Ariosoma-type Leptocephali (Congridae: Bathymyrinae) in the Mentawai Islands region off western Sumatra, Indonesia

Background: Ariosoma-type leptocephali of the congrid subfamily Bathymyrinae are typically present in tropical and subtropical regions where they reach large sizes and are often abundant compared to other taxa. Different morphological species types of these larvae have been documented in the Indo-Pacific region, but few have been matched with their adult species, and their life histories are not known. Results: A sampling survey for leptocephali off western Sumatra, Indonesia, collected 297 Ariosoma-type leptocephali of at least 12 different species of both the exterillium gut and non-exterillium gut types that could be distinguished using morphological characteristics. These leptocephali were collected at 23 of 24 stations over deep water but were not caught at two stations over the shelf between Sumatra and Java. They were most abundant in the 15- to 50-mm size range, but six species were also collected with sizes of > 140 mm, some of which were undergoing metamorphosis at sizes of 143 to 324 mm. Larvae of Ariosoma scheelei were most abundant, with small larvae of <= 25 mm present near Sumatra and the Mentawai Islands and larger larvae also present offshore. The other abundant larval types, sp. 1 and Ariosoma sp. 7 showed a different tendency with their small larvae being present more offshore, which suggests different spawning locations possibly linked to differing adult habitats or spawning behaviors. Conclusions: The presence of at least moderately small leptocephali of most of these species suggests that their adults are present in this region, which indicates that there is a high biodiversity of these small benthic eels on the shelf and slope of the Mentawai Islands region off western Sumatra in the eastern Indian Ocean.