TY - JOUR
AU - Blakeslee, April M H
AB - Abstract Shoreline development can alter natural habitats, create novel habitats, and affect coastal community structure. Our study compares abundances and tidal variation patterns of palaemonid shrimps in artificial and natural estuarine habitats of Long Island, NY, USA. We sampled shrimps at three habitat types: floating docks, bulkheads, and natural shorelines, at high and low tides. Several possible predictor variables were explored to explain shrimp abundances across habitat types and tides, including wave action, vegetation cover, local taxonomic richness, temperature, and salinity. Shrimp abundances were mainly influenced by habitat type, temperature, taxonomic richness, and vegetation cover. Shrimp abundances were higher in artificial in contrast to natural habitats (particularly along bulkheading), higher in areas with intermediate levels of taxonomic richness, and higher in habitats with vegetation cover compared to areas without. Shrimp abundance also had a weak, but positive, correlation with temperature. Along bulkheads and in natural habitats, abundances were higher at low versus high tide; however, there was no tidal variation along floating docks. These results demonstrate how artificial structures within the intertidal zone, along with associated fouling organisms, may enhance populations of palaemonids, which are a critical component of coastal and estuarine ecosystems. INTRODUCTION Present-day increases in shoreline development in the northeastern United States are driving drastic habitat alterations and the creation of new artificial habitats, which together greatly influence coastal community structure (Seitz et al., 2006; Bilkovic & Roggero, 2008; Morley et al., 2012; Patrick et al., 2014). In particular, shoreline hardening alters soft-bottom habitats by replacing a soft substrate with a hard structure, which in turn enhances wave action, substrate erosion, and turbidity (Douglass & Pickel, 1999). Turbidity reduces light levels in the water column and the percent cover of subaquatic vegetation, an important shelter, nursery, and food source for many fish and crustacean species (Heck et al., 2003; Patrick et al., 2014). As a result, numerous studies have found altered population densities, taxonomic richness, and species diversity along hardened compared to natural shorelines (e.g. Seitz et al., 2006; Bilkovic & Roggero, 2008; Morley et al., 2012). Two common types of artificial structures found in coastal habitats are bulkheading and floating docks. Bulkheading is a vertical surface made of wood, metal, or concrete designed to stabilize the shoreline. While bulkheading prevents erosion on its landward side, it increases wave action and substrate scouring on its seaward side, consequently altering the beach profile and increasing turbidity (Douglass & Pickel, 1999). Natural gently sloping intertidal zones are as a result converted into turbulent, vertical habitats (Douglass & Pickel, 1999; Nordstrom et al., 2009). Floating docks additionally create novel floating habitats that provide a hard attachment surface for fouling algae and sessile invertebrates (e.g., bivalves, tunicates, bryozoans, barnacles, hydroids, and sea anemones) and protection to mobile and sessile organisms, especially underneath the docks and within their crevices (Caine, 1987; Freeman et al., 2016; personal observation). Mobile animals (e.g., polychaetes, gastropods, amphipods, crabs, and shrimps) may also use the sessile fouling organisms as shelter (Caine, 1987; Cerda & Castilla, 2001; Ferreira et al., 2001; Krohling et al., 2006), as prey, as well as attracting nektonic predators (e.g., fishes and cnidarians) to the site (Relini et al., 2002; Osman & Whitlatch, 2004; Krohling et al., 2006). In contrast to other intertidal habitats, floating docks move vertically with the tide so that the attached biota are essentially constantly submerged. Due to their high abundances in coastal ecosystems, palaemonid shrimps (commonly known as grass shrimps) are one of many organisms that may be impacted by increases in artificial habitats along coastlines. Palaemonids tend to live among vegetation such as seaweeds and seagrasses (Warkentine & Rachlin, 2010), among fouling organisms and other submerged structures (Heard, 1982; Anderson, 1985; Massie, 1998), and in shallow-water habitats with a depth to about 30 cm (Clark et al., 2003). Palaemonids are about 2 to 4 cm long, and are opportunistic feeders, consuming detritus, seaweeds, seagrasses, animals, and phytoplankton (Heard, 1982; Anderson, 1985; Massie, 1998). Like many other coastal species, these shrimps are eurythermal and euryhaline, with optimal survival at salinities of 4–16 and temperatures of 18–25 °C (although they can range from 0 to 38 °C), and from 0 to > 30 salinities) (Wood, 1967; Beck & Cowell, 1976; Heard, 1982; Anderson, 1985). Due to their small size, palaemonids are generally not captured either commercially or recreationally; however, they are a food source for many commercially and ecologically important fishes and crustaceans, including the summer flounder (Paralychthys dentatus Linnaeus, 1766), killifish (Fundulus heteroclitus (Linnaeus, 1776)), and blue crabs (Callinectes sapidusRathbun, 1896), as well as some shore birds (Rozas & Hackney, 1984; Heard, 1982; Anderson, 1985), making them an integral