So similar and yet so different. A new species of the genus Socarnopsis (Amphipoda: Lysianassidae) from the coral belt of Mayotte (Indian Ocean)

Abstract During a 2023 survey of higher crustacean biodiversity along the reefs of Mayotte, small fish-baited bottle traps were deployed. One of these traps contained a considerable number of a previously unknown amphipod species. Although the specimens were readily recognized as belonging to the superfamily Lysianassoidea Dana, 1849, their assignment to family and genus was initially uncertain. Through extensive comparisons with other species of this species-rich superfamily, we concluded that the material represents a sister species of Socarnopsis catacumba (Clark & Barnard, 1985) from the Bahamas, as it shows very close morphological similarities. Both species share a serrated posterior margin with deep teeth on the basis of pereopods 5–7. The most conspicuous difference is observed on the merus of the pereopod 5, which is approximately one- third wider in the new species, herein described as Socarnopsis mayottensis sp. nov. Most other differences between the two species are marginal and concern the setation of the inner plate of maxilla 2 and the mandibular palp. We generated and deposited the first COI sequence for the genus Socarnopsis Chevreux, 1911, as well as the first complete set of four molecular markers (COI, 16S rRNA, 28S rRNA, and histone H3) for a single species of the family Lysianassidae Dana, 1849 in BOLD and GenBank. Genetic analyses revealed high intraspecific divergence and a deeply divergent lineage represented by a single specimen. Although no morphological differences were detected among the genetic lineages, congruent mitochondrial and nuclear divergence and consistent species delimitation results indicate the presence of cryptic diversity.