the larvae are unifonnl}- distributed in the water 

 and swiin most of the time, a certain percentage 

 of them will be carried away and lost in the sea. 

 Manj- more are lost as prey to enemies, disease, 

 and other causes. 



In the light of present knowledge only two 

 general assumptions regarding the larval behavior 

 can be made: oyster larvae are able to move by 

 their own power within only a very limited area, 

 and they are dispersed by tidal currents beyond 

 the immediate vicinity of spawning grounds. 



A survival relationship exists between the age 

 of the larvae and tidal cycles. After analyzing 

 daily counts of larvae of O. ediilis in plankton 

 samples taken in Oostersheld, Holland, Korringa 

 (1941) concluded that the longer the duration of 

 the pelagic period the greater is the loss of larvae 

 and the lower is the percentage reaching maturity: 

 In 6 to 7 days, equal to 13 tides, 10 percent reach 

 maturity; in 10 days, equal to 19 tides, 5 percent 

 reacli maturity; and in 12 days, equal to 23 tides, 

 2.5 percent reach maturity. If the original num- 

 ber of larvae is A and the rate of dispersal and 

 other losses of larvae are equal during their free- 

 swimming period, the number of larvae at the 

 completion of pelagic life is A(l-l/p)" where 1/p 

 is the decrease during one tidal cycle and n is 

 the number of tides. The loss dm-ing one tidal 

 cycle is estimated by Korringa at between 13 to 

 15 percent. About 10 percent of the losses he 

 attributed to predators and only about 4 percent 

 to tides. Because of the greater duration of the 

 pelagic life of the oviparous f. inrritiuca, it is 

 reasonable to expect that losses of larval popula- 

 tions of this species probably exceed those deter- 

 mined by Korringa for the larviparous 0. edulis. 



It is generally known that mortality among the 

 planktotrophic larvae during their pelagic life is 

 tremendous and that only an insignificant per- 

 centage of them reach metamorphosis. Korringa 

 made an interesting computation which shows 

 tliat out of one milUon 0. edulis larvae produced 

 in Oostershelde only about 250 attach themselves 

 and metamorphose, and of this newly set spat 95 

 percent die before the onset of winter. 



The rate of survival of larvae of C. rirginica 

 and the percentage reaching attachment are not 

 known, but the principles of Korringa 's method 

 can be applied to the American species. His 

 studies show that the success of oyster setting 

 depends on prolific and shnultaneous spawning of 

 oysters in an estuary. By determining the abun- 



LARVAL DEVELOPMENT AND METAMORPHOSIS 



dance of larval population and the rate of exchange 

 of water during a tidal cycle, an estimate can be 

 made of the intensity of the forthcoming setting, 

 barring, of course, unforeseen circumstances which 

 may destroy the larvae. 



REACTION OF LARVAE 

 TO EXTERNAL ENVIRONMENT 



Little is known about the reactions of larvae to 

 changes in temperature and salinity of water. 

 Temperature fluctuations dm-ing the reproductive 

 season apparently have no direct effect on the 

 beha^^or of larvae of C. virginica, 0. edulis, and 

 C. gigas. Davis (1958) has demonstrated in a 

 series of laboratory tests that the reduction of 

 sahnity from the normal (for Long Island Sound 

 oysters) level of 267oo to 277oo to 157oo has no ef- 

 fect on the growth of larvae and that inhibition of 

 wrowth became noticeable in salinities of 12.5°/oo 

 and lower (Davis and Ansell, 1962). In water of 

 107oo salinity 90 to 95 percent of the larvae died 

 by the 14th day, and at a salinity of 5°/oo they 

 appeared to be moribund within 48 hours. In 

 these experunents the behavior of larvae was not 

 recorded. It would be interesting to repeat these 

 studies and determine the reactions of larvae to 

 sudden and to gradual changes of salinities. 



Vertical distribution of larvae of 0. edulis ap- 

 parently is not affected by light (Korrmga, 1941). 

 This is probably true also for the larvae of C. 

 virginica, but because no experiments have been 

 made under controlled laboratory conditions, it is 

 prematm-e to assume that larvae of the American 

 oyster are not sensitive to light. The phototactic 

 responses of larvae to light intensity and color 

 have not been explored, but the presence of the 

 eye in the fully developed larva suggests that this 

 oi-gan is somehow used during the last days of 

 lanal life. Before attachment the larva crawls 

 over the surface exploring the substratum with 

 its foot, whicli acts as a tactile organ. It has not 

 been established that the eye participates in this 

 exploration. Nelson (1926) beUeves, however, 

 that the "eyed" larvae of C. virginica are stimu- 

 lated by light and continue to move until they 

 reach a shaded place where they become quiescent. 

 Hopkins (1937) expresses the opposite view and 

 states that in settmg of 0. lurida light is not an 

 orienting factor. He inclines toward Prytherch's 

 (1934) view that the larval eye has an entirely 

 different function. Since neither of the quoted 

 authors can corroborate theu- impressions by ex- 



371 



