Comparison of early life-history in two catadromous sculpin, Trachidermus fasciatus and Cottus kazika

The freshwater sculpins, Trachidermus fasciatus and Cottus kazika, inhabiting the Japanese Archipelago, exhibit a catadromous life style, The former is found only in the rivers facing Ariake Bay in northwestern Kyushu Island. The latter is found in Honshu Island facing Japan Sea and Pacific Ocean, southern Shikoku and eastern Kyushu Islands. Comparison of early life-history between the two species was done, observing swimming levels and development of characters related to swimming functions at the 6 developmental stages from yolk-sac larva to juvenile. The larvae of T. fasciatus from yolk-sac to postflexion stages swam just below the water surface of the aquaria. The postflexion larvae which all fin rays attained at their full counts and juveniles swam in the middle and bottom layers of the aquaria, respectively. Whereas, the yolk-sac larvae of C. kazika swam in the middle layer of the aquaria, and the preflexion larvae swam just below the water surface of the aquaria. Thereafter, the swimming levels of flexion and postflexion larvae deepened with growth, and that of juveniles changed to a demersal habit in the aquaria. The development of 31 structures of fin supports and vertebral column in both species were observed in the transparent specimens double-stained with alcian blue 8 GX and Alizarin red S. The stages of 31 structures fully calcified in T. fasciatus were later than those in C. kazika. These results and the distribution of larvae and juveniles in natural waters in the Kashima estuaries system (Saga Prefecture) for T. fasciatus and Gonokawa estuaries system (Shimane Pref.) for C. kazika suggested that T. fasciatus changed from pelagic to demersal habits after running up the tidal reaches of the river, and C. kazika started to run up the river after changing from pelagic to demersal habits in the surf zone. Thus, T. fasciatus has longer pelagic period than C. kazika. This ecological difference in T. fasciatus may be caused by an adaptation of a habitat with large tidal range in Ariake Bay.