Habitat use of the gnomefishes Scombrops boops and S. gilberti in the northwestern Pacific Ocean in relation to reproductive strategy

The Japanese gnomefishes Scombrops boops and S. gilberti closely resemble each other but their geographical distributions in the northwestern Pacific Ocean are markedly different. In order to understand the determinants of reproductive isolation, we examined habitat use in relation to the reproductive strategy employed by these species in the Izu-Islands region, where adults of both species coexist. We examined the species compositions of the gnomefishes based on mitochondrial sequence differences, and the maturation process of gonads using the gonado-somatic index (GSI) and by histological observation of the gonads. Genetic analysis showed that large individuals (>400 mm SL) of both species were present in the region, whereas all small gnomefish (