The life history, population and reproductive variables of the southern red tabira bitterling Acheilognathus tabira jordani were investigated in a lowland reach of the River Ohara in Shimane Prefecture, western Honshu, Japan. Acheilognathus t. jordani, like all other species of bitterling, lays its eggs on the gills of freshwater mussels. It was the only species of bitterling present in the study reach, and three species of bivalve mussel were available to it for spawning: Anemina arcaeformis, Anodonta lauta and Corbicula leana. Spawning by A.t. jordani was recorded between early April and early July in 2003 and began at a size of 38. 0 mm standard length (L(S)) in the 1+ age class. Ovipositor length (L(OP)) during oviposition was positively correlated with female L(S), and showed significant seasonal variation, with a mean +/-s.d.L(OP) of 27. 5 +/- 5. 3 mm ranging from 16. 8 to 42. 0 mm during the spawning period, which was shorter than that of a previously studied A. t. tabira population. Eggs of this subspecies are relatively long and elliptic in shape, with a volume of c. 2. 4 mm(3). Egg number correlated positively with female L(S) and both egg shape and volume changed significantly with season. The population size of adults was estimated to be 850 individuals, and comprised age 0+ to 3+ individuals with L(S) ranging from 12. 0 to 72. 2 mm. The population sex ratio was significantly female biased, with seven females: three males. Egg shape and size and L(OP) during oviposition in the present A. tabira population may be the result of local adaptations to the mussel species utilized and no competition with other bitterling species for spawning sites.
Animals Bivalvia