Reproduction in the basking shark, Cetorhinus maximus (gunner)

Abstract

In the male part of the ductus deferens is an ampulla nearly two metres in length, containing numerous transverse septa each with a central opening. In pockets between the septa spermatozoa are formed into spermatophores up to about 3 cm. in diameter, each with a core of sperm and a firm translucent cortex. They float in a clear fluid, and about four gallons of them are transferred to the female. In the female the right ovary alone is functional; it contains at least six million ova 0-5 mm. or more in diameter, most of which degenerate and are replaced by bodies resembling corpora lutea. The yolky eggs must be ripe at a diameter of about 5 mm., for the diameter of the anterior end of the oviduct, which has inelastic fibrous walls, is too narrow for anything larger to pass. The uterus, about 1 m. in length, is lined throughout its greater part with trophonemata up to TO cm. long. The structure of the ovary would suggest that Cetorhinus is oviparous, but that of the uterus clearly shows that it is viviparous (or ovo-viviparous). There are no modern records of pregnancies, although the sex ratio in the commercial catch is thirty or forty females to one male. Sexual maturity is not reached until at least the third year of age. Females are impregnated in surface inshore waters during early summer, but pregnant females evidently migrate elsewhere, either horizontally or vertically or both, and do not reappear until after parturition.