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The Japanese gnomefishes Scombrops boops and S. gilberti closely resemble each other but their geographical distributions in the northwestern Pacific Ocean are markedly differ- ent. 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 (<400 mm SL) were identified only as S. boops. Thus, the Izu-Islands region is likely utilized as a nursery by S. boops but not by S. gilberti. The GSI of adult S. gilberti showed signif- icantly higher values in March compared to the consistently low values detected for S. boops. In addition, the GSI-elevated S. gilberti had mature ovaries and testes. These results suggest that the Izu-Islands region is utilized as a spawning ground by S. gilberti but not by S. boops. It appears that S. boops spawns in the upper reaches of the Kuroshio Current and that the offspring are trans- ported to the Izu-Islands region by the current. The differences between the species-specific spawning grounds, in conjunction with the transport provided by the currents, likely determine the interspecific differences in geographical distribution.

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