the gill filaments (against the normal 

 feeding current) and accumulate near the 

 inhalent chamber. Rapid and repeated con- 

 tractions of the adductor muscle then 

 forcefully eject the eggs a considerable 

 distance. The latter mechanism is also 

 used to expel unwanted particulate mate- 

 rial (pseudofeces) from the mantle cavity. 



Fertilization occurs in the water 

 column via chance encounters of eggs and 

 sperm, and larval development ensues. 

 Thus begins the free living phase of oys- 

 ter larvae. These larvae function as zoo- 

 plankters (meroplankton) in the water col- 

 umn, and probably are significant as a 

 food source for planktivores in local 

 areas. 



After passing through blastula and 

 gastrula stages, the young oyster develops 

 into a trochophore larva characterized by 

 a band of locomotory cilia called the pro- 

 totroch. As development continues, the 

 larval oyster secretes a pair of shells, 

 and the prototroch becomes the larval 

 velum, a ring of locomotory and feeding 

 cilia characterizing the veliger larva. 

 The first shelled larval stage is also 

 termed the straight-hinge (veliger) stage. 



The straight-hinge stage is succeeded 

 by the umbo (veliger) stage, in which the 

 larval "beak" on the left valve overhangs 

 the hinge line. During the latter part of 

 this stage, the larval oyster develops a 

 foot and a byssus gland with which it will 

 eventually attach itself to the substra- 

 tum. With the development of the foot the 

 larvae becomes known as a pedi veliger. 

 During the latter part of the pedi veliger 

 stage, the larval oyster develops a pair 

 of darkly pigmented eyes. The presence of 

 these eyes indicates that the free-swim- 

 ming oyster is ready to attach and meta- 

 morphose into the adult form. At that 

 time the larva is termed an eyed pedive- 

 liger. 



Depending on water temperature and 

 food availability, the larval life stage 

 of C^. virginica will last approximately 7 

 to 10 days. However, some larvae will 

 remain planktonic for up to 2 months dur- 

 ing cooler periods or in the absence of 

 sufficient food. Early winter sets of 

 oyster larvae in the northern Gulf of Mex- 

 ico may be attributed to this phenomenon 



(Edwin W. Cake, Gulf Coast Research Lab., 

 Ocean Springs, Mississippi; pers. comm. ). 



Feeding activities in larval oysters 

 are generally well understood due to 

 recent advances in commercial oyster cul- 

 ture. In the artificial conditions of an 

 oyster hatchery, mixed cultures of various 

 small "naked" flagellates (algae) produce 

 adequate nutrition for the growing oys- 

 ters. It is important to emphasize the 

 value of mixed cultures, as opposed to 

 monocultures, for oyster food sources. 

 There are apparently synergistic reactions 

 among various food items that are as yet 

 unknown but that are very important to 

 oyster growth (Epifanio 1979). This is 

 hardly suprising because the diet of 

 oyster larvae in the natural state is 

 obviously far from a pure culture and 

 probably includes bacteria and small de- 

 trital particles as well as algae and pro- 

 tozoa. The diet could also include dis- 

 solved organic matter. 



After a variable planktonic period 

 (about 2 weeks) from initial fertiliza- 

 tion, the surviving oyster larvae prepare 

 for settlement and metamorphosis. At this 

 stage the "mature" larvae are signifi- 

 cantly larger than the younger straight- 

 hinge, early umbo, and late umbo stages; 

 and they are experimentally separable by a 

 160-y mesh sieve that retains the mature 

 stages but not the immature (Hidu and 

 Haskin 1971). 



Several environmental factors influ- 

 ence the settlement of larval oysters, 

 including the physico-chemical and biolog- 

 ical factors discussed by Hidu and Haskins 

 (1971). They maintained that light, sa- 

 linity, temperature, and current velocity 

 all affect "prospective" spat (newly set- 

 tled oysters). Thorson (1964) proposed 

 that the settling response of marine 

 invertebrates is often cued by light. For 

 example, oyster larvae tend to be photo- 

 positive throughout their larval life span 

 but may become photonegative in response 

 to a temperature increase. Late settling 

 oyster larvae also tend to be more demer- 

 sally distributed than earlier larvae, 

 possibly because of their heavier shells. 



Along the Atlantic coastal regions 

 south of Virginia, spatfall appears to be 

 denser in intertidal areas. Hidu and 



23 



