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There are numerous species in this genus and DNA-sequence analyses indicate that it is likely that the genus is polyphyletic (M. Craig). Nevertheless, many of these small basslets are very similar in general appearance and overlap in meristics, making it a difficult group for species identifications. Most of the regional species share the median-fin ray formula of D-X,12 A-III,7. DNA sequencing is likely necessary to separate larvae of some of the species, however identifications can be narrowed down by pectoral-fin ray counts and subtle differences in markings.
Unlike many other reef fishes, the Serranus species lose their larval melanophore pattern rapidly at transition and develop transitional melanophores, often in a completely different pattern. The distinction in melanophore size is not as obvious as in many other fishes, and the transitional melanophores can often look the same as the larval melanophores they are replacing. This is especially true on the head and on the fin-ray membranes. Patches of fin-membrane melanophores disappear and new patches can arise on different parts of the fin. In general, the transitional melanophores on the body are smalller and more finely scattered than the large, often single, larval melanophores. Like the Hypoplectrus, larvae of Serranus often show less than their full complement of larval melanophores, and occasional individuals have extra spots. In addition, transitional larvae lose their larval melanophores in no consistent order, leading to a spectrum of spot patterns. This variation makes identification more complicated since variants are to be expected.