Tissue & Cell 35 (2003) 339–351
Ultrastructural study and ontogenesis of the appendages and
related musculature of Paraspadella (Chaetognatha)
J.-P. Casanova
a,∗
, M. Duvert
b
, T. Goto
c
a
EA 2202 Biodiversité, Laboratoire de Biologie Animale (Plancton), Université de Provence, 3 Place Victor Hugo, 13331 Marseille Cedex 3, France
b
Laboratoire de Cytologie, INSERM EMI 99-29, Université Victor Segalen Bordeaux II, 146 rue Léo Saignat, F-33076 Bordeaux Cedex, France
c
Department of Biology, Faculty of Education, Mie University, Tsu, Mie 514, Japan
Received 18 February 2003; received in revised form 5 June 2003; accepted 6 June 2003
Abstract
A lineage of benthic chaetognaths has developed limb-like appendages on the caudal part of the body, resulting from a local modification
of the lateral fins, which are folds of the epidermis and have a role in balance when swimming. The most complex are those of Paraspadella
gotoi which are used as props with the tip of the tail, allowing an elaborated mating behaviour comprising different movements: complete
erection of the body, swings and jumps, astonishing for so simple-bodied animals. In the tail, the epidermis and the connective tissue,
together with the longitudinal musculature, are involved in this innovation. All the components of the fins, i.e. connective tissue, fin rays
and multilayered epidermic cells are conserved, but their function has changed. The movements of appendages are adjusted by one pair
of small appendicular muscles localised in the body wall, while posture movements of the body are allowed by four longitudinal bundles
of raising muscle. These two new muscles have successively appeared in the evolutive series previously described in Paraspadella. They
have definitely arisen from the secondary muscle: the two lateral bundles for the former, and the two dorsal and two ventral ones for the
latter. All are supercontracting muscles, a muscle kind also observed in the other benthic genus Spadella, but unknown in planktonic and
benthoplanktonic chaetognaths.
© 2003 Elsevier Ltd. All rights reserved.
Keywords: Chaetognatha; Paraspadella; Appendage; Ultrastructure; Ontogenesis; Muscles
1. Introduction
Chaetognaths are a small marine phylum comprising a lit-
tle more than one hundred species among which a fifth be-
longing to the family Spadellidae are strictly benthic. Their
bauplan (Fig. 1) is built around a hydroskeleton which plays
a central role in movement, in association with four longitu-
dinal muscle masses in trunk and tail (Duvert, 1989). These
masses are mainly constituted of primary muscle and of six
discrete bundles of secondary muscle, representing less than
1% of the body wall, one in each quadrant of primary mus-
cle and one at the limit between each ventral quadrant and
the adjacent lateral field (Duvert, 1989, 1991); they stretch
uninterrupted from neck to tip of tail. The species which live
on or near the seabed are moreover provided with a ventral
transverse musculature situated in the trunk and tail in the
Heterokrohniidae and only in the trunk in the Spadellidae.
Chaetognaths have one or two pairs of lateral fins and a
tail one (Fig. 1a) which only have a role in balance (Hyman,
∗
Corresponding author.
1959; Duvert and Salat, 1990). Planktonic species swim ow-
ing to bursts of brief and rapid jerks of the body followed by
passive sinking in the sea water (Bone and Duvert, 1991).
The Spadellidae, which are truly benthic, are also able to
swim on short distances; moreover, they exhibit a trend to-
wards a benthic locomotion pattern, via a key innovation,
i.e. the emergence of paired limb-like appendages from the
posterior part of lateral and/or tail fins in coastal species
(Casanova, 1990).
Spadella has patches of adhesive cells on the ventral body
side (John, 1933); specimens lie on the substrate and can
only erect their trunk vertically. Paraspadella has adhesive
processes of different aspects on the ventral half posterior
part of the tail, the most ‘achieved’ being those of Paras-
padella gotoi (Fig. 2a); they allow the complete erection
of the body and jumping, in relation with an elaborated
mating behaviour. Extant chaetognaths suggest a scenario
helping to understand the aspect and evolution of these kind
of appendages (Casanova et al., 1995): (1) as compared to
planktonic species, the sole major difference in the muscle
apparatus of Spadella cephaloptera consists in the myofib-
rillar apparatus of the secondary muscle. Indeed, the Spadel-
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doi:10.1016/S0040-8166(03)00055-7