summer of 1959. Antheunisse states that in spite 

 of the parallelism between the neurosecretory and 

 reproductive C3'cles in zebra mussels spawning and 

 neurosecretion are not interrelated. Extirpation 

 experiments were difficult to perform, and most of 

 the mussels died some days after the operation. 

 It is therefore apparent that further studies are 

 needed tf> determine the role of the neurosecretion 

 in reproduction of mussels, oysters, and other 

 biv^alves. 



FECUNDITY OF THE OYSTER 



The intensity of spawning as judged by the 

 number of eggs or spermatozoa discharged in each 

 instance is variable. In both sexes the number 

 of sex cells produced by a ripe female or male 

 depends on the size of the oyster and the degree 

 of development of the gonad. The range of 

 variation is enormous. If the female gonad is 

 poorly developed, only a few thousands of eggs 

 may be released. On the other hand, the number 

 of eggs produced and discharged by a well- 

 developed gonad may reach many millions. 

 Potential capacity of the ovary, i.e., the total 

 number of eggs produced by a female during the 

 breeding season, is not indicative of its actual 

 reproductive ability which is expressed by the 

 number of eggs actually spawned. The following 

 procedure is used for estimating the number of 

 eggs released by the female. The oyster is placed 

 in a 20 1. tank and spawnuig is stimulated by 

 warming the water and adding sperm suspension. 

 After the completion of spawning five samples of 

 100 ml. each are taken while the water is agitated 

 by an electric stirrer. Eggs in the sample are 

 killed by adding two to three drops of 1 percent 

 osmic acid, allowed to settle, and are counted 

 in a Sedgwick-Rafter chamber. The oysters used 

 in four separate tests varied from 9.2 to 13.3 cm. 

 in height. The number of eggs (in millions) 

 discharged in one spawning were 15, 30.3, 70.3, 

 and 114.8 (Galtsoff, 1930b). After discharging 

 over 100 million eggs the last oyster had a gonad 

 about 5.5 mm. thick containing vast numbers of 

 eggs. 



The results of these counts were questioned on 

 the basis that the computed volume of the 

 discharged eggs exceeds the total volume of the 

 body (Burkenroad, 1947). Rechecking the data 

 confixmed my estimate. The counts are correct 

 within ± 10 percent, the principal source of error 



being the difficulty in obtaining uniform distri- 

 bution of eggs in the tank. 



In the ovaiy the eggs are tightly packed and 

 compressed; upon their release the diameter of 

 their rounded part is increased. The spawned 

 eggs in the above tests averaged 40 /i in diameter. 

 The vf)lume of a given number of eggs can be 

 computed by using the conversion table from 

 diameters to volumes of spheres given in Perry 

 (1941). Since the volume of one egg of 40 m 

 diameter is 33,510.3 M^ the volume of 115 millions 

 of eggs, solidly packed would correspond to 3.8 

 cm.' With an allowance of 25 percent for inter- 

 spaces the volume of spawned eggs in the ovarj^ 

 would be about 4.8 cm.' The latter figure is 

 within the range of magnitude of the volume of 

 the gonad obtained by the displacement method. 



Not all the ovocytes become mature at the 

 same time. During the intervals between spawn- 

 ing some of them grow and replace those 

 discharged by the preceding ovulation. 



The fecundity of C. gigas is even greater. The 

 five large oysters of this species forced to spawn 

 in the laboratory averaged 55.8 million eggs per 

 oyster; post mortem examination showed that 

 after ovulation they retained the major part of 

 the gonadial material. In comparison to C. 

 virginita and C. gigas the fecunditj- of the larvi- 

 parous European oyster is rather low. Estimates 

 of the mean number of larvae per oyster were 

 made by Dantan (1913), Cole (1941), Cerruti 

 (1941), and Millar (1961). In British waters the 

 mean number of larvae vary from 90,000 for a 

 1-year-old oyster to over a million for a 4-year-old 

 oj'ster. French oysters relaid in West Loch 

 Tarbert, Scotland, after 1 year produced as 

 many larvae as the native oysters on English beds. 

 The number of larvae is dependent, of course, on 

 the size of the oyster, as can be seen from the table 

 given by Millar. 



The fully grown 0. lurida bear broods of 250,000 

 to 300,000 larvae, the number depending generally 

 upon the size of the maternal oyster (Hopkins, 

 1936, 1937). 



ORGANS OF REPRODUCTION 



313 



