LARVAL AND EARLY POSTLARVAL DEVELOPMENT OF THE PROTOBRANCH BIVALVE SOLEMYA-VELUM (MOLLUSCA, BIVALVIA)

Early development in the north-west Atlantic awning clam, Solemya velum (Solemyoida:Solemyidae) was examined by light and scanning electron microscopy. Embryonic and pericalymma larval stages, typical of protobranch bivalves, developed within individual adhesive gelatinous egg capsules and offspring emerged at hatching as benthic crawl-away juveniles Fertilized eggs were light orange in colour, spherical, and had a yolk-mass diameter of approximately 190-mu-m. Entirely ciliated cylindrical pericalymma larvae, with a length and width of 318-mu-m and 208-mu-m, respectively, were obtained within 24 h. These larvae lacked an apical ciliary tuft, possessed nine rows of calymma cells, and actively rotated within the egg capsule. During metamorphosis, which occurred within 48 h of fertilization, the transitory larval ectoderm was entirely ingested. The prodissoconch measured roughly 320-mu-m in length by 215-mu-m in height. The foot and ciliated gill buds were well developed by four days after fertilization and subsequent growth contributed to the dissoconch or adult shell. Hatching began at 13 d after fertilization when the shell measured approximately 402-mu-m in length by 251-mu-m in height. Actively crawling and burrowing juveniles were cultured for six months after fertilization and an ontogenetic sequence of encapsulated and juvenile shell morphology was obtained for identification purposes. Current contradictory hypotheses regarding the phylogeny of marine bivalve developmental types are reviewed in light of new information on the Order Solemyoida.