The numbers of several meristic characters, fecundity and distribution of the sandeel (Genus Ammodytes) population distributed around Japan were studied, in order to know its population structure.
It turns out that the sandeel population around Japan is composed of three brood groups, Group I to III, which are characterized by the Mahalanobis distance, applying the discriminant function method to the meristic counts. These three groups also reveal differences in fecundity, which increases in the proceeding order from Group I to III. Group I is distributed to the north of northern Hokkaido, Group II from off southern Hokkaido to Sendai Bay on the Pacific side and to off Tottori Pref. on the Sea of Japan side, and Group III ranges in the more sounthern seas.
So far, Group I has been classified as a species, A. hexapterus, while Froups II and III have been considered different subpopulations belonging to another species, A. personatus. However, it is difficult to distinguish these forms by their external appearance and their differences in mor-phology and ecology are not substantial enough to assign different species. It is necessary to con-sider if these forms are designated subspecies or not.