Sex-change in the Native Oyster (O. edulis)

Abstract

IT is well known that sex-change in the native oyster (O. edulis) occurs at some period of its life. This mollusc apparently always begins life as a male, and may change into a female at the age of one or two years. Very little is, however, known about the change of sex afterwards. In following up the indications given from a general study of breeding (see J. H. Orton, “Sea-temperature, Breeding, and Distribution in Marine Animals”, Journal of the Marine Biological Association, vol. xii., July, 1920, pp. 339–66), it seemed certain that an oyster ought to continue breeding in the same season even after becoming white-sick, i.e. after extrusion of ova into the mantle cavity. Thus if a breeding oyster were marked and examined afterwards, it should be possible to find out something about a possible annual change of sex. Accordingly on July 30, 1920, two white-sick oysters were isolated in a tank at Plymouth, and one of them was cut open and examined on August 26, 1920. At the latter date the one examined¹ was found to have its gonad full of wholly ripe sperm-morulæ, which disintegrated into separate active and apparently ripe sperm as soon as they were placed in sea-water. Thus a female-functioning oyster had changed into a male-functioning oyster within less than a month. An indication of this change had already been given on July 29, when the gonad of a white-sick oyster—examined at the moment when it contained embryos in the mantle cavity—showed developing sperm-morutæ and some actively tailed sperm-morulæ.