part of the marine and estuarine food webs and a trophic link to terrestrial ecosystems. Long Island is a heavily populated region on the US east coast where artificial substrates like bulkheads and floating docks are very common, along with associated fouling organisms (e.g. tunicates, bryozoans, gastropods, bivalves, barnacles) and mobile species such as palaemonids (Freeman et al., 2016). Several native species of palaemonids, including Palaemon pugio (Holthuis, 1949), P. mundusnovusDe Grave & Ashelby, 2013, and P. vulgaris Say, 1818 are found in Long Island Sound along with P. macrodactylusRathbun, 1902, which was introduced to the New York City metropolitan area in 2001 (Warkentine & Rachlin, 2010), but currently appears to be in low abundance in Long Island (MR, unpublished data). It should be noted that these species had been included in the genus PalaemonetesHeller, 1869 for many years, but the genus was recently synonymized with PalaemonWeber, 1795 (De Grave and Ashelby, 2013), which will be used herein. Long Island has an extensive shoreline spanning hundreds of kilometers with varying degrees of habitat alteration, making Long Island an ideal location to study the effects of artificial versus natural habitats on population abundance and distribution of palaemonids. Moreover, the recent invader P. macrodactylus is very similar in morphology, diet, and habitat preferences to its native congener (Fofonoff et al., 2003). Understanding how artificial habitats affect the distribution of native palaemonids may subsequently provide insight into how such substrates may also influence the spread of this non-native species. We asked whether artificial structures differ from natural substrates in influencing shrimp densities in the region, and what other abiotic and biotic factors may also be influencing shrimp densities and distributions among these different habitat types. We collected abundance data of palaemonids from several Long Island sites and compared them between natural and artificial habitats at low and high tides. Several other abiotic and biotic variables that could help explain observed trends were analyzed, including wave action, temperature, salinity, vegetation cover, and local taxonomic richness (primarily of stationary or fouling organisms, but also a few mobile animals). Altogether, our results for this ecologically important group of local grass shrimps add to the growing literature demonstrating the impact of increasing shoreline development on marine ecosystems worldwide. MATERIALS AND METHODS Sample sites We sampled 10 sites between 8 July and 6 August 2014 in Long Island, six of which were along the north shore, one on the south shore, one in the Bronx on the mainland coast northwest of Long Island, and two on the south shore of the north fork (Fig. 1). Each site contained at least one of the three habitat types (bulkhead, floating dock, and natural habitats) (Table 1). Bulkheading was composed of wood, concrete, or metal, whereas floating docks were all composed of wood planks on top of submerged plastic floats. Natural habitats were comprised of a gently sloping intertidal zone with sand, mud, or rocks as substrate, with or without vegetation. We sampled within 1.5 hours of high or low tide during daylight hours, and we sampled each habitat type at each tide at five different sampling sites (e.g., five of the sample sites contained a bulkhead habitat sampled at high tide) (Table 1; see Supplementary material Appendix S1 for details). Figure 1. View largeDownload slide The sampling sites along the coasts of Long Island, NY, USA. Figure 1. View largeDownload slide The sampling sites along the coasts of Long Island, NY, USA. Table 1. Sampling location, tide, habitat type, number of samples collected, and shrimp densities (m–3) at each sampling time. OBMC, Oyster Bay Marine Center; OBWC, Oyster Bay Waterfront Center; PJH, Port Jefferson Harbor; CSH, Cold Spring Harbor. Date (2014) Time Site Tide Habitat Samples Shrimp densities (m–3) per sample 08 Jul 1445 Northport Bay Low Natural 3 (0, 0, 101) 11 Jul 0544 OBMC Low Docks 5 (0, 40, 0, 27, 54) 16 Jul 1026 Northport Bay Low Docks 5 (47, 0, 67, 0, 7) 17 Jul 1122 PJH Low Bulkhead 5 (7, 0, 0, 0, 7) 17 Jul 1122 PJH Low Docks 5 (0, 0, 0, 0, 0) 18 Jul 1027 Flanders Bay Low Bulkhead 5 (175, 67, 436, 503, 409) 18 Jul 1216 Peconic Bay Low Bulkhead 5 (604, 282, 342, 1087, 269) 18 Jul 1216 Peconic Bay Low Docks 5 (40, 13, 161, 20, 222) 23 Jul 1500 CSH Low Natural 5 (7, 20, 20, 67, 47) 24 Jul 1010 Hempstead Harbor High Natural 5 (0, 0, 0, 0, 0) 24 Jul 1010 Hempstead Harbor High Bulkhead 5 (0, 0, 0, 0, 0) 24 Jul 1200 Pelham Bay High Docks 5 (7, 20, 7, 20, 0) 24 Jul 1200 Pelham Bay High Bulkhead 5 (0, 20, 7, 0, 0) 25 Jul 1753 OBMC Low Natural 5 (0, 67, 20, 188, 269) 25 Jul 1651 Hempstead Harbor Low Natural 5 (0, 0, 0, 7, 0) 25 Jul 1309 Jones Inlet Low Docks 5 (40, 27, 87, 0, 20) 25 Jul 1309 Jones Inlet Low Bulkhead 5 (148, 0, 87, 0, 20) 30 Jul 1325 CSH High Bulkhead 5 (94, 13, 0, 47, 7) 30 Jul 1325 CSH High Natural 5 (0, 0, 0, 0, 40) 30 Jul 1425 OBMC High Natural 5 (0, 7, 0, 0, 47) 30 Jul 1425 OBMC High Bulkhead 5 (20, 7, 0, 0, 0) 30 Jul 1425 OBMC High Docks 5 (0, 0, 0, 0, 0) 30 Jul 1526 OBWC High Natural 5 (0, 0, 0, 0, 0) 31 Jul 1500 Northport Bay High Bulkhead 5 (13, 101, 1134, 215, 0) 31 Jul 1500 Northport Bay High Docks 5 (20, 315, 295, 