26 THE SCALLOP FISHERY 



should outnumber the eggs 1,600 to 1, as it only requires one sperma- 

 tozoon to fertilize one egg. But as the small male cell has the active 

 part of finding the egg, this again is a wise provision of nature, whereby 

 this proportion vastly increases the chances of natural fecundation. 



If only 1 scallop arrives at maturity from 3,000,000 eggs, it is suffi- 

 cient, under normal conditions, to perpetuate the species. Naturally 

 there is a vast destruction of eggs and young scallops, an important 

 part of which is due to the loss of eggs through non-fecundation, i.e., 

 the eggs and spermatozoa not meeting in the water. There are a great 

 many chances in nature against fertilization of the egg. Scallops may 

 be some distance apart and the spermatozoa must travel far before 

 they can meet the egg. Water Currents, winds and other weather con- 

 ditions may prevent this union. Fertilization is partly by chance, as 

 the male cell can only be attracted to the egg from a short distance. 

 Thus, if it were not for the abundant supply of sex products the race 

 of scallops would soon be exterminated. 



In artificial fertilization a large number of the eggs are not fertilized, 

 and, failing to develop, soon decompose and pollute the water, thus 

 causing the death of the more advanced larvae. This shows that per- 

 haps all the eggs given forth at one time from the scallop are not 

 ready for fertilization and cannot develop, and it may be supposed 

 that, under natural conditions, an indeterminable per cent, of the ex- 

 truded eggs are incapable of development. 



(b) Self-fecundation. Pecten irradians is hermaphroditic, i.e., both 

 sex elements are found in the same individual. Pelseneer (9) asserts 

 that : " In hermaphrodite mollusks the spermatozoa ripen before the ova ; 

 the hermaphroditism is therefore protandric. The hermaphroditism 

 also is not self-sufficient, and the ova of one individual must normally 

 be fertilized by the spermatozoa of another individual." Pecten ir- 

 radians is an exception to this in that both the eggs and spermatozoa 

 mature at the same time, and that self-fertilization frequently occurs 

 although it is not the common method of reproduction. 



In nature it is not usual for scallops to produce both male and female 

 cells at the identical moment, and self-fertilization is therefore not 

 as common an occurrence as when scallops are confined in aquaria. 

 Scallops often shoot eggs and spermatozoa within as short an interval 

 as 15 to 30 minutes apart. In numerous cases self-fertilization has 

 been observed during the spawning experiments. The spermatozoa 

 and eggs of the same scallop have been artificially mixed, and the early 

 embryological stages followed. Whether these self -fertilized eggs 

 would develop into mature scallops was not determined, as the devel- 

 opment was only followed as far as the trochosphere larva, up to 

 which period it was normal. Drew (1) and Risser (2) also have made 

 observations on the self -fertilizing powers of the individual Pecten irra- 

 dians. 



Fertilization of the Eggs of Two-year-old Scallops. In spite of 



