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200 



Figure 289. — Section of a small portion of an hermaphro- 

 ditic gonad of C. virginica. Bouin, hematoxylin-eosin. 



October and November; the swarming of the At- 

 lantic Palolo [Odontosyllii enopla Verrill) at 

 Bermuda and Eunice fucata Ehlers, at Tortugas, 

 Florida (Mayer, 1908); and the breeding habits of 

 Heteronereis form of Nereis limbata at Woods 

 Hole (LilHe and Just, 1913). Legendre (1925) 

 gives an interesting historical account of the 

 effect of the moon on marine organisms. A 

 comprehensive review of the instances of lunar 

 periodicity of breeding among many marine 

 invertebrates, including several species of pele- 

 cypods, is given by Korringa (1947). 



Evidence of a relationship of breeding of 0. 

 edulis to moon phases was first presented bj^ 

 Orton (,1926), who examined weekly samples of 

 adult oysters from Fal estuary and found two 

 important maximums in spawning ,at the full 

 moon spring tides in the year 1925. He further 

 concluded that the population as a whole gave 

 maximal percentage of spawn (based on presence 

 of embryos in the oysters) in the weeks after the 

 July and September full moons. Later observa- 

 tions by Korringa (1941, 1947) in the commercial 

 oyster district of Oosterscbelde, Holland, con- 

 firmed the existence of a relationship between 

 breeding of 0. edulis and moon phases. He 

 found that the full moon exercises the same in- 

 fluence on the breeding of oysters as does the 



new moon. Korringa based his studies on deter- 

 minations of the time and abundance of oyster 

 larvae in plankton and found a marked periodicity 

 in the maximums of oyster larvae occurring about 

 10 days after full and new moon. Fluctuations 

 in water temperatm-es, according to Korringa's 

 view, are apparently of little or no importance in 

 causing the periodicity in swarming which appears 

 to be correlated with the spring tides. Unfor- 

 tunately no experimental evidence is available to 

 substantiate this inference which is based entirely 

 on the concurrence of the two phenomena. 



Spawning of C. virginica has no relationship to 

 lunar phases. The existence of such a relation- 

 ship was postulated by Prytherch (1929), who 

 stated that Long Island Sound oysters spawn 

 "at the end of full moon tidal period, or eight 

 days after the time of full moon," but the correla- 

 tion could not be corroborated by careful studies 

 of Loosanoff and Nomejko (1951a), who continued 

 the observations in Long Island Sound over a 

 period of 13 years after the termination of 

 Prytherch's work. Negative results were also 

 reported by Hopkins (1931) in Galveston Bay, 

 Tex., and by R. 0. Smith in South Carolina waters 

 (unpublished reports on file in the Bureau of 

 Commercial Fisheries). 



BIOLOGICAL SIGNIFICANCE OF 

 SPAWNING REACTION 



The most outstanding single factor in oyster 

 reproduction is the difference in spawning behav- 

 ior of the two sexes. The males are more respon- 

 sive to sexual stimulation than the females and 

 are easily stimulated to spawn by rising tempera- 

 tures and by a great variety of organic sybstances, 

 some of them not found in natural sea water. 

 The spawning response of the male is nonspecific. 



The less responsive, sexually mature females 

 require stronger stimuli and are highly specific 

 to chemical stimulation; they respond only to 

 suspensions of sperm of the same or related 

 species and are indifferent to the sperm of other 

 bivalves and various chemical substances tested. 

 The specificity of females is an insurance that 

 eggs cannot be discharged when there is no sperm 

 in the water. 



Males are usually the first to initiate spawning; 

 the discharge of sperm by even one individual 

 induces spawning by those next to it, and the 

 process spreads over the entire oyster community. 

 This sequence has been observed among oysters 



ORGANS OF REPRODUCTION 



319 



