Comparison of reproductive cycle between two genetically distinctive groups of sand lance (Genus Ammodytes) from northern Hokkaido [Japan]

Abstract

Seasonal changes of gonosomatic indices (GSI) and histological characteristics of ovaries in two genetically distinctive sympatric groups of sand lance from the region, east of Cape Soya, were compared. GSI values of one group which was fixed for Ldh-2(300) (W-b group) started to increase at least three months earlier than the other one group which was fixed for Ldh-2(100) (W-a group). In both sexes of W-b group, their GSI exceed 20.0 in November, while GSI of females in W-a group in this month were less than 5.0 in most case. Several specimens of W-a group collect ed in May, had ripe eggs or sperm which had the ability to fertilize and develop. Remaining specimens after spawning whose ovaries were occupied by oocytes at the peri-nucleolus stage, and many empty follicles and several remained ripe eggs were observed. While, in ovaries of the W-b group in the same sample, oocytes at the oil droplet stage were dominant, and no empty follicle or remained eggs were found. From these results the spawning period is supposed to be late Novem ber to December in W-b group, and late March to early May in the W-a group. The difference in their reproductive cycles would play an important role as a mechanism for reproductive isolation between these two sympatric groups.