Bathynomus apothecarius (Isopoda, Cirolanidae), a new species of giant isopod from the deep Northwestern Atlantic Ocean

Abstract A new species of deep-sea cirolanid isopod, Bathynomus apothecarius Gallagher & Riehl, sp. nov., from the Northwestern Atlantic Ocean (The Bahamas), is described based on integrative taxonomy. This species represents the sixth described species of Bathynomus from the tropical and subtropical Western Atlantic. Bathynomus apothecarius sp. nov. is characterized by a body that is parallel-sided in the pereon mid-section; the cephalothorax with the clypeus apical margin broadly rounded; the pleotelson with the posterior margin bearing 7–11 broad and short spines, including a bifid central spine and setation covering the entire spine-bearing region; pereopod 7 with the coxal plate posterior margin concave; pereopods 5–7 with the coxal plates having a densely setose ventro-posterior margin; the uropod exopod with the lateral margin sinuate and a long, continuous setal fringe covering 85–94% of the margin; and the uropod endopod with the medial margin setose. Genetic analysis indicates a ~ 6.26% and 6.58% COI sequence divergence from sympatric species B. giganteus and B. maxeyorum , respectively. Morphologically, the sympatric species can be readily distinguished by the evenly rounded, semicircular pleotelson outline, which is more bluntly and broadly rounded in the sympatric congeners. A virtual holotype created by means of micro-computer tomography (µCT) is made publicly available. An identification key to the Atlantic Bathynomus species is presented. The sympatric occurrence of three Bathynomus species sparks questions on the evolution of this charismatic deep-sea genus.