0, 60) 31 Jul 1604 Northport Bay High Natural 5 (0, 0, 0, 0, 0) 05 Aug 1322 OBWC Low Natural 5 (0, 0, 0, 27, 0) 05 Aug 1811 Huntington Harbor High Docks 5 (161, 60, 7, 20, 7) 05 Aug 1912 OBWC High Docks 5 (7, 7, 0, 13, 7) 06 Aug 1447 Huntington Harbor Low Bulkhead 5 (846, 638, 336, 906, 618) Date (2014) Time Site Tide Habitat Samples Shrimp densities (m–3) per sample 08 Jul 1445 Northport Bay Low Natural 3 (0, 0, 101) 11 Jul 0544 OBMC Low Docks 5 (0, 40, 0, 27, 54) 16 Jul 1026 Northport Bay Low Docks 5 (47, 0, 67, 0, 7) 17 Jul 1122 PJH Low Bulkhead 5 (7, 0, 0, 0, 7) 17 Jul 1122 PJH Low Docks 5 (0, 0, 0, 0, 0) 18 Jul 1027 Flanders Bay Low Bulkhead 5 (175, 67, 436, 503, 409) 18 Jul 1216 Peconic Bay Low Bulkhead 5 (604, 282, 342, 1087, 269) 18 Jul 1216 Peconic Bay Low Docks 5 (40, 13, 161, 20, 222) 23 Jul 1500 CSH Low Natural 5 (7, 20, 20, 67, 47) 24 Jul 1010 Hempstead Harbor High Natural 5 (0, 0, 0, 0, 0) 24 Jul 1010 Hempstead Harbor High Bulkhead 5 (0, 0, 0, 0, 0) 24 Jul 1200 Pelham Bay High Docks 5 (7, 20, 7, 20, 0) 24 Jul 1200 Pelham Bay High Bulkhead 5 (0, 20, 7, 0, 0) 25 Jul 1753 OBMC Low Natural 5 (0, 67, 20, 188, 269) 25 Jul 1651 Hempstead Harbor Low Natural 5 (0, 0, 0, 7, 0) 25 Jul 1309 Jones Inlet Low Docks 5 (40, 27, 87, 0, 20) 25 Jul 1309 Jones Inlet Low Bulkhead 5 (148, 0, 87, 0, 20) 30 Jul 1325 CSH High Bulkhead 5 (94, 13, 0, 47, 7) 30 Jul 1325 CSH High Natural 5 (0, 0, 0, 0, 40) 30 Jul 1425 OBMC High Natural 5 (0, 7, 0, 0, 47) 30 Jul 1425 OBMC High Bulkhead 5 (20, 7, 0, 0, 0) 30 Jul 1425 OBMC High Docks 5 (0, 0, 0, 0, 0) 30 Jul 1526 OBWC High Natural 5 (0, 0, 0, 0, 0) 31 Jul 1500 Northport Bay High Bulkhead 5 (13, 101, 1134, 215, 0) 31 Jul 1500 Northport Bay High Docks 5 (20, 315, 295, 0, 60) 31 Jul 1604 Northport Bay High Natural 5 (0, 0, 0, 0, 0) 05 Aug 1322 OBWC Low Natural 5 (0, 0, 0, 27, 0) 05 Aug 1811 Huntington Harbor High Docks 5 (161, 60, 7, 20, 7) 05 Aug 1912 OBWC High Docks 5 (7, 7, 0, 13, 7) 06 Aug 1447 Huntington Harbor Low Bulkhead 5 (846, 638, 336, 906, 618) View Large Table 1. Sampling location, tide, habitat type, number of samples collected, and shrimp densities (m–3) at each sampling time. OBMC, Oyster Bay Marine Center; OBWC, Oyster Bay Waterfront Center; PJH, Port Jefferson Harbor; CSH, Cold Spring Harbor. Date (2014) Time Site Tide Habitat Samples Shrimp densities (m–3) per sample 08 Jul 1445 Northport Bay Low Natural 3 (0, 0, 101) 11 Jul 0544 OBMC Low Docks 5 (0, 40, 0, 27, 54) 16 Jul 1026 Northport Bay Low Docks 5 (47, 0, 67, 0, 7) 17 Jul 1122 PJH Low Bulkhead 5 (7, 0, 0, 0, 7) 17 Jul 1122 PJH Low Docks 5 (0, 0, 0, 0, 0) 18 Jul 1027 Flanders Bay Low Bulkhead 5 (175, 67, 436, 503, 409) 18 Jul 1216 Peconic Bay Low Bulkhead 5 (604, 282, 342, 1087, 269) 18 Jul 1216 Peconic Bay Low Docks 5 (40, 13, 161, 20, 222) 23 Jul 1500 CSH Low Natural 5 (7, 20, 20, 67, 47) 24 Jul 1010 Hempstead Harbor High Natural 5 (0, 0, 0, 0, 0) 24 Jul 1010 Hempstead Harbor High Bulkhead 5 (0, 0, 0, 0, 0) 24 Jul 1200 Pelham Bay High Docks 5 (7, 20, 7, 20, 0) 24 Jul 1200 Pelham Bay High Bulkhead 5 (0, 20, 7, 0, 0) 25 Jul 1753 OBMC Low Natural 5 (0, 67, 20, 188, 269) 25 Jul 1651 Hempstead Harbor Low Natural 5 (0, 0, 0, 7, 0) 25 Jul 1309 Jones Inlet Low Docks 5 (40, 27, 87, 0, 20) 25 Jul 1309 Jones Inlet Low Bulkhead 5 (148, 0, 87, 0, 20) 30 Jul 1325 CSH High Bulkhead 5 (94, 13, 0, 47, 7) 30 Jul 1325 CSH High Natural 5 (0, 0, 0, 0, 40) 30 Jul 1425 OBMC High Natural 5 (0, 7, 0, 0, 47) 30 Jul 1425 OBMC High Bulkhead 5 (20, 7, 0, 0, 0) 30 Jul 1425 OBMC High Docks 5 (0, 0, 0, 0, 0) 30 Jul 1526 OBWC High Natural 5 (0, 0, 0, 0, 0) 31 Jul 1500 Northport Bay High Bulkhead 5 (13, 101, 1134, 215, 0) 31 Jul 1500 Northport Bay High Docks 5 (20, 315, 295, 0, 60) 31 Jul 1604 Northport Bay High Natural 5 (0, 0, 0, 0, 0) 05 Aug 1322 OBWC Low Natural 5 (0, 0, 0, 27, 0) 05 Aug 1811 Huntington Harbor High Docks 5 (161, 60, 7, 20, 7) 05 Aug 1912 OBWC High Docks 5 (7, 7, 0, 13, 7) 06 Aug 1447 Huntington Harbor Low Bulkhead 5 (846, 638, 336, 906, 618) Date (2014) Time Site Tide Habitat Samples Shrimp densities (m–3) per sample 08 Jul 1445 Northport Bay Low Natural 3 (0, 0, 101) 11 Jul 0544 OBMC Low Docks 5 (0, 40, 0, 27, 54) 16 Jul 1026 Northport Bay Low Docks 5 (47, 0, 67, 0, 7) 17 Jul 1122 PJH Low Bulkhead 5 (7, 0, 0, 0, 7) 17 Jul 1122 PJH Low Docks 5 (0, 0, 0, 0, 0) 18 Jul 1027 Flanders Bay Low Bulkhead 5 (175, 67, 436, 503, 409) 18 Jul 1216 Peconic Bay Low Bulkhead 5 (604, 282, 342, 1087, 269) 18 Jul 1216 Peconic Bay Low Docks 5 (40, 13, 161, 20, 222) 23 Jul 1500 CSH Low Natural 5 (7, 20, 20, 67, 47) 24 Jul 1010 Hempstead Harbor High Natural 5 (0, 0, 0, 0, 0) 24 Jul 1010 Hempstead Harbor High Bulkhead 5 (0, 0, 0, 0, 0) 24 Jul 1200 Pelham Bay High Docks 5 (7, 20, 7, 20, 0) 24 Jul 1200 Pelham Bay High Bulkhead 5 (0, 20, 7, 0, 0) 25 Jul 1753 OBMC Low Natural 5 (0, 67, 20, 188, 269) 25 Jul 1651 Hempstead Harbor Low Natural 5 (0, 0, 0, 7, 0) 25 Jul 1309 Jones Inlet Low Docks 5 (40, 27, 87, 0, 20) 25 Jul 1309 Jones Inlet Low Bulkhead 5 (148, 0, 87, 0, 20) 30 Jul 1325 CSH High Bulkhead 5 (94, 13, 0, 47, 7) 30 Jul 1325 CSH High Natural 5 (0, 0, 0, 0, 40) 30 Jul 1425 OBMC High Natural 5 (0, 7, 0, 0, 47) 30 Jul 1425 OBMC High Bulkhead 5 (20, 7, 0, 0, 0) 30 Jul 1425 OBMC High Docks 5 (0, 0, 0, 0, 0) 30 Jul 1526 OBWC High Natural 5 (0, 0, 0, 0, 0) 31 Jul 1500 Northport Bay High Bulkhead 5 (13, 101, 1134, 215, 0) 31 Jul 1500 