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Adaptations to Benthic Development W T > T Bi0l°giCal MoL Dis“ve?in Woods Hole Adaptations to Benthic Development: Functional Morphology of the Attachment Complex of the Brachiolaria Larva in the Sea Star Asterina gibbosa Author(s): Delphine Haesaerts, Michel Uangoux, Patrick Flammang Reviewed work(s): Source: Biological Bulletin, Vol. 211, No. 2 (Oct., 2006), pp. 172-182 Published by: Marine Biological Laboratory Stable URL: http://www.jstor.org/stable/4134591 Accessed: 10/01/2012 03:53 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at http://www.jstor.org/page/info/about/policies/terms.jsp JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected] . Marine Biological Laboratory is collaborating with JSTOR to digitize, preserve and extend access to Ii Biological Bulletin. http://www.jstor.org Reference: Biol. Bull. 211: 172-182. (October 2006) © 2006 Marine Biological Laboratory Adaptations to Benthic Development: Functional Morphology of the Attachment Complex of the Brachiolaria Larva in the Sea Star Asterina gibbosa DELPHINE HAESAERTS1, MICHEL JANGOUX1’2, AND PATRICK FLAMMANG2’* 1 Université Libre de Bruxelles, Laboratoire de Biologie Marine, Académie universitaire Wallonie- Bruxelles, Brussels, Belgium; and 2 Université de Mons-Hainaut, Laboratoire de Biologie Marine, Académie universitaire Wallonie-Bruxelles, Mons, Belgium Abstract. The asteroid Asterina gibbosa lives all its life of attachment to the substratum. The end of this period in close relation to the sea bottom. Indeed, this sea star corresponds to the complete regression of the external larval possesses an entirely benthic, lecithotrophic development. structures, which also coincides with the opening of the The embryos adhere to the substratum due to particular mouth. This sequence of stages, each possessing its own properties of their jelly coat, and hatching occurs directly at adhesive strategy, is common to all asteroid species having the brachiolaria stage. Brachiolariae have a hypertrophied, a benthic development. In A. gibbosa, morphological adap­ bilobed attachment complex comprising two asymmetrical tations to this mode of development include the hypertro­ brachiolar arms and a central adhesive disc. This study aims phic growth of the attachment complex, its bilobed shape at describing the ultrastructure of the attachment complex forming an almost completely adhesive sole, and the regres­ and possible adaptations, at the cellular level, to benthic sion of the sensory equipment. development. Immediately after hatching, early brachio­ lariae attach by the arms. All along the anterior side of each Introduction arm, the epidermis encloses several cell types, such as secretory cells of two types (A and B), support cells, and Brachiolaria larvae, which are characterized by a special­ sensory cells. Like their equivalents in planktotrophic lar­ ized attachment complex comprising three larval arms (bra­ vae, type A and B secretory cells are presumably involved chia) and an adhesive disc, occur in the life cycle of many in a duo-glandular system in which the former are adhesive asteroids whatever their nutritional mode (planktotrophy and the latter de-adhesive in function. Unlike what is ob­ versus lecithotrophy) or their developmental habitat (pe­ served in planktotrophic larvae, the sensory cells are unspe­ lagic, benthic, or intragonadal) (McEdward and Janies, cialized and presumably not involved in substratum testing. 1993, 1997; Byrne, 1999; McEdward and Miner, 2001; During the larval period, the brachiolar arms progressively Byrne et al, 2003). Among the different development pat­ increase in size and the adhesive disc becomes more prom­ terns, pelagic planktotrophy is traditionally considered to be inent. At the onset of metamorphosis, brachiolariae cement the ancestral one (see, e.g., McEdward and Janies, 1993, themselves strongly to the substratum with the adhesive 1997). Successive evolutionary transitions would then have disc. The disc contains two main cell types, support cells occurred to give rise to various types of nonfeeding larval and secretory cells, the latter being responsible for the development by (a) loss of larval feeding structures such as cement release. During this metamorphosis, the brachiolar ciliated bands and functional gut, and gain of large, yolky arms regress while post-metamorphic structures grow con­ eggs, followed by (b) loss of planktonic dispersal, and then siderably, especially the tube feet, which take over the role (c) gain of parental brood protection. Recently, however, molecular phylogenetic studies conducted on the family Received 27 July 2005; accepted 