kolbasov et al., 2020) and male Ascothorax synagogoides, with troughs 4.9–7.5 µm long (herein). Those of the ascothoracid-larvae of A. synagogoides are nearly twice as long, at 14 µm, and are the longest of any larval or juvenile ascothoracidan yet reported. The longest of the narrow lattice organs, with troughs reaching up to 21 µm in length, are found in adult Synagoga spp. and Waginella spp. (Grygier & Itô, 1995; Grygier & Ohtsuka, 1995; Kolbasov & Newman, 2018; Kolbasov et al., 2019; herein). The range of trough lengths seen in ascothoracidan lattice organs is similar to those in most cirripede cyprid larvae except for those of lepadid barnacles, in which they are much longer (Jensen et al., 1994b). Figure 5 summarizes the distribution and orientation of the ascothoracidan lattice organs so far described by SEM. Despite the free or partly free tubes of some lattice organs reported herein, all except those of ascothoracid-larva II of Dendrogaster astericola (Fig. 5P; Kolbasov et al., 2008) belong, at least loosely, to the keel-in-a-trough type. The details are quite diverse, however, partly due to the availability of data from more life stages than those corresponding to the cyprid and y-cyprid larvae of Cirripedia and Facetotecta. The number of lattice organs reported per valve varies from perhaps none (Dendrogaster ascothoracid-larva I) or either none or one (a male Baccalaureus) to four (ascothoracid-larvae I and II and males of Ascothoracidae; ascothoracid-larva II and males of Dendrogaster) or five (juvenile and adult Synagoga, SessilogogaGrygier, 1990, Waginella, GorgonolaureusUtinomi, 1962; ascothoracid-larva of Baccalaureus; and ascothoracid-larva II of UlophysemaBrattström, 1936). Six lattice organs, two anterior and four posterior, were found by light microscopy on one valve of an unidentified ascothoracid-larva attributed to the family Lauridae (Grygier, 1992a). Kolbasov et al. (2008) failed to find lattice organs in ascothoracid-larva I of the same two species we studied, D. leptasteriae and D. murmanensis, but did find four pairs on ascothoracid-larva II of D. astericola, while we did not, and on males of D. dichotomaWagin, 1950 and D. astropectinis. The latter was a deep-water form not necessarily conspecific with the shallow-water D. astropectinis we examined. We probably missed them in ascothoracid-larva II because of their atypical nipple-like shape at that stage (see Kolbasov et al., 2008: fig. 6) and our attempt to examine the carapace surface exposed in dissected preparations of ascothoracid-larva I. An unpublished SEM micrograph received by MJG from P. Palmer in late 1996 shows three pairs of minute (4 μm) lattice organs in ascothoracid-larva II of D. otagoensisPalmer, 1997, including one anterior pair and two closely spaced posterior pairs, but the entire dorsal margin of the valves is not visible, so there may be more. The dissertation by P. Palmer (Palmer, 2009) might have more information (the abstract mentions an ultrastructural study of lattice organs in males) but we were unable to access the dissertation. Otherwise, except in our male Baccalaureus, there are always two anterior pairs when lattice organs are present. A 2 + 3 pattern has been considered to be plesiomorphic for Cirripedia (Jensen et al., 1994b) as well as for the entire Thecostraca (Høeg & Kolbasov, 2002). Based on the condition in ascothoracid-larvae of Ulophysema and Baccalaureus, these authors considered posteriorly pointing tubes with distal (i.e., posterior) pores as plesiomorphic for LO1 and LO2. Grygier & Itô (1995) nevertheless noted that these organs may be strongly angled or nearly perpendicular to the long axis of the valve in an adult female Waginella metacrinicola. More or less perpendicular ones were observed in the present study as well, in adult Waginella spp., a male Baccalaureus sp. (only one lattice organ present), and an ascothoracid-larva II and male of Ascothorax synagogoides. Moreover, LO1 points anteriorly in a juvenile male of Synagoga millipalusGrygier & Ohtsuka, 1995(see Grygier & Ohtsuka, 1995), and LO2 points anteriorly in ascothoracid-larva I of Ascothorax gigas and in a near-adult male of Gorgonolaureus muzikae (herein), although terminal pores were not confirmed in the two latter cases. Several criteria involving either absolute or relative positioning of lattice organs might be used to address the question whether the two pairs found in Ascothoracidae are LO3 and LO4, or LO4 and LO5. The “extra” lattice organ found by Grygier (1992a: fig. 