CHAPTER 1 



LARVAE OF MARINE BIVALVES 



(MORPHOLOGY, PHYSIOLOGY, 



AND BEHAVIOR) 



EARLY DEVELOPMENT 



The development of bivalves has been best studied in Dreissena polymorpha, 

 a species inhabiting fresh and brackish waters and exhibiting a development 

 typical for marine bivalves (Meisenheimer, 1901); in Unionidae — freshwater 

 mollusks endowed as larvae, at the glochidium stage, with an adaptation to 

 temporary parasitism (Lillie, 1895); and in Crassostrea gigas — a marine bi- 

 valve in which the larva is a typical veliger (Fujita, 1929). 



Egg 



The eggs of bivalves contain relatively little yolk, which is uniformly 

 distributed throughout the egg cell. The position of an animal pole is indicated 

 by the extrusion of polar bodies in the egg cell. Bivalve eggs have a diameter 

 of 40-360 jxm. Like the eggs of other Bilateria with external fertilization, they 

 are surrounded by a vitelline and a jellylike membrane. The thickness of the 

 vitelline membrane is 1-2 |4.m; the thickness of the jellylike membrane may 

 be more than 10 |im. The jellylike membrane is generally transparent, delicate, 

 and is poorly defined after fixation (Drozdov and Kasyanov, 1985b). The eggs 

 have a jellylike membrane in addition to the vitelline one (Allen, 1953, 1961). 



Fertilization : Eggs and spermatozoa are released in water where fertiliza- 

 tion and subsequent development take place. In species that brood the embryo 

 and larva in the mantle cavity or gills, sperms in the water pass through the 

 incurrent siphon of the maternal organism into the mantle cavity or oviduct 

 where fertilization occurs. In Mysella tumida spermatozoa fall in the gill 

 chambers of the female and the transported sperms become attached to the gill 



