A SINGLE GENETIC POPULATION OVER 24,000 KM: The Unusual Cohesive Genetic Pattern in Abyssorchomene distinctus (Birsten & Vinogradov, 1960)

Cryptic diversity among deep-sea malacostracans is increasingly unveiled through molecular analyses, helping to reassess biodiversity in abyssal and hadal zones and establish baselines before inevitable mineral exploitation. Cryptic diversity, which is the presence of morphologically similar but genetically distinct lineages within what appears as a single species, is marked by genetic variation, structured populations, and high differentiation among geographically distant populations, often isolated over evolutionary timescales. Although cryptic diversity is prevalent in Lyssianassoidea amphipods, Abyssorchomene distinctus emerges as an exception. Analysing mitochondrial COI and nuclear 28S genes from 373 specimens across three ocean basins, we observed no cryptic diversity in A. distinctus. Instead, our results indicate a single, widely distributed population spanning ~24,000 km across the Southeastern Indian and Northeastern Pacific Oceans. Evidence includes a predominant ancestral haplotype in a star-shaped COI network, a skewed nucleotide mismatch distribution, and deviations from neutrality tests, all suggesting a unique population expansion event. This finding positions A. distinctus as one of only five known deep-sea amphipod species with confirmed wide cross-ocean distribution. To explore if this genetic pattern extends to other Lyssianassoidea amphipods, we are conducting similar analyses on Orchomenella pinguides, a circumpolar Antarctic species with minimal prior genetic characterization. Our preliminary study includes 48 specimens from the Ronne Ice Shelf, examining COI and 28S genes to assess genetic structure, cryptic diversity, and intra-specific variation. We plan to expand our sample size to compare genetic differentiation between populations from the Ronne Ice Shelf and publicly available COI sequences databases from the Australian Antarctic Territory, and Southeastern Filchner area in the Weddell Sea.

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