Larvae and juveniles of Leptoderma lubricum and L. retropinnum (Argentiformes: Alepocephalidae) collected from Suruga Bay, Japan

Larvae and juveniles of the alepocephalid fishes, Leptoderma lubricum [26.9-69.0 mm in standard length (SL)] and Leptoderma retropinnum (21.1-67.2 mm SL), collected within 1-8 m of the seafloor in Suruga Bay, southern Japan, are described. They can be easily distinguished from each other by the following adult-like characters: membrane morphology between the vertical fin rays and procurrent caudal-fin rays (separated in L. lubricum vs. continuous in L. retropinnum), numbers of dorsal-fin rays (34-40 vs. 45-52) and anal-fin rays (50-57 vs. 65-72), and caudal peduncle length (11.7-13.4% SL vs. 4.5-5.9% SL), in addition to several other body proportional differences. Unique characters in the larval stages of Leptoderma include a translucent occipital region, horizontally elongated eye, and head below the upper margin of the orbit and abdominal cavity densely covered by melanophores, ontogeny being characterized by the acquisition of general adult characters to the postflexion stage, indistinct transformation, and the retention of few larval characters until almost the end of the juvenile stage, as in other known alepocephalids. In addition to the near-bottom larval and juvenile collections of both species, the occurrences of benthic or near-bottom taxa, including Harpacticoida, in their gut contents confirmed the early life history dependence of the former on the near-bottom.