Northport Bay High Docks 5 (20, 315, 295, 0, 60) 31 Jul 1604 Northport Bay High Natural 5 (0, 0, 0, 0, 0) 05 Aug 1322 OBWC Low Natural 5 (0, 0, 0, 27, 0) 05 Aug 1811 Huntington Harbor High Docks 5 (161, 60, 7, 20, 7) 05 Aug 1912 OBWC High Docks 5 (7, 7, 0, 13, 7) 06 Aug 1447 Huntington Harbor Low Bulkhead 5 (846, 638, 336, 906, 618) View Large Sampling methods We took point measurements for salinity and water temperature (°C) at each site using a hand-held YSI meter (YSI, Yellow Springs, OH, USA). For each habitat type at a site, we randomly selected five sampling locations and oriented a dip net at a 45 ̊angle to the substrate with the net just fully submerged, scooped along a 3 m transect line, and immediately removed the net to count captured shrimps. The substrate in the artificial habitats was the side of the floating dock or bulkheading. In the natural habitats, we held the net still for 1 min at the starting end of the transect before sampling to minimize sediment disturbance. It appears that the roughness or unevenness of substrate, which could have affected the accuracy of our sampling, had a negligible effect on our results. The evenness of the substrates at the sampling sites varied depending on the substrate type. Uneven substrates were mainly caused by fouling organisms on artificial structures, and by small rocks and grasses in natural habitats. Even so, the extent of this variability was relatively uniform across habitat types, so it was unlikely that surface unevenness affected our ability to catch shrimp in one habitat more than another. In addition to shrimp counts per scoop, we also recorded the following: substrate type, wave action, vegetation cover, types of vegetation, and other biota present on the substrate (to obtain a measure of taxonomic richness per sample). Substrate type was categorized as concrete, metal, or wood for bulkheading, and as sand, rock, sand and rocks, or mud and rocks in natural habitats. The substrate was the same for all floating docks: wood on top and plastic on the bottom. Vegetation cover was considered low when the substrate along the transect was < 1/3 covered, medium when it was 1/3 to 2/3 covered, and high when it was > 2/3 covered in vegetation. Wave action was qualitatively categorized as low, medium, or high based on wave height. Local taxonomic richness, or the number of taxa found within a transect, was determined at the class or subphylum level and included the following: Phaeophyceae (brown algae), Rhodophyta (red algae), Chlorophyta (green algae, primarily Ulva spp.), cord grass (primarily Spartina spp.), Osteichthyes (bony fishes), Tunicata (tunicates), Bivalvia (bivalves, primarily mussels), Crustacea (crabs and shrimps), Gastropoda (snails), Anthozoa (sea anemones), Hydrozoa (hydroids), Polychaeta (polychaete worms), Ctenophora (ctenophores), Medusozoa (jellyfishes, or jellies), and Merostomata (horseshoe crabs). We calculated shrimp densities per tow by determining the volume of water sampled per tow using the formula: V = (πr2/2) H, where r = radius of the net (17.78 cm) and H = tow distance (3 m). The value for πr2 was divided in half to represent the sampling orientation at a 45o angle. Altogether, this yielded a volume of 0.15 m3 of water sampled per tow. This number was then solved for unity by dividing 1 by 0.15 m3, which equaled 6.67. Each sample was then multiplied by this number to extrapolate the density of palaemonids m–3 (see Table 1). Although it is likely that we collected both native and non-native palaemonid species in our study (based on field observations), a related quantitative study (MR, unpublished data) in the same region revealed that only about 0.3% of the sampled shrimps were the non-native species (P. macrodactylus) and thus represented a very small proportion of the grass shrimps at our sites. As a result of this finding, native and non-native palaemonids were collectively analyzed in our study here. Data analysis To understand the important drivers of the distribution of palaemonid shrimps within our sampling region (Long Island), we fit a negative binomial generalized linear mixed model (GLMM) with shrimp abundance as our response variable, site as a random effect, and the following parameters as fixed effects: tide (low, high), temperature (21 °C to 29 °C, mean 25.5 °C, median 26 °C), salinity (5 to 27, mean 22.0, median 23.5), habitat type (natural, docks, bulkhead), wave action (low, medium, high), vegetation cover (none, low, medium, high), and taxonomic richness (from 0 to 7), and the interaction of vegetation cover and richness. An exhaustive suite of models with these seven fixed effects were then evaluated, and the fully orthogonal model and its subsets were compared using the corrected Akaike’s Information Criterion (AICc) to select the best performing model. Once the lowest AICc was determined, we calculated each model’s ΔAICc value and its corresponding AICc weight (wi), the latter providing a measure of model likelihood normalized by the sum of all model likelihoods (i.e., values close to 1 indicate a measure of the strength of the best fitted model against all other evaluated models). We also evaluated the Bayesian Information Criterion (BIC) to provide an additional model selection criterion for each model. Using these two model criteria, we determined a likely best fit model for our dataset (based on the parameters measured) as the model with the highest AICc weight and the lowest BIC value. SPSS v. 24 and Sigma Plot 13.0 were used to run our analyses. RESULTS Shrimp density was highly variable across our sampling region, ranging from 0 to 1134 shrimps m–3 with a mean of 83 (± 197 SD) m–3, a median of seven shrimps m–3, and mode of 0 shrimps m–3 (see Table 1). We explored in our GLMM an exhaustive suite of models for our seven-fixed effect parameters and site as a random effect with shrimp abundance as the response variable. Altogether, this generated a number of models with low AICc weights, and there were only four models with AICc weights > 0.01. These four models included the following parameters: temperature, habitat, vegetation cover, taxonomic richness, and vegetation cover*richness (Table 2). Table 2. The parameters included in the four best performing models and their associated Bayesian Information Criterion (BIC), corrected Akaike Information Criterion (AICc), ΔAICc, and AICc weight (wi). Model Parameters BIC AICc ΔAICc wi 1 habitat, temperature, taxa richness, vegetation cover 596.28 593.37 0.00 0.57 2 habitat, temperature, vegetation cover 597.67 594.75 1.38 0.28 3 habitat, temperature, taxa richness, vegetation cover, vegetation cover*taxa richness 599.61 596.72 3.35 0.11 4 habitat, taxa richness, vegetation cover 602.01 599.09 5.72 0.03 Model Parameters BIC AICc ΔAICc wi 1 habitat, temperature, taxa richness, vegetation cover 596.28 593.37 0.00 0.57 2 habitat, temperature, vegetation cover 597.67 594.75 1.38 0.28 3 habitat, temperature, taxa richness, vegetation cover, vegetation cover*taxa richness 599.61 596.72 3.35 0.11 4 habitat, taxa richness, vegetation cover 602.01 599.09 5.72 0.03 View Large Table 2. The parameters included in the four best performing models and their associated Bayesian Information Criterion (BIC), corrected Akaike Information Criterion (AICc), ΔAICc, and AICc weight (wi). Model Parameters BIC AICc ΔAICc wi 1 habitat, temperature, taxa richness, vegetation cover 596.28 593.37 0.00 0.57 2 habitat, temperature, vegetation cover 597.67 594.75 1.38 0.28 3 habitat, temperature, taxa richness, vegetation cover, vegetation cover*taxa richness 599.61 596.72 3.35 0.11 4 habitat, taxa richness, vegetation cover 602.01 599.09 5.72 0.03 Model Parameters BIC AICc ΔAICc wi 1 habitat, temperature, taxa richness, vegetation cover 596.28 593.37 0.00 0.57 2 habitat, temperature, vegetation cover 597.67 594.75 1.38 0.28 3 habitat, temperature, taxa richness, vegetation cover, vegetation cover*taxa richness 599.61 596.72 3.35 0.11 4 habitat, taxa richness, vegetation cover 602.01 599.09 5.72 0.03 View Large The best fit model (F7,140 = 4.94, P < 0.001) in our analysis had the highest AICc weight at wi = 0.57 and the lowest BIC (596.28) of all evaluated models and included four parameters: habitat (F2,140 = 9.12; P < 0.001), temperature (F1,140 = 2.521; P = 0.12), taxonomic richness (F1,140 = 5.13, P = 0.03), and vegetation cover (F3,140 = 1.80; P = 0.15). The second model had an AICc weight of wi = 0.28 and a BIC of 597.67, which was similar to that of the previous model, suggesting it may also be a likely model. This model (F6,141 = 4.46, P < 0.001) included three parameters: habitat (F2,141 = 8.87, P < 0.001), temperature (F1,141 = 0.88, P = 0.35), and vegetation cover (F3,141 = 4.54, P = 0.01). The third model (F6,137 = 3.71, P < 0.001) had an AICc weight of wi = 0.11 and a BIC of 599.61 and included five parameters: habitat (F2,137 = 8.51, P < 0.001), temperature (F1,137 = 4.16, P = 0.04), taxonomic richness (F1,137 = 6.750, P = 0.01), vegetation cover (F3,137 = 4.036, P = 0.01), and vegetation cover*taxonomic richness (F3,137 = 3.311, P = 0.02). The fourth model (F6,141 = 4.493, P < 0.001) had an AICc weight of wi = 0.03 and a BIC of 602.01; it included three parameters: habitat (F2,141 = 9.31, P < 0.001), taxonomic richness (F1,141 = 3.306, P = 0.07), and vegetation cover (F3,141 = 2.74, P = 0.05) (Table 2). The significant parameters (habitat, temperature, taxonomic richness, and vegetation cover) are graphically demonstrated in Figs. 2–5. Figure 2. View largeDownload slide Box plot of palaemonid shrimp densities (m–3) in three habitats: natural (N = 48), floating docks (N = 50), and bulkheads (N = 50). In post-hoc pairwise analyses, bulkheads had significantly higher (P < 0.05) shrimp densities than natural habitats or floating docks. Figure 2. View largeDownload slide Box plot of palaemonid shrimp densities (m–3) in three habitats: natural (N = 48), floating docks (N = 50), and bulkheads (N = 50). In post-hoc pairwise analyses, bulkheads had significantly higher (P < 0.05) shrimp densities than natural habitats or floating docks. Figure 3. View largeDownload slide Relationship between taxonomic richness (number of taxa within the transect) and density of palaemonid shrimps (m–3) across all three habitats combined. Richness ranged between 0 taxa and 7 taxa across the study region. The relationship between richness and density fit better to a quadratic (r2 = 0.100) versus linear (r2 = 0.059) curve. Figure 3. View largeDownload slide Relationship between taxonomic richness (number of taxa within the transect) and density of palaemonid shrimps (m–3) across all three habitats combined. Richness ranged between 0 taxa and 7 taxa across the study region. The