PHYSIOLOGY OF REPRODUCTION OF OSTREA VIRGINICA

Abstract

The gonad of an oyster consists of a large number of branching tubules containing sex cells and emptying into genital canals, the lumen of which is lined on one side with the ciliated epithelium. There are two gonoducts, one on each side of the gonad, which open into suprabranchial chambers. Spawning of the female consists in the discharge of eggs from the ovary (ovulation); rhythmical contractions of the adductor which cause opening and closing of the shell valves; and in the contraction and change in the position of mantle borders which almost completely close the pallial cavity. The last two phases of spawning are accessory sexual reactions which provide mechanism for better dispersal of eggs in the water. Muscular behavior during spawning is characterized by its remarkable rhythmicity and maintenance of a constant tonus level. It cannot be reproduced by artificial stimulation. Eggs discharged into the suprabranchial chamber are not carried away by the cloacal current but penetrate through the gills into the pallial cavity and are expelled by vigorous movements of the shell valves. Their passage through the gills coincides with the periods of greatest relaxation of the adductor and widest gaping of shell. If the shell movement is prevented by severing the hinge, eggs fail to pass through the gills and are discharged by the cloacal current. The possibility of the reversal of the ciliary motion and of the suction produced by shell movement as the forces responsible for the passage of eggs through the gills are discussed and rejected as untenable. The following tentative explanation is offered. The discharge from the ovary and the closure of the cloaca produce an increased pressure in the suprabranchial chamber which forces the eggs through the water tubes and gill ostia into the pallial cavity. The difference in pressure on both sides of the gill reaches its maximum when the valves open and the gill lamellæ spread apart. This accounts for the penetration of eggs through the gill during the periods of greatest relaxation of the adductor. Ejaculation is not accompanied by the rhythmical contractions of the adductor and the sperm discharged through the spermiduct is carried away by the cloacal current. There is no increase in the rate of pumping of water during spawning. The discharge of sperm is probably controlled by the contractions of numerous muscle fibres in the walls of the spermiduct. Definite sphincter-like structure is absent.