Morphological and genetic variation in North Atlantic synaphobranchid eels in relation to species diagnostics

Synaphobranchus eels are difficult to identify at species level. A literature study of previous taxonomic work on these species revealed several inconsistencies concerning the diagnostic characters of Synaphobranchus kaupii and Synaphobranchus affinis. One hundred and eleven specimens from the mid-Atlantic ridge and a type specimen of S. affinis were examined morphologically, and the DNA barcode region of the mitochondrial COI gene was sequenced from 60 of these specimens. The results of this examination showed an ontogenetic change in most morphometric characters and that most of the previously used diagnostic characters, namely dorsal fin origin in relation to the vent, predorsal length, scale shape and pattern, and dentition, cannot distinguish between the species S. kaupii and S. affinis. All the examined MAR-ECO specimens are in accordance with the original description of S. kaupii by Johnson (1862) and have a high number of vertebrae, hence all are identified as S. kaupii. The type specimen of S. affinis may represent a member of a cryptic species that closely resembles S. kaupii, separable by having a lower number of vertebrae, but a geographical expansion of this study is needed to resolve this taxonomic issue