Embryonic, Larval, and Post-Larval Development in the Symbiotic Clam Codakia orbicularis (Bivalvia: Lucinidae)

Codakia orbicularis is a large Caribbean lucinid clam with chemoautotrophic bacteria in its gill cells. Its development from spawning up to 2.5 mm shell-length juveniles is described using light, scanning, and transmission electron microscopy. Embryonic development, from large eggs with an abundant vitelline supply, takes place within individual glycoprotein capsules up to the veliger stage. After hatching, 48 hours after fertilization, swimming veligers develop to swimming-crawling pediveligers, then to benthic, crawling plantigrades in 16 days without developing an umbonate secondary larval shell. This first phase of metamorphosis is completed without any specinal environmental stimulation. However, without addition of a suitable substrate, a delay of metamorphosis occurs at the end of the plantigrade stage whereas, in planktotrophic bivalves, a developmental hiatus has been described at the end of the pediveliger stage. When a sterile sand fraction is added, plantigrades enter a second phase of metamorphosis, differentiate gill filaments, a siphonal septum, and a byssal gland, and secrete a fastgrowing juvenile shell, which becomes umbonate. Metamorphosis is completed with the differentiation of the unique excurrent siphon in 2 mm shell-length juveniles. Such a developmental pattern is different from the planktotrophic and from the lecithotrophic developments already described for bivalves. C. orbicularis is a facultative planktotroph with a two-step metamorphosis including long planktonic and benthic stages without significant growth, contrasting with the rapid growth of the post-larval shell. The whole larval and post-larval development occurs without the presence of chemoautotrophic symbionts. Therefore, symbiosis is not necessary to achieve metamorphosis. Additional key words: facultative planktotrophy, metamorphosis Codakia orbicularis (LINNE 1758) is a large lucinid clam, up to 90 mm in shell-length, ranging from Florida through the Caribbean region to Brazil (Abbott 1974). Adult individuals, which live in shallow-water sea-grass beds, harbor sulfur-oxidizing chemoautotrophic bacteria in their gill cells (Berg & Alatalo 1984; Frenkiel & Moueza 1995) like every species of the eight genera of the family Lucinidae so far examined (Reid 1990). Descriptions of larval development in members of the eulamellibranch superfamily Lucinacea are scarce. Two developmental patterns are known to date, one in a member of the family Thyasiridae, Thyasira gouldi (Blacknell & Ansell 1974) and the other in a tropical member of the family Lucinidae, C. orbicularis (Alatalo et al. 1984). Both descriptions are supported by observations on living and fixed larvae a Author for correspondence. E-mail: Liliane.Frenkiel @univ-ag.fr using light microscopy. The first one was done before the discovery of sulfur-oxidizing bacterial symbiosis in Thyasiridae (Dando & Southward 1986) or in other marine invertebrates (Cavanaugh et al. 1981; Felbeck et al. 1981; Cavanaugh 1983). In the second one, Alatalo et al. (1984) postulated that larval development may be sustained by chemoautotrophic nutrition, but gave no information about the presence of bacteria during developmental stages. To date, four species of bivalves known to host chemoautotrophic sulfur-oxidizing bacteria in adult gillcells have been raised successfully (Blacknell & Ansell 1974; Alatalo et al. 1984; Gustafson & Reid 1986, 1988a; Gustafson & Lutz 1992), and interest in the relationships between hosts and their bacterial endosymbionts during developmental stages is increasing. Gill endosymbionts are vertically transmitted from parents to offspring in the protobranchs Solemya reidi (Cary 1994) and S. velum (Krueger et al. 1996), and This content downloaded from 157.55. 39.153 on Mon, 19 Sep 2016 05:11:12 UTC All use subject to http://about.jstor. org/terms Ontogenesis in a lucinid clam environmentally transmitted to the new host generation in C. orbicularis (Gros et al. 1996). Embryonic and larval development of the members of the protobranch family Solemyidae, S. reidi (Gustafson & Reid 1986, 1988a,b), and S. velum (Gustafson & Lutz 1992), have been described using electron microscopy whereas information is lacking on embryonic stages, metamorphosis, and juvenile features of C. orbicularis. Planktic larval development has been described in several bivalve species including mussels (Bayne 1976), oysters (Andrews 1979), clams, and scallops (Sastry 1979; Cragg & Crisp 1991). The most common sequence is external embryonic development resulting in a free-swimming ciliated trochophore, which, upon secretion of the first larval shell, prodissoconch I, becomes a planktonic, D-shaped veliger larva. In planktotrophic species, the free-swimming D-larva grows rapidly and becomes an umbonate veliger, or veliconcha, as its mantle secretes prodissoconch II. The pediveliger is defined as a transitional stage due to the simultaneous presence of functional velum and foot, allowing for alternating swimming and crawling. Regression of the velum commits the larva to benthic life; the crawling postlarva develops functional gill filaments and settles as a crawling, early spat, called a plantigrade (Bayne 1976). The most critical step in metamorphosis appears to be the shift of ciliary feeding and oxygenation functions from velum to gills. In the development of some planktotrophic veligers, a hiatus takes place and a delay of metamorphosis may be observed when pediveligers do not receive appropriate cues for settlement (Bayne 1965, 1976; Hadfield 1978). During such a delay of metamorphosis, metamorphic competence is retained during a period determined by the nutritional status of the pediveligers; growth ceases, and mortality increases, probably due to the lack of ciliary cleansing mechanisms no longer performed by the velum and not yet assumed by the gills. As soon as the gill filaments increase in size and number, plantigrades become resistant to previously deleterious bacteria and acquire specific behavior; oysters cement onto a suitable substrate (Cranfield 1973, 1974), mussels stick to it with byssus threads (Bayne 1976), whereas burrowing species crawl onto the sediment and acquire siphons (Quayle 1952; Ansell 1962; D'Asaro 1967). The crawling plantigrade appears as a short transitory benthic stage common to species which will develop various specific behaviors and complete metamorphosis. The onset of permanent benthic life is also contemporary to the secretion of the dissoconch, which exhibits specific shape and ornamentation. However, the development of gill filaments appears as a prerequisite for metamorphosis, whereas secretion of the dissoconch appears as a result of successful metamorphosis. According to Ockelmann (1965), lecithotrophic development described in several species, is correlated with a short pelagic life and with the absence, or poor development, of prodissoconch II, which distinguishes the shells of lecitotrophic larvae from those of late, planktotrophic larvae. Alatalo et al. (1984) observed that C. orbicularis had some features of lecithotrophic species but a long planktonic stage, and postulated that it could have a mixed nutrition, involving lecithotrophy, facultative planktotrophy, and chemoautotrophy. These authors took for granted that metamorphosis could proceed to completion in C. orbicularis, as in other species with a planktonic larval stage, as soon as the plantigrade early spat had acquired benthic behavior. In the present paper, we describe, with scanning and transmission electron microscopy, the embryonic, larval, and post-larval development of C. orbicularis, obtained repeatedly up to the fully metamorphosed juveniles, 2.5 mm in shell-length.