28 May 2006. Asterinidae (Hart et al, 1997, 2004) showed that ordered * To whom correspondence should be addressed. Patrick.Flammang@ transformations between the four modes of development umh.ac.be comprising brachiolariae (i.e., planktotrophic-pelagic, leci- 172 THE ATTACHMENT COMPLEX OF A BENTHIC ASTEROID LARVA 173 thotrophic-pelagic, lecithotrophic-benthic, and lecithotro- plex of A. gibbosa and to possible adaptations, at the cel­ phic-intragonadal) could not be easily reconstructed, and lular level, to benthic development. that many parallel changes in larval form, habitat, and dispersal potential occurred. For example, in this family, Material and Methods benthic lecithotrophy evolved three times independently, in Parvulastra (Patiriella) exigua, in Aquilonastra (Asterina) Larval rearing minor and in the two sister species, Asterina gibbosa and Specimens of Asterina gibbosa (Pennant, 1777) were Asterina phylactica (Hart et al., 2004; Byrne, 2006). In collected intertidally at Roscoff (Brittany, France) in 2001, these species, brachiolariae all possess a hypertrophied at­ 2002, and 2003 during spring tides. They were kept in tachment complex (Ludwig, 1882; MacBride, 1896; Ko­ marine aquaria (14 °C, 33 psu) at the marine biology labo­ matsu et al.y 1979; Marthy, 1980; Byrne, 1995, 2006).ratory of the University of Mons-Hainaut. Each year, at the Larvae of A. gibbosa differ, however, from those of A. end of April, groups of about 60 individuals were trans­ minor and P. exigua because their attachment complex ferred into a 10-1 tank filled with unfiltered aerated seawater develops as a sole-like structure rather than as a tripod-like at room temperature. When maintained in such conditions, structure as it is the case in the other two species. individuals spawned spontaneously after a few hours to a Very little is known about the fine structure and func­ few days, depending on the group. Fertilized eggs were tioning of the attachment complex in asteroids with benthic collected the day they were laid and transferred into large larvae; most ultrastructural and immunocytochemical infor­ petri dishes at 14 °C and 21 °C. Dishes were filled with mation on this organ comes from planktotrophic larvae. In filtered seawater (0.22 gm). Water was changed daily, and planktotrophic developers, the brachiolar arms provide tem­ the larvae were maintained to the juvenile stage. porary attachment to and sensory testing of the substratum during settlement, whereas the adhesive disc is involved in Morphological observations the permanent attachment that marks the onset of metamor­ phosis (Barker, 1977; Gondolf, 2000; Haesaerts et al., Specimens of A. gibbosa were observed and photo­ 2003). In planktonic larvae, brachiolar arms are tipped with graphed in vivo with a Leica MZ8 binocular microscope several sensory-secretory papillae, in which both the epi­ equipped with a Nikon Coolpix digital camera. For scan­ dermis and the underlying nerve plexus are greatly thick­ ning electron microscopy (SEM), larvae were fixed in ened. The papillary epidermis encompasses support cells, Bouin’s fluid for 12 h, dehydrated in a graded ethanol series, serotonergic sensory cells, and secretory cells (Barker, dried by the critical point method (with C02 as transition 1978; Byrneet al., 2001; Haesaerts et al., 2005a). The latter fluid), mounted on aluminum stubs, coated with gold in a function as a duo-glandular system with some cells acting as sputter coater, and observed with a JEOL JSM-6100 scan­ adhesive cells and some others as de-adhesive cells (Her­ ning electron microscope. Images were digitized with the mans, 1983; Flammang, 1996; Haesaertset al., 2005a). As SEMafore 3.0 Pro software (JEOL, Tokyo, Japan). Some for the adhesive disc, it is a rounded, concave structure lying additional larvae were fixed in glutaraldehyde and postfixed between the brachiolar arms. It is made up of support cells, in osmium tetroxide (see below), a method that does not ciliated secretory cells, and neurons (Barker, 1978; Gon­ preserve the cuticle and thus reveals the underlying struc­ dolf, 2000; Byrne et al., 2001; Haesaerts et al., 2005a). In tures (Ameye et al., 2000). benthic developers, on the other hand, hypertrophied growth For light microscopy (LM) and transmission electron of the brachiolar arms has been described as an adaptative microscopy (TEM), specimens were fixed in 3% glutaral­ character to benthic habitat, but it is not known if other dehyde in cacodylate buffer (0.1 mol U 1, pH 7.8, adjusted adaptations occur at the cellular level. to an osmolality of 1030 mOsmol k g '1 with NaCl) for 30 The present
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