sperm of the species. In this case the latent 

 period of spawning reaction is much longer, vary- 

 ing from 6 to 27 minutes at 20° to 21° C. The 

 interesting fact is that in the case of sperm 

 stimulating male spawming the latent period is of 

 the same order of magnitude as that of the female 

 spawning reaction. Possibly the sperm acts as a 

 stimulant only after it has been absorbed by the 

 oyster, while eggs and egg water act upon the 

 receptors located on the body surface. 



The active principle of sperm of C. virginica can 

 be extracted with ethyl alcohol and benzene. The 

 residual powder of the e.xtract can be mixed with 

 sea water and added to the gills of the female to 

 induce a typical spawning reaction (Galtsoff, 

 1940). Spermatozoa of C. virginica carry another 

 hormonelike substance which may be recovered 

 after alcohol extraction. The substance was 

 named "diantlin" by Nelson and Allison (1940), 

 who found that it dilates the ostia and stimulates 

 the increase of water flow through the gills. 



FREQUENCY OF SPAWNING 



Under laboratory conditions male oysters may 

 be induced to spawn many times at very brief 

 intervals. It is, therefore, reasonable to assume 

 that they behave in a similar way in their natural 

 environment. The females spawn only a limited 

 number of times within one breeding period. Out 

 of several hundreds of oysters tested in the 

 laboratory, the majority of the females w-ere 

 induced to spawn two or three times within a 

 6-week period (July-August), and only one 

 spawmed seven times. Similar conditions exist 

 with C. gigas. Frequently a substantial percent- 

 age of females of these two species fail to shed 

 eggs in spite of a ripeness of their ovaries and 

 favorable environmental conditions. The spawn 

 may be retained in the gonads until late fall w^hen 

 it is reabsorbed. 



The number of times the adult female oysters 

 spawn under natural conditions can only be 

 surmised from examination of gonads and the 

 occurrence of larvae in plankton. It is very 

 difficult to decide from plankton observations 

 whether the entire oyster population spawned 

 several times or if different groups of oysters pro- 

 duced the larvae appearing at intervals in plank- 

 ton. In Long Island Sound, for instance, four or 

 more "waves" in tlie occurrence of straight hinge 

 larvae were recorded (Loosanoff and Nomejko, 

 1951a) in 1943, but the described periodicity may 

 have been due to spawning of different populations 



living in shallow and deep water. Inasmuch as 

 the laboratory experiments show that repeated 

 spawning may be induced in the same female, it is 

 reasonable to infer that in their natural habitat 

 oysters spawn more than once during every 

 breeding season. 



Laboratory observations show that spawning of 

 sexually mature C. virginica is sometimes inhibited 

 and that the oysters fail to respond to all known 

 methods of stimulation. Similar conditions exist 

 with C. gigas, which sometimes fail to spawn in 

 spite of full gonad development. Artificial stimu- 

 lation by suspension of sex cells may facilitate 

 spawning but is not always successful. The reason 

 for this inhibition of spawning in sexually ripe 

 oysters has not been established, but the work of 

 Lubet (1955) on Chlamyft and Mytilus throws some 

 light on the problem. Lubet discovered that the 

 excision of cerebral ganglia in these mollusks 

 provokes precocious spawning and that the muti- 

 lated animals spawn much earlier than the con- 

 trols. Excision of the visceral ganglia seems to 

 retard spawning. These experiments suggest that 

 spawning is under the control of the nervous 

 system. It also appears significant that neuro- 

 secretion in the ganglia cells precedes gameto- 

 genesis and that maximum accumulation of the 

 neurosecretory products occurs at the time of the 

 maturation of sex cells. In the species studied by 

 Lubet the neurosecretory granules were absent in 

 the ganglia of the recently spawned out animals. 

 Whether Lubet 's findings on neurosecretion apply 

 to sexually mature oysters is not known, but his 

 work seems to indicate that in the bivalves he 

 studied, the release of sex cells was facilitated by 

 the removal of internal inhibition (excision of 

 cerebral ganglia) and that the disappearance of 

 the neurosecretory products from the cerebral 

 ganglia was necessary for the moUusk to become 

 receptive to spawning stimuli. The latter infer- 

 ence is based on the observation that partial 

 disappearance of neurosecretory granules always 

 occurs a few days before spawning. After the 

 completion of spawning all neurosecretory cells 

 are empty. These findings are not in accord with 

 the results of studies conducted by Antheunisse 

 (1963) on zebra mussels {Dreifsena poh/morpha 

 Pallas) from the Amstel River near Amsterdam. 

 The mussels were collected once a month, from 

 November 1957 to November 1958, for histological 

 examination. For extirpation experiments only 

 adult females were used during the spring and 



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FISH AND WILDLIFE SERVICE 



