Reproductive cycle of the geoduck clam Panopea generosa at its southernmost distribution limit

Abstract

The reproductive cycle of Panopea generosa at the southernmost limit of its distribution in the State of Baja California, Mexico was studied, using standard histological techniques, oocyte cohort analysis and a gonadosomatic index. The reproductive cycle of P. generosa begins in late autumn, with a developing phase in winter and continuous proliferation and spawning from April throughout October. During this period, several oocyte cohorts were developed, but only three attained maturity, the rest being reabsorbed into the gonad. Gamete reabsorption within the gonad might reflect a conservative pattern of nutrient recycling. Ripe gametes were continuously released with a main spawning at the beginning of summer, when temperature was increasing. Spawning of less intensity during late summer occurred when the temperature reached its seasonal peak. The gonadosomatic index correlated well with development of the species' reproductive season. The sex ratio was dominated by females (2.8:1) although sexually undifferentiated geoduck clams were the most abundant. This is the first description of the reproductive activity of P. generosa at its southernmost distribution. The results showed significant differences in gametogenic patterns relative to northern congeners, which suggest that fishery of geoducks in this region needs specific strategies.