6E) on one valve of a laurid ascothoracid-larva is clearly the second of that valve’s four posterior lattice organs, because the other three are positioned like those of a “normal” specimen (Grygier, 1992a: fig. 6D). Kolbasov et al. (2008), attempting to apply a “spacing” criterion to Dendrogaster spp., which have a 2 + 2 disposition of lattice organs, remarked on the considerable interspecific variation in LO1/LO2 spacing among ascothoracidans with a 2 + 3 pattern, and similar variation in spacing between the three posterior pairs. They deferred any conclusion about the identity of the “missing” posterior lattice organ, but “for convenience” (Kolbasov et al., 2008: 162) labelled the posterior pairs as LO3 and LO4. Here we try to flesh out their analysis on the basis of measurements made from published illustrations. Among the forms with five pairs of lattice organs, LO1 and LO2 are closely set (13.2–17.6 μm apart) in female Waginella spp., male Synagoga millipalus, and a laurid ascothoracid-larva (Grygier, 1992a; Grygier & Itô, 1995; Grygier & Ohtsuka, 1995; herein), but much farther apart (64.2–135 μm) in adults of other Synagoga spp. and Sessilogoga captiva and in ascothoracid-larva II of Ulophysema oeresundense (Jensen et al., 1994b; Kolbasov & Newman, 2018; Kolbasov et al., 2019, 2020). In those with four pairs, LO1 and LO2 are close together (16–27 μm apart) in Ascothorax synagogoides (ascothoracid-larva II and male; present study) and A. rybakovi (male; Kolbasov & Petrunina, 2018), but much farther apart (90–170 μm) in A. gigas (ascothoracid-larva I; herein) and Dendrogaster spp. (ascothoracid-larva II and males; Kolbasov et al., 2008). When only two pairs of posterior lattice organs are present, they are either 20–53 μm apart (similar to or not more than twice as far apart as the same individual’s LO1 and LO2) or nearly adjacent (2.3–5.5 μm apart) in D. astericola (ascothoracid-larva II; Kolbasov et al., 2008) and sometimes A. synagogoides (ascothoracid-larva II and male; herein). The spacing is 16.7–45 μm between LO3 and LO4, and 21.6–70 μm between LO4 and LO5, in most of the above-mentioned ascothoracidans with three posterior pairs of lattice organs, but distinctly greater (56–60 μm and 83–106 μm, respectively) in adult Sessilogoga captiva. Spacing as such thus provides little unambiguous help in identifying the two posterior lattice organs in species of AscothoraxDjakonov, 1914 and Dendrogaster; nonetheless, Kolbasov & Petrunina (2018) explicitly regarded LO5 as absent in male A. rybakovi. The position of the posterior lattice organs relative to the contour break between the dorsal and posterodorsal margins of the valve is a second possible criterion. Two patterns are seen in species with three pairs: 1) LO3 anterior to or at the contour break and LO4 and LO5 behind it; and 2) LO3 and LO4 anterior to the contour break and LO5 at or posterior to it. Pattern 1 is seen in Gorgonolaureus muzikae and Waginella sandersi (herein), as well as in previously described laurid ascothoracid-larvae (Itô & Grygier, 1990; Grygier, 1992b). Pattern 2 is seen in adult Synagoga grygieriKolbasov & Newman, 2018 (Kolbasov & Newman, 2018: fig. 14e) and S. arabesqueKolbasov, Petrunina, Ho & Chan, 2019 (Kolbasov et al., 2019: fig. 18A, E), as well as Sessilogoga captiva (Kolbasov et al., 2020). In Ascothorax synagogoides, the position of both posterior pairs behind the end of the hinge in the male and behind the valve margin’s contour break in ascothoracid-larva II (Fig. 4C, G) matches pattern 1 and may indicate that they represent LO4 and LO5, with LO3 being absent, which is the interpretation we adopt. In Dendrogaster, however, Kolbasov et al. (2008: figs. 6F, 7D) seem to show both posterior pairs ahead of the contour break in ascothoracid-larvae II of D. astericola, and one pair ahead of and the other behind the contour break in male D. dichotoma, implying that LO5 is absent in that genus. The last word on whether Ascothorax and Dendrogaster have lost the same lattice organ, and which one is missing in each, will probably have to come from a study of their nervous connections modelled after, e.g., Kalke et al. (2020). These authors suggested, but could not provide conclusive evidence, that the anterior lattice organs are innervated by the third ramification field, and the posterior ones by the fourth ramification field, in the peripheral nervous system of the cyprid larva of a species of balanid barnacle. Jensen et al. (1994b) considered that, as the plesiomorphic state in the