New insights into the molecular and morphological diversity and distribution of Liljeborgia in the North Atlantic and Southern Ocean (Amphipoda, Amphilochidea, Liljeborgiidae)

The amphipod genus Liljeborgia Spence Bate, 1863 comprises 79 species with a global distribution, occurring from shallow coastal zones to abyssal depths exceeding 6000 metres. To shed new light on the diversity within this speciose genus, we analysed the mitochondrial cytochrome c oxidase subunit I (COI) gene in 137 individuals mainly from the North Atlantic and Southern Ocean, complemented by 28S rDNA sequences for selected specimens and detailed morphological investigations of potentially cryptic species. Molecular analyses include phylogenetic reconstructions, haplotype networks and species delimitation via ASAP. Our results show potential cryptic species diversity within two species complexes: L. ossiani d’Udekem d’Acoz & Vader, 2009 and L. georgiana Schellenberg, 1931. For L. ossiani the two putative species are differentiated by 8% in COI and are consistently delimited in 28S, which together with their sympatric distribution clearly supports their status as separate species. However, even detailed morphological analyses, including confocal laser scanning microscopy (CLSM), failed to reveal distinguishing external traits. Furthermore, we support that L. caliginis d’Udekem d’Acoz & Vader, 2009 is a junior synonym of L. charybdis d’Udekem d’Acoz & Vader, 2009, indicating that the morphological characters previously used to differentiate these species change during ontogeny. These morphological changes during ontogeny as well as the observed morphological stasis during speciation makes species identification based solely on morphology challenging. Our findings highlight the value of an integrative taxonomic framework in resolving species boundaries.