Overlooked and forgotten: A discovery of serial thoracopodal epipods in two species of Boreomysis, with a review of similar findings across the Mysidae (Mysidacea, Malacostraca)

This study investigates the presence and interpretation of thoracopodal epipods in mysidaceans, especially in Mysida. Traditionally, Mysida have been described as lacking thoracopodal epipods, with the notable exception of the first thoracopod. This absence of serial thoracopodal epipods starkly contrasts with the branched epipodal gills observed in Lophogastrida and most non-peracaridan Malacostraca. Despite the broadly stated absence of thoracopodal epipods in Mysida, a few mentions in mysid literature suggest the potential existence of epipodal processes in certain species. In this paper, we present two new instances of serial epipodal vestigia found on the thoracopods of Boreomysis spp., alongside a review of various observations of putative thoracopodal epipods and gills reported in the existing mysid literature. • Serial anterolateral distocoxal processes are found on 2-7 in both sexes in 2 of 3 investigated species of Boreomysis. • The processes are discussed and interpreted as vestigial homologues of epipods, so far considered as missing in Mysida. • Tufts of setae found in comparable position in various other mysid are discussed as possible remnants of reduced epipods • Reports of “epipods”, processes and gills found across mysid literature are reviewed and compared to the new findings.