relationship between richness and density fit better to a quadratic (r2 = 0.100) versus linear (r2 = 0.059) curve. Figure 4. View largeDownload slide Box plot of density of palaemonid shrimps (m–3) in areas with varying levels of vegetation cover (none (N = 28), low (N = 77), medium (N = 16), and high (N = 27)) across all three habitats combined. Post-hoc pairwise analyses demonstrated low vegetation cover to have significantly higher (P < 0.05) shrimp densities than no vegetation cover, but no difference between low, medium, and high vegetation cover. Figure 4. View largeDownload slide Box plot of density of palaemonid shrimps (m–3) in areas with varying levels of vegetation cover (none (N = 28), low (N = 77), medium (N = 16), and high (N = 27)) across all three habitats combined. Post-hoc pairwise analyses demonstrated low vegetation cover to have significantly higher (P < 0.05) shrimp densities than no vegetation cover, but no difference between low, medium, and high vegetation cover. Figure 5. View largeDownload slide Relationship between water temperature (°C) and palaemonid shrimp densities (m–3) across all three habitats combined. Water temperatures ranged from 21 °C to 28 °C across our study period from 8 July to 6 August; r2 = 0.029. Figure 5. View largeDownload slide Relationship between water temperature (°C) and palaemonid shrimp densities (m–3) across all three habitats combined. Water temperatures ranged from 21 °C to 28 °C across our study period from 8 July to 6 August; r2 = 0.029. DISCUSSION The densities of palaemonid shrimps were influenced by several factors, especially habitat type, local taxonomic richness, temperature, and vegetation cover. Shrimp densities were higher in artificial compared to natural habitats, particularly along bulkheading (Fig. 2). Moreover, shrimp density was highest at intermediate numbers of taxa (i.e., moderate taxonomic richness) (Fig. 3). Density was also higher in areas that contained vegetation cover, whereas few to no shrimps were caught at sites with no vegetation cover (Fig. 4). In one of our best fit models (Table 2), the inclusion of the interaction between vegetation cover and richness improved the model fit. Shrimp density was weakly but positively correlated with temperature (Fig. 5). Altogether, the importance of these multiple factors demonstrates the complexity of the abiotic and biotic environment in influencing the distribution of shrimps throughout our study region. Several prior studies demonstrated a significant effect of artificial habitats on species abundance, both positively (Caine,1987; Freeman et al., 2016) and negatively (Seitz et al., 2006; Bilkovic & Roggero, 2008; Morley et al., 2012; Patrick et al., 2014). In the case of palaemonids, the increased hard substrate provided by bulkheads and floating docks could support more complex fouling communities that the shrimps can use as a food source or as shelter from predators (Caine, 1987; Freeman et al., 2016). We expected to find higher shrimp densities along artificial substrates due to more complex fouling communities associated with these structures. This is partially supported by the finding that densities were greater with moderate to higher taxonomic richness. For example, many of the samples that had no shrimps present only had green algae, primarily Ulva spp. on the substrate. The samples with high shrimp densities, however, often contained additional algal species and a variety of fouling and mobile organisms, with tunicates and brown algae being especially prevalent in these higher density samples. All three habitat types (bulkheading, floating docks, and natural habitats) contained green and brown algae, bivalves, crustaceans, and bony fishes in at least some of their samples. There were a few small differences between habitat types: red algae and cord grass (primarily Spartina spp.) were unique to the natural habitat; ctenophores, jellyfishes, and horseshoe crabs were only found along bulkheading, though these were relatively scarce; and hydroids, sea anemones, and polychaete worms were only found along floating docks, but were also relatively scarce. The most common fouling organisms were tunicates, which were absent in any of the natural habitat samples, where shrimp densities were lower than in artificial habitats. Higher taxonomic richness may indicate higher food supply, variety of food items, and availability of shelter created by the tunicates and other sessile organisms found on these structures (Caine, 1987). Sessile organisms provide a diverse and abundant food supply, as palaemonids prey on a variety of organisms including detritus, epiphytic microalgae and fauna, and mysids (Odum & Heald, 1972; Morgan, 1980). Shrimps may be using the three-dimensional structure formed by other sessile organisms, especially tunicates, as a shelter from predators (Caine, 1987; Cerda & Castilla, 2001; Krohling et al., 2006; Ferreira et al., 2001). Furthermore, the inclusion of vegetation cover, along with the interaction between vegetation cover and taxonomic richness, improved model fit and confidence (Table 2), suggesting that vegetation cover was also an important aspect in driving shrimp densities. Few shrimps were found where there was no vegetation cover, whereas there was high variability in shrimp density in low vegetation cover, which had the highest mean shrimp densities (Fig. 4). Moderate densities were found for medium and high vegetation cover (Fig. 4). This finding is supported by other studies showing higher abundances of shrimps in habitats containing vegetation, particularly seagrasses (Heard, 1982; Anderson, 1985; Pérez-Castañeda et al., 2010; Blanco-Martínez & Pérez-Castañeda, 2016). Subaquatic vegetation provides an important shelter, nursery, and food source for a variety of crustaceans, including palaemonids, as well as for other marine organisms (Heck et al., 2003; Patrick et al., 2014). The natural habitats sampled were composed of a variety of substrate types: sand, rock, sand and rocks, or mud and rocks. It could be argued that sampling a variety of substrate types may have affected our results by increasing variability in shrimp densities for natural habitats. It was nevertheless found that densities were more variable along bulkheading, which had a more uniform substrate, suggesting that variability in substrate types had little effect on our results. Densities among the artificial habitats were higher along bulkheading compared to floating docks (Fig. 2). This was a surprising finding considering there is often less vegetation on bulkheading to use as shelter or food. Lower densities along docks, however, may be attributable to greater human activity such as increased boat traffic or movement of docks with human use. Such activities were frequently observed at most of the docks we sampled and thus may affect shrimp usage of these docks. Alternatively, shrimp density could actually be higher underneath the docks as compared to along the side of the docks. While not included as data for this study, we did take one sample from underneath a dock toward the end of our study period and found relatively high densities of shrimps in that sample. While this sample is not representative of all docks, it does provide some circumstantial evidence. Shrimps are preyed upon by fishes (Diener et al., 1974; Overstreet & Heard, 1982) and shore birds, including herons, rails, and ducks (Ogden et al., 1976; Kushlan, 1979; Heard, 1982; Howard & Lowe, 1984); thus, they may be more protected under the docks where they could be safer from such predators and may have more crevices in which to hide. Shrimp density demonstrated a weak but positive relationship with increasing temperature (Fig. 5). This could be attributed to higher shrimp metabolic rates at higher temperatures (Brown, 2004); however, given their ability to thrive in a wide range of temperatures, this explanation may be less likely as water temperature during our study only ranged from 21 °C to 29 °C. The palaemonid shrimps in our study are all shallow-water estuarine species that survive in highly variable environments. Palaemon pugio can survive in temperatures ranging from 5 °C to 38 °C, although they thrive from 18 °C to 25 °C (Beck & Cowell, 1976); similarly, P. vulgaris can survive in temperatures ranging from 5 °C to 35 °C (Beck & Cowell, 1976). The observed relationship between temperature and shrimp density could be due to the timing of our study. While densities of shrimps may have increased in our later sampling period as a result of recent recruitment events due to warmer temperatures (i.e., a seasonal effect), there was neither a significant relationship between date and temperature, nor a significant interaction between the two. All samples were taken within a one-month period from sites that were fairly close together, so there was little temperature variation between sites (ranged from 21 °C to 29 °C; mean 25.5 °C; median 26 °C). A study conducted over a greater geographic and/or temporal scale would be needed to better understand this relationship. While wave action was not a statistically significant factor influencing shrimp densities, it is noteworthy mentioning that no shrimps were caught in any of the high-wave-action samples. In addition to creating unfavorable conditions for shrimp to live, high wave action may also limit the number and types of fouling organisms that can survive and provide shelter to palaemonids (Vadas et al., 1990; Pulgar et al., 2012; Garner, 2013). As was previously noted, the presence of substantial three-dimensional structure created by fouling organisms is an important factor enhancing palaemonid populations. The inclusion of tide in our study did not improve model fit in our four best fit models. We had expected little influence of tide on our floating docks since they shift vertically with the tide, resulting in the bottom of the dock and associated fouling community being constantly submerged. Accordingly, means and standard deviations between low and high tide for docks were fairly similar. We expected there could be a tidal effect on densities for bulkheads and natural habitats. Along bulkheads, the shrimps are more spread out at high tide throughout the water column, whereas, the water depth is much shallower at low tide, causing the shrimps to be concentrated in a smaller area, which may be expected to result in higher shrimp density within the transect area. Similarly, more vegetation is submerged in natural habitats at high tide, and shrimps are spread out throughout these vegetated areas. At low tide, however, there is less vegetation for individuals to hide in for protection from predators. Subsequently, there may be a higher concentration of