REPRODUCTIVE CYCLES AND SUPERFETATION IN PCILIÏD FISHES

¹ From the Department of Zoölogy, Northwestern University

A graphic summary of the relations of growing ovocytes, mature ova, the fertilization process, the broods of embryos retained in the ovary and the time of birth is offered in Table IV. Each group represents the situation within a single ovary at the height of the reproductive season and at the moment in which a brood is born. The term brood is used to indicate a group of growing and differentiating ovocytes of approximately equal development up to the time of fertilization and also the embryos produced by the fertilization of these ova up to the time for birth. No quantitative relations are implied concerning comparative size of ovocytes or embryos in different species nor of comparative lengths of time that embryos are retained in the ovary.

A study of Table IV leaves one with the impression that the extreme type of superfetation in Heterandria formosa might have arisen from the simpler type as illustrated in Pcilistes pleurospilus. The argument also appears to be defensible that the simpler type of superfetation found in Pcilistes might have arisen from a condition like that illustrated by Quintana atrizona in which a new group of ova are ready for

[See table in the PDF file]

fertilization when a brood is born but in which fertilization is delayed for some hours. All of this implies that the extreme cases of superfetation should be considered as the specialized and derived types while those without superfetation are the simpler. Since the Pciliidæ undoubtedly arose from an oviparous cyprinodont such an assumption does not seem to be overdrawn. That group of the Pciliidæ (Gambusia and Mollienisia) with the ovarian cell cycle which most resembles the oviparous cycle may by the same sign be considered the most unspecialized.

The evidence which now exists concerning the evolutionary relations of the different genera of the Pciliidæ gives no support to the thought that those genera in which superfetation occurs are more closely related to each other than to any other genera. Rather, it appears that superfetation has arisen more than once and that the present cases represent convergent evolution. The occurrence of superfetation therefore has no taxonomic significance.

1. All species of the Pciliidæ are ovo-viviparous.

2. At the height of the breeding season broods are produced at regular intervals varying from about forty-five days in some species to five or six days in others.

3. In laboratory-reared specimens and in specimens breeding in temperate zones the shortest interval between broods occurs in spring and early summer and the longest interval during fall and winter.

4. In the reproductive cycle of Mollienisia and some other species, the oldest group of ovocytes remaining in the ovary just after a brood has been extruded is very small. Growth is rapid and the ova are ready for fertilization about eight days after the voiding of the last brood of embryos.

5. In Lebistes and other species with similar reproductive cycles, the oldest group of ovocytes remaining in the ovary just after a brood has been voided are much larger than in Gambusia and fertilization occurs a few days earlier than in Gambusia.

6. In Quintana atrizona the oldest group of ova remaining in the ovary are fully developed and ready for fertilization at the time of birth of the previous brood of embryos.

7. Superfetation occurs in Pcilistes pleurospilus and several other species. Fertilization of the oldest group of ova takes place before the voiding of the brood already in the ovary and two broods at different levels of development are found in the ovary.

8. In Heterandria formosa and possibly other species there is an extreme development of superfetation with six or more small broods at different levels of development occurring in the ovary at one time at the height of the breeding season.

9. Older groups of ova and embryos retained in the ovary retard the development of younger groups of cells and a balance is maintained between this retarding influence and the agent, assumed to be a follicle-stimulating hormone from the pituitary, which forces the cells to grow from the lower to the upper levels of development.

10. The short reproductive cycles of the Pciliidæ are the equivalent of the longer annual or biennial reproductive cycles of the fishes of the temperate zones.

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