Two new deep-sea species of burrowing anemones (Cnidaria: Actiniaria: Edwardsiidae) from Whittard Canyon off the southwestern coast of Ireland

Burrowing sea anemones have a simple morphology with an elongate body and a round aboral end that anchors the animal into mud, sand, or gravel, leaving only the tentacle crown exposed. Edwardsiids are easily differentiated from other burrowing sea anemones by their distinctive mesentery arrangement of eight unpaired macrocnemes at midcolumn with microcnemes restricted to the distal column at the base of the tentacles. Though edwardsiids may be frequently collected in biodiversity surveys, oceanographic expeditions, and ecological monitoring projects, their identification is particularly hampered by their small size, the need for histology, the high number of undescribed species, and the few specialists able to identify them. Scolanthus belongs to the subfamily Edwardsiinae, which is characterized by nemathybomes; it is differentiated from other members of the subfamily by having nemathybomes with basitrichs and periderm in the proximal end, at least eight microcnemes, and 16 or more tentacles in adults. The 14 valid species of Scolanthus are distributed worldwide, but only four species have been recorded from waters deeper than 100 m (S. ingolfi, 1461 m; S. nidarosiensis, 125-150 m; S. intermedius, 223 m; S. triangulus, 71-271 m). Here we describe Scolanthus shrimp, sp. nov., and S. celticus, sp. nov., the first two sea anemones recorded from the deep-sea Whittard Canyon off the coast of Ireland. We provide detailed morphological descriptions of the new species, including micro-CT scanning of S. celticus, and differentiate them from other species in the genus. We also generate a phylogeny using five molecular markers (12S, 16S, 18S, 28S, CO3) to establish the phylogenetic position of the new species. Based on our results, we discuss the relationship of Scolanthus to other edwardsiid genera and implications for the morphology and evolution of the group.