shrimps within the vegetation near the water’s edge, resulting in more shrimps caught along the sampling transect. Such an observation was found in a prior study of the palaemonid P. vulgaris in natural coastal environments: the shrimps were more dispersed throughout submerged vegetation during high tide and more congregated near the water’s edge at low tide (Horváth & Varjú, 1995). Means and standard deviations in our study did differ for both natural habitats and bulkheading between low and high tide: the mean density for both these habitats was about five times higher in low versus high tide, and the standard deviations were broader. Various species of shrimps (including palaemonids), crabs, and fishes experience tidal variation in their distribution within tidal creeks (Anderson, 1985; Hampel et al., 2003). The densities recorded in our study can be compared to those reported in an assessment survey by Carlton & Weigle (2015). The survey utilized very similar methods to those in our study to measure densities of palaemonids along floating docks at sites from New Jersey to Maine. South of Cape Cod (MA), surveyors reported maximum densities of 24.9 m–3 for P. macrodactylus (Fairhaven, MA), 101.8 m–3 for P. pugio (Toms River, NJ), 47.5 m–3 for P. mundusnovus 2013 (Mystic, CT), and 280.5 m–3 for P. vulgaris (Mystic, CT) (Carlton & Weigle, 2015). North of Cape Cod, they reported maximum densities of 20.4 m–3 for P. macrodactylus (Plymouth, MA), 4.5 m–3 for P. pugio, and 24.9 m–3 for P. vulgaris (Marshfield, MA) (Carlton & Weigle, 2015). Maximum shrimp densities along floating docks were higher in our study than many of the above locations; however, this is likely because we measured the collective densities of shrimps as opposed to the densities of individual species along floating docks (see Carlton & Weigle, 2015). In some instances, however, maximum shrimp densities were similar, especially those along Long Island Sound. For example, Carlton & Weigle (2015) recorded a maximum density of 280.5 m–3 for P. vulgaris in Mystic, CT (on the northern banks of Long Island Sound), whereas our maximum collective shrimp density was 315 shrimps m–3 in Northport, NY (on the southern banks of Long Island Sound; Table 1). Overall, our data adds to a growing body of literature on the importance of understanding densities of palaemonid shrimps in the region, and also suggests that the methods used in the present study were effective at capturing shrimps and accurately measuring their densities. Altogether, our study provides novel evidence that artificial habitats can enhance populations of palaemonid shrimps by providing alternate or additional habitat space. Artificial habitats provide additional surface area for the attachment of fouling organisms that then may attract associated mobile species (possibly enhancing food supply and shelter for shrimps). Continued increase in artificial structures along our coastlines will likely continue to influence the sizes of shrimp populations and/or alter the distribution of their populations. Due to their importance as detritivores and as prey for a variety of fishes, invertebrates, and shore birds (Heard, 1982; Rozas & Hackney, 1984; Anderson, 1985), changes in palaemonid populations could influence nutrient cycling as well as the abundances and distributions of associated species (potentially including commercially important species like the summer flounder and blue crab). Since artificial structures support large shrimp populations, increased shoreline development has the potential of also enhancing the spread of the non-native species P. macrodactylus that shares similar habitat preferences to its native congener. As shoreline development rapidly expands along our coastlines, it is vital that we understand how these changes will affect populations of ecologically important species like the palaemonids that play an important role in supporting coastal marine food webs. SUPPLEMENTARY MATERIAL Supplementary material is available at Journal of Crustacean Biology online. S1 Appendix. Habitat types in which each treatment for each secondary parameter was represented. ACKNOWLEDGEMENTS The research was funded by the Summer Internship Program at Long Island University (LIU) Post, NY. This work served as the basis of LM’s honors thesis at Rider University under the guidance of GWS, and with the mentorship of AMHB (now at East Carolina University) and MR (now at the University of Massachusetts Boston). The results of this research were presented at the 44th and 45th Benthic Ecology Meetings. Travel to the conferences was funded by Rider University and the University of Massachusetts Dartmouth. We thank Drs. Paul Jivoff and Hongbing Sun (Rider University) for assistance with the statistical analyses, and all the summer interns and volunteers (particularly I. Kernin, K. Bartley, S. Diaz, and B. Lahoud) for assistance with field work. 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TI - Distribution and tidal variation of palaemonid shrimps (Decapoda: Caridea: Palaemonidae) in artificial and natural habitats
JF - The Journal of Crustacean Biology
DO - 10.1093/jcbiol/ruy017
DA - 2018-03-01
UR - https://www.deepdyve.com/lp/oxford-university-press/distribution-and-tidal-variation-of-palaemonid-shrimps-decapoda-v6KIbWV2uQ
SP - 1
EP - 301
VL - Advance Article
IS - 3
DP - DeepDyve
ER -