Thecostraca, LO3–5 all have the tube pointing posteriorly with a terminal pore. While this is true for ascothoracid-larvae of Ulophysema and Baccalaureus, as well as the two posterior pairs of Ascothorax gigas, and may indeed be the plesiomorphic condition in such larvae, other life-history stages in other taxa show different patterns (Fig. 5). Adult Synagoga millipalus and S. grygieri (but not S. arabesque or Sessilogoga captiva) have a reversed tube in LO3. So does the near-adult male of Gorgonolaureus muzikae studied herein. LO3 and LO4 are reversed in adult female Waginella ?metacrinicola. In the male Ascothorax synagogoides we studied, the tube is reversed in those rear lattice organs for which the orientation could be determined. The orientation of the tube is, therefore, not constant within Ascothoracida for any pair of lattice organs. The situation in Ascothorax synagogoides, in which the pore is not always at the distal end of the tube, suggests that the position of the pore must be considered separately from the tube orientation. It is, therefore, important to know whether the long groove in some lattice organs of our male of G. muzikae and a male A. rybakovi of Kolbasov & Petrunina (2018) is actually a slit-like aperture or an artifact of drying. It can be argued that the only meaningful comparison of lattice organs is between those of homologous instars, such as between cyprid larvae and first and/or second ascothoracid-larvae. Among the ascothoracidans we considered, members of Ascothoracidae and Dendrogastridae are known to have two such instars, whereas Lauridae seem to have only one. There is still no unanimity between larvae of Ulophysema and Lauridae on the one hand, and those of various Dendrogaster and Ascothorax species on the other, in terms of either the number of organs or, for Ascothorax gigas, the orientation of LO2. As a result, Ascothoracida cannot be used very well as an outgroup to estimate the character-state polarity of nearly any feature of the lattice organs in Cirripedia, except for there being two anterior pairs (assuming that the larval condition in Baccalaureus is more plesiomorphic than that in the adult male) and the tube-in-a-trough morphology. For other features an additional outgroup is needed, and subclass Facetotecta is the only available candidate. For now, there is no unanimity concerning even the number of lattice organs in facetotectan cyprids (“cypris y”), and a thorough survey in this group remains to be done. Høeg & Kolbasov (2002) described five pairs, including two anterior and three posterior, in specimens from the White Sea later identified as Hansenocaris itoiKolbasov & Høeg, 2003, and in two unidentified specimens from Norway (Bergen) and the Bahamas, but Kolbasov et al. (2007) could not confirm the presence of LO3 in H. papillata Kolbasov & Grygier in Kolbasov, Grygier, Ivanenko & Vagelli, 2007. All lattice organs so far described in the Facetotecta do have posterior terminal pores, a condition that Høeg & Kolbasov (2002) regarded, so far without dispute, as part of the thecostracan ground-plan. If so, ascothoracidans display an assortment of deviations from it. Turning now to general morphology, Jensen et al. (1994b) maintained, and Høeg & Kolbasov (2002) confirmed by transmission electron microscopy (TEM), that the tube is not free in ascothoracid-larvae of Ulophysema, but is a keel attached lengthwise to the floor of the trough. Although our SEM photos of Waginella ?metacrinicola (Fig. 1A–F) provide no evidence one way or the other, much of the length of the tube appears to be free in some of the other species examined. The damaged lattice organ in Gorgonolaureus muzikae (Fig. 2D) clearly shows that the tube is free in males of that species. A free tube increases the structure’s resemblance to a seta, and therefore tends to strengthen the hypothesis that lattice organs represent modified sensory setae (see Rybakov et al., 2003). The “nipple in a pit” form of the lattice organs of ascothoracid-larva II of Dendrogaster astericola (see Kolbasov et al., 2008: fig. 6), however, is much less suggestive of a setal precursor, unless the seta regresses greatly during the transformation process. Unfortunately, the form in adult males of D. astericola is not known. Precisely which naupliar setae become lattice organs is still a matter for discussion. Comparison with naupliar setae Jensen et al. (1994b) contended that the lattice organs represent modified setae. Following the report by Collis & Walker (1994) of five pairs