South(East) by Southwest: Identification of a New Halocaridina rubra Holthuis, 1963 (Decapoda: Atyidae) Genetic Group From O‘ahu, Hawai‘i

Abstract: The anchialine ecosystem, comprised of coastal landlocked habitats containing brackish water, experiences tidal fluctuations due to simultaneous underground connections with the sea and terrestrial aquifer system. Anthropogenic impacts have led to substantial habitat degradation and loss, potentially making the anchialine ecosystem and its biota one of Hawai‘i’s most threatened. Prior analyses of mitochondrial (mtDNA) cytochrome oxidase subunit I (COI) sequences from the Hawaiian anchialine atyid shrimp Halocaridina rubra revealed a potential cryptic species complex with strong regional endemism. This study hypothesized that a Halocaridina population discovered in 2018 in southeast O‘ahu, an area with no historically documented anchialine habitats but where a specimen was collected in 1998, represents a unique lineage or genetic group endemic to the region. Comparison of newly generated mtDNA COI sequences to previous ones identified the population as belonging to the South O‘ahu lineage of Halocaridina. However, fixed mutations and significant genetic differentiation distinguish it from the closely related ‘Ewa genetic group of southwest O‘ahu. Given this, we propose the new Maunalua genetic group for Halocaridina from southeast O‘ahu, with an apparent split between groups occurring at approximately the Holocene-Pleistocene Epoch boundary, corresponding well to the geological age of where the new population was discovered. Notably, the 1998 specimen shared the same distinctive coloration as the Maunalua and ‘Ewa genetic groups, implying inclusion within the South O‘ahu lineage. Conservation efforts should consider the currently recognized 15 Halocaridina genetic groups since they represent unique units from all others in both their genetics and geographic distribution.