930 



ZOOLOGICAL SOCIETY BULLETIN. 



SPINY DOGFISH 



Embryo still attached to the egg : reduced one half. 

 Photograph by R. C. Osburn. 



FISHES THAT PRODUCE LIVING 

 YOUNG 



THE statement that some fishes bring forth 

 their young alive is usually a startling- 

 one to the person who is not familiar with 

 ichthyological lore. The common sorts of fishes, 

 it is true, lay their eggs either broadcast in the 

 water or in various makeshifts for nests, and 

 the fertilization takes place after the eggs are 

 laid. But in a number of groups the eggs are 

 retained until the young are developed, and it 

 is of interest that these viviparous fishes are 

 often not closely related, but belong to widely 

 separated families. The development of the 

 life-bearing function in such unrelated groups 

 forms one of the best examples of parallel 

 evolution. 



It is an equally interesting fact that most 

 fishes that bear living young are closely related 

 to others that reproduce in the usual manner; 

 they are, therefore, individual species or genera 

 which have adopted this mode of reproduction 

 without undergoing a sufficient change in struc- 

 tural characteristics to separate them from the 

 parent stock. Thus the sharks, rays and killie- 

 fishes have representatives of both classes. The 

 surf-perches (Embiotocidae) of the Pacific 

 Ocean represent a single family in which all 

 the members are live-bearing. Among the 

 sharks, the majority of the species are vivi- 

 parous, but the Port Jackson and bullhead 

 sharks lay large eggs with tough, horny shells. 

 The majority of the rays or skates lay eggs with 

 horny shells, but certain members, as the sea- 

 bat or sea-devil (genus Manta) and the butter- 

 fly ray (genus Pteroplatea) bring forth living 

 young. Among the killie-fishes, the more com- 

 mon genera (Fundulus and Zygonectes) lay 



eggs, but in the genus Gambusia, etc., living 

 young are produced. 



In the viviparous sharks and rays, the eggs 

 are very large — as large as those laid by their 

 oviparous relatives. The eggs contain sufficient 

 nutriment, or nearly so, to bring the young to 

 a proper condition for birth and but little 

 nourishment is ordinarily derived from the 

 mother. In the live-bearing bony fishes where 

 the eggs are small, the young receive their 

 nourishment, or a portion of it at least, from 

 the maternal tissues. In the surf-perches, partic- 

 ularly, as shown years ago by Professor Eigen- 

 mann, the eggs are reduced in size to such an 

 extent that they contain very little yolk, the 

 nourishment in this case being derived from the 

 membranes of the mother. The eggs of the 

 viviparous fishes are always comparatively few 

 in number for very good reasons. First, since 

 the young at birth are larger and more highly 

 developed than those hatched in the ordinary 

 way and so are better able to take care of them- 

 selves, it has not been necessary to produce such 

 a large number in order to continue the species. 

 Second, a larger number of young would be too 

 great a strain upon the vitality of the parent, 

 which must be preserved if the young are to 

 be produced in good condition. It is as though 

 each type of fish possesses a certain amount of 

 energj' for reproduction, which, in the case of 

 egg-lajing fishes, can be devoted to the produc- 

 tion of a large number of eggs, but which in 

 the live-bearing fishes is devoted to the special 

 nourishment and protection of a much smaller 

 number. 



The common little shark, known as the dog- 

 fish, produces several young at a time. These, 

 when born, are about eight inches in length, 

 while the adult fish reaches not more than 

 three feet. It will thus be evident that the 

 younger generation is well on its way to ma- 

 turity at the time of birth and has passed most 

 of the dangers that surround the ordinary type 

 of fish during its hatching and growth periods. 



The surf-perches again seldom reach more 

 than a foot in length, and bring forth a small 

 number of young, which range in length ac- 

 cording to the species from one and a half to 

 two and a half inches, so these young are well 

 on the way to maturity. 



Among the killie-fishes, the top-minnow 

 (Gambusia affinis) of our southern Atlantic 

 States bears a larger number of young, but these 

 at birth are only about one-third of an inch, 

 while the adult mother reaches a length of about 

 two inches. According to a recent article in 

 Science, by Dr. Hugh M. Smith, the average 

 number of young in families produced in June 