of dorsal sensilla on the nauplius of the rhizocephalan Sacculina carciniThompson, 1836 (i.e., four pairs of setae and one pair of “spines”), Walossek et al. (1996) suggested that the five pairs of dorsal setae on nauplii of another rhizocephalan, Briarosaccus tenellusBoschma, 1970, are the ontogenetic predecessors of the cyprid’s lattice organs. Rybakov et al. (2003) tested this hypothesis using SEM to compare the potential positional homologies of the dorsal setae of the final naupliar instars of these two species and nine other kentrogonid rhizocephalans, supplemented by information on setal shape and pore position, with the lattice organs of the corresponding cyprids. Except for Sacculina polygeneaLützen & Takahashi, 1997 with one pair, two anterodorsal pairs of setae termed S1 and S2 were universally present, as well as in the other thecostracan nauplii studied by Rybakov et al. (2003). Most of the rhizocephalans actually had six, not five, pairs of dorsal setae on their last nauplius, including a pair lateral to S2 (called S2a, sometimes modified or absent) and three more posterior pairs, of which the first (S3) could be located either fairly close to S2, or else well separated from it and grouped instead with S4 and S5. Rybakov et al. (2003) considered S1–S5 of these rhizocephalan nauplii to correspond ontogenetically to LO1–LO5 of the cyprids, and implied that S2a corresponds to a diffuse oval pore field located lateral to LO2 in these cyprids. It is understandable, but unfortunate, that homologies were not established more directly by observations of formation of the later structure within the earlier one or by confirmation that the nervous connections are the same. These authors also failed to account for the additional pair of setae in the S4 position reported by Rybakov et al. (2002) from naupliar instar 2 onward in Peltogasterella gracilis (Boschma, 1927). Although this model works tolerably well for kentrogonid rhizocephalans, Rybakov et al. (2003) noted that only the S1 and S2 aspects apply well to the other investigated thecostracans (Ascothoracida, Facetotecta, Acrothoracica, Thoracica). In their experience, nauplius larvae of all of these lacked any equivalent of S3-S5 except possibly for one posterior pair of setae (S5?) in late nauplii of Facetotecta. For Ascothoracida, their only point of explicit reference was the possibly atypical brooded nauplius of Ulophysema oeresundense. The two posterior pairs of dorsal setae in later naupliar instars of Baccalaureus falsiramus, which are free-swimming larvae, were mentioned, but without comment on their homologies. For Ascothoracida, the literature provides rather few relevant data. NVI nauplii of Baccalaureus falsiramus (the ascothoracid-larvae of which have five pairs of lattice organs) have four pairs of setae, two at about one-quarter length and two posteriorly (Itô & Grygier 1990). The supposed NVI nauplius of a petrarcid ascothoracidan (Grygier, 1993), however, has five pairs, corresponding the four above plus one pair near the anterior margin (ascothoracid-larvae and lattice organs remain unknown in the Petrarcidae). Such setal arrangements pose an immediate problem for this model of lattice organ ontogeny because the naupliar setae are not grouped into two anterior pairs and three posterior pairs, as in the case of lattice organs. In the petrarcid nauplius, for example, the third pair seems to be situated rather too far anteriorly to become LO3 at the posterior end of the hinge in the ascothoracid-larva. This pair is nonetheless thicker than the other dorsal setae, which are hair-like, and it may actually represent the precursors of the tubes of one pair of lattice organs, even if the other setae do not. This pair is presumably homologous to the pair of curled setae seen in Briarosaccus tenellus and to the pair of “spines” seen in Sacculina carcini. A similar thick pair of setae is found in late instars of unidentified laurid-type ascothoracidan nauplii from the Red Sea (Fig. 6), and in such larvae there can be six, seven, or even eight pairs of dorsal setae in all (MJG, unpublished data). This high number of setae again argues against a kentrogonid-like near-one-to-one correspondence between them and lattice organs; even if five of them are precursors of the latter, it is not clear which five. In contrast, as already noted by Rybakov et al. (2003), everything known about the dorsal setation of the nauplius larvae of acrothoracican and thoracican barnacles implies the opposite problem: there are