Current literature states that family Scombropidae consists of a single genus Scombrops comprising three species worldwide, with two of them, Scombrops boops and Scombrops gilberti, distributed in the waters around the Japanese Archipelago. Although these two scombropids are commercially important species, little is known about the ecology of these fishes. It is difficult to discriminate between these two species based on external characteristics because of their morphological similarity. Here, we report two different morphotypes characterized by the relative growth between the otolith size and the standard length (SL) of the scombropid specimens caught in southern waters off Kyushu Island, Japan, and show the genetic relationship between the morphotypes by means of phylogenetic analyses using complete DNA sequences of the cytochrome b gene. The relationship between otolith weight and SL was significantly different between specimens < 505 mm SL and those > 550 mm SL. Phylogenetic analysis demonstrated that the sequences from these scombropid specimens formed three clades: two corresponded to S. boops and S. gilberti, while the third did not correspond to any sequence recorded in databases, suggesting that these specimens are undescribed scombropid species. Almost all the specimens with SL < 505 mm (n = 76) were identified as S. boops, and only nine as S. gilberti. On the other hand, almost all the specimens with SL > 550 mm (n = 41) fell in the unidentified group except for four specimens, whose sequences were identical to that of S. boops.
Keywords: Gnomefishes; Scombrops boops; Scombrops gilberti; cryptic species.