too few posterior naupliar setae (actually none) compared to the cyprid’s three pairs of posterior lattice organs. Figure 6. Open in new tabDownload slide Distribution of eight pairs of dorsal setae (arrows) on undescribed late-instar (NV or NVI) ascothoracidan (probably Lauridae) metanauplius from plankton in the Red Sea, RV Meteor cruise 5, stn. 641. One member each of two pairs not clearly seen; their sockets indicated by arrowed dots. Third pair of setae (bold arrows) thicker than others, thus judged mostly likely to be precursors of lattice organ tubes. Figure 6. Open in new tabDownload slide Distribution of eight pairs of dorsal setae (arrows) on undescribed late-instar (NV or NVI) ascothoracidan (probably Lauridae) metanauplius from plankton in the Red Sea, RV Meteor cruise 5, stn. 641. One member each of two pairs not clearly seen; their sockets indicated by arrowed dots. Third pair of setae (bold arrows) thicker than others, thus judged mostly likely to be precursors of lattice organ tubes. SEM-based descriptions by Høeg & Kolbasov (2002) and Kolbasov et al. (2007), together with unpublished information by MJG, P.G. Jensen, N. Dreyer, and J. Olesen, confirm that facetotectan cyprids (“cypris y”) ordinarily have five pairs of lattice organs (of diverse morphologies) in a 2 + 3 pattern. Grygier (1992b, 1995: fig. 3), summarizing the naupliar development of numerous forms of Facetotecta based largely on unpublished observations of material collected by T. Itô, indicated that in almost all cases late naupliar instars have only four pairs of dorsal setae: one anterior, two middorsal before midlength, and one posterior, the last pair rarely being absent. Rybakov et al. (2003) identified these, respectively, as S1, S2 and S2a, and possibly S5. Even this outgroup thus fails to provide evidence of a one-to-one correspondence between naupliar setae and the lattice organs of cypridiform larvae. No thecostracan taxon does. Pore-plates and pore-fields The other organs we describe besides the lattice organs seem to be manifestations of one or two fundamental kinds of cuticular organ. Similar organs have been reported in other ascothoracidans by light microscopy and SEM, but have never been properly categorized. The submarginal pore-plates of Gorgonolaureus muzikae and the anterodorsal “pits” of Ascothorax synagogoides differ in the reticular nature and the much more clearly defined outer border of each cluster in G. muzikae, the often higher number of pores per pore-plate (two to 31) than per “pit” (usually five or six), and their precise location on the valves (Figs. 2A, 2E, 4D). Both structures are, however, reminiscent of the “compound gland cones” (term coined by Grygier & Itô, 1995) on the ventral submarginal part of the carapace in Flatsia walcoochorum Grygier, 1991, which have only been examined by light microscopy (Grygier, 1991b). They appear to represent clusters of numerous pores on round protruberances, but their appearance under SEM is unknown. Although we may categorize all three organs as “submarginal pore plates”, the absence of any histological or ultrastructural information about the internal structure of any of them permits only a preliminary suggestion that they represent mutually homologous pore clusters. The male Dendrogaster astropectinis treated herein (Fig. 3J, 3K) and the male holotype of Synagoga millipalus (see Grygier & Ohtsuka, 1995: fig. 4A) both have a small cluster of minute pores at the posterior angle of the valve, on the inner side near the margin. Position and form suggest that these pore-fields may be mutually homologous. The volcano-like pits near the lattice organs in Waginella spp. might represent a transition between these minute organs and the large pore-plates of the outer surface in G. muzikae. If the pits have several pores inside, as a micrograph suggests (Fig. 1B), then they could be interpreted as sunken pore-plates or as homologues of the minute posterior pore fields. An intermediate step in this transition might be represented by the aberrant, multi-perforate (6 pores) anterior mesh on the exterior of the polygonally sculptured carapace of an unidentified laurid ascothoracid-larva (Grygier, 1992a: fig. 6C). The volcano-like pits are positioned similarly to the “central pores” in cyprid larvae of some, perhaps most, Cirripedia and y-cyprids of Facetotecta. Each pore (or pair or small cluster of pores, as the case may be) is situated in the dorsal midline, forming a “lattice organ complex” sensuLerosey-Aubril & Meyer (2013) with four associated lattice organs: the right and left LO1 and LO2 anteriorly, and the right and left LO3 and LO4 posteriorly (Høeg et al., 1998; Høeg & Kolbasov, 2002; Kolbasov & Høeg, 2007). In some cirriped cyprids, e.g. Lepadidae (see Jensen et al., 1994b), the central pores are open and clearly visible using SEM, but in others, they are cuticle-covered (Høeg et al., 1998). In contrast, the volcano-like pits of Waginella spp. are paired, being situated near but not on the midline, wide open, and seem to contain multiple pores. Unlike cyprids and y-cyprids, the carapace of Waginella is truly bivalved, with a flexible hinge occupying the dorsal midline. The presence of unpaired central pores may thus be precluded, but because central pores are situated within the immovable hinge-line of lepadid cyprids (Jensen et al., 1994b), they should be searched for in intact ascothoracidans. Their eventual absence could suggest that each central pore represents the fusion of an originally paired organ similar to the volcano-like pits of Waginella spp. If so, such fusion must have happened independently in the Facetotecta and Cirripedia. We suspect that reticular pore-plates of Gorgonlaureus muzikae are vestiges of a once overall reticulation, like that now seen in most laurid ascothoracid-larvae (“Tessmann’s larvae”; see Itô & Grygier, 1990), of a type had a pore within each mesh; see Grygier (1988) for such a case, and also Figure 3I for male Baccalaureus. A stage at which the cuticular meshwork has become restricted to the margins of the valves may be represented by an unidentified ascothoracid-larva from the tropical bathyal Atlantic (Grygier, 1988). The patches of tiny pores at or near the valve edge in various species may either represent a further consolidation of the pore-plates or various expressions of an organ that originally filled a single mesh, as in the above-mentioned laurid ascothoracid-larva of Grygier (1992a). It appears that there are no similar organs reported in any cyprid larva in Cirripedia. The closest are the “wheel organs” described by Elfimov (1986, 1995a, 1995b) in two species of cyprids, and described as “round openings 6–8 µm in diameter with a thickened rim” on the outer surface of the carapace (Elfimov, 1995b: 145). Their cuticle has radial folds which could give the appearance of pores. ACKNOWLEDGMENTS The amassing of the study material by MJG was made possible in part by a University of California Pauley Grant for student travel to Hawaii, several invited stays as a Visiting Foreign Researcher at the former Sesoko Marine Science Center, University of the Ryukyus, Japan, and a National Academy of Sciences Soviet and East European Exchange Fellowship. The ascothoracidan metanauplius from the Red Sea was provided by R. Böttger-Schnack from collections made under Deutsche Forschungsgemeinschaft grant 695/12 to H. Weikert. MJG’s SEM work, supported by a stipend from the Austrian Academic Exchange Service (Österreichischer Akademischer Austauschdienst), was done at WK’s laboratory in the Abteilung für Ultrastrukturforschung of the Institute of Zoology at the University of Vienna, now the Core Facility Cell Imaging and Ultrastructure Research, member of the Vienna Life-Science Instruments (VLSI), where technical assistance was provided by D. Gruber and A. Losert. The first draft of the manuscript was written during MJG’s term as a Center of Excellence Visiting Foreign Researcher at the Tropical Biosphere Research Center Sesoko Laboratory, University of the Ryukyus, and J.T. Høeg is thanked for a critique of it. The photo plates were composed by T. Deguchi during MJG’s long period of employment at the Lake Biwa Museum, with support from an internally funded research project; later modifications of these plates and production of the digital versions of MJG’s line drawings were done by N. Dreyer. 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Google Scholar OpenURL Placeholder Text WorldCat © The Author(s) 2020. Published by Oxford University Press on behalf of The Crustacean Society. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model)
TI - Lattice organs and newly characterized submarginal pore-plates and pore-fields of the carapace in Ascothoracida (Crustacea: Thecostraca)
JF - The Journal of Crustacean Biology
DO - 10.1093/jcbiol/ruaa068
DA - 2020-12-31
UR - https://www.deepdyve.com/lp/oxford-university-press/lattice-organs-and-newly-characterized-submarginal-pore-plates-and-f4cR2bQKeG
SP - 781
EP - 794
VL - 40
IS - 6
DP - DeepDyve
ER -