Burrowing Sea Anemones are characterized by an elongate body and round proximal end that anchors the animal into mud, sand, or gravel, leaving only the tentacle crown exposed. Edwardsiids are easily distinguishable from other Burrowing Anemones by their distinctive mesentery arrangement of eight unpaired macrocnemes (divisions within the gastrovascular cavity created by folded membranes) at midcolumn with microcnemes located only distally at the base of tentacles. The Edwardsiidae is among the most speciose taxon in the Order Actiniaria with 10 genera and approximately 95 valid species distributed worldwide from polar to tropical zones, including hypersaline environments and Antarctic ice, and from depths of 0–3550 m. Though Edwardsiids may be frequently collected in biodiversity surveys, oceanographic expeditions, and ecological monitoring projects, their identification is difficult due to the high number of undescribed species, their small size and need for histological examination for specific identification, and the small number of specialists able to identify them.
In a paper published in the American Museum Novitates on 12 February 2020, Luciana Gusmão of the Division of Invertebrate Zoology at the American Museum of Natural History, Cherie Qu of the Science Research Mentoring Program at the American Museum of Natural History, and Bard High School Early College, Sadie Burke, also of the Science Research Mentoring Program at the American Museum of Natural History, and of Baruch College Campus High School, and Estefanía Rodríguez, also of the Division of Invertebrate Zoology at the American Museum of Natural History, describe two new species of Edwardsiid Burrowing Sea Anemones from Whittard Canyon off the southwestern coast of Ireland.
The genera Scolanthus and Edwardsianthus belong to the subfamily Edwardsiinae, which is characterised by having nemathybomes; hollow, epidermis-lined, nematocyst-dense pockets sunk into the mesoglea of the column. Species of Scolanthus are differentiated from other members of the subfamily by having nemathybomes with basitrichs and periderm not only on the scapus but also the proximal end (not a true physa), at least eight microcnemes at the base of tentacles, and 16 or more tentacles in adults. Half of the diversity of Scolanthus has been described in the last 10 years, with most species recorded from the Pacific Ocean. The remaining species are known from the Mediterranean, and Caribbean seas, the southwestern Atlantic, and the North Atlantic. Scolanthus intermedius is the only species recorded in more than one ocean; it is found in the Atlantic portion of the Southern Ocean and Chilean Pacific. Most Scolanthus spp. have been recorded from shallow waters, with only four species known from waters deeper than 100 m: Scolanthus ingolfi (1461 m), Scolanthus nidarosiensis (125–150 m), Scolanthus intermedius (223 m), and Scolanthus triangulus (71–271 m).
Specimens were collected by the R/V Celtic Explorer during the 2014 Ecosystem Functioning and Biodiscovery Expedition at Whittard Canyon, a multibranch system of submarine canyons some 400 km southwest of Cork, Ireland.
Map of type localities for Scolanthus shrimp (triangle), and Scolanthus celticus (star), at Whittard Canyon in the North Atlantic. Gusmão et al. (2020).
The first new species described is named Scolanthus shrimp, where 'shrimp' honours the Science Research Mentoring Program at the American Museum of Natural History, which suports high school students, known informally as srmpers. The Science Research Mentoring Program supported Cherie Qu, Sadie Burke, and Luciana Gusmão. The specimen from which the species is described is elongate, wider proximally than distally, 2.3–5.1 mm in diameter and 15 mm in height. The epidermis of the entire body is covered by a thin periderm and nemathybomes. The column is dark beige with eight mesenterial insertion visible when alive; the preserved specimen is yellow with mesenterial insertion visible. The proximal end is rounded, and not externally differentiated from rest of column; the body divided into in aboral end, scapus, scapulus, and capitulum. The periderm is thin, tightly adherent, not deciduous, and covers the column and nemathybomes from the distal scapus to the proximal end. The nemathybomes single, conspicuous, and irregularly scattered on the entire body, but forming eight rows more visible in distal column perhaps due to contraction. There are sixteen small tentacles, arranged in two cycles, presumably all of same size; tentacle length is up to 1.5 mm; these were light orange in the live specimen, and white in the preserved state. Though the description of Scolanthus shrimp is based on a single specimen, the specimen was well preserved and all external and internal characters were easily described.
External anatomy of Scolanthus shrimp. (A) Lateral view of live specimen; (B) lateral view of preserved specimen showing nemathybomes arranged in rows on scapus; (C) aboral view of the holotype showing nemathybomes on proximal end; (D) detail of distal scapus with nemathybomes scarcely present distributed in rows; (E) detail of nemathybomes on column; (F) oral view of holotype with oral disc and 16 tentacles. Scale bars: (A), (E) 1 mm; (B) 5 mm; (C) 3.5 mm; (D) 2.5 mm; (F) 2 mm. Gusmão et al. (2020).
The second new species is named Scolanthus celticus, where 'celticus' honours the Celtic Explorer, a multipurpose research vessel operated by the Marine Institute in Galway, Ireland, which collected specimens of the new species. This species is described from four specimens. All specimens are elongate but stout, robust, wider proximally than distally, 5–10 mm in diameter, and 14–24 mm in height. The proximal end is rounded, externally differentiated from rest of column but not true physa; the body divided in aboral end, scapus, scapulus, and capitulum. All preserved specimens with distal column and oral disc retracted, including part of scapus, scapulus, and capitulum; the tentacles not visible. The epidermis of the entire body is covered by a thick periderm and nemathybomes. the periderm is thick, rusty brown, tightly adherent, deciduous, and covers the column and nemathybomes from distal scapus to proximal end. The nemathybomes conspicuous, irregularly scattered on body, single or compound, and more aggregated on the proximal end due to contraction of body. There are sixteen small tentacles, arranged in two cycles, presumably all of same size; tentacle length is up to 3 mm. The four specimens were collected in a single collection site in the Whittard Canyon approximately 450 km from the southwest coast of Ireland, North Atlantic, at 1130 m depth. Scolanthus celticus was collected less than 30 km from the type locality of Scolanthus shrimp, suggesting a pattern of sympatry common among Edwardsiids
External anatomy of Scolanthus celticus. (A) Lateral view of live specimen; (B) lateral view of preserved specimen showing thick periderm on scapus; (C) oral view of proximal end with periderm and nemathybomes; (D) detail of nemathybomes on proximal scapus. Scale bars: (A) 10 mm; (B) 6 mm; (C)–(D) 3.8 mm. Gusmão et al. (2020).
The vermiform body without marginal sphincter or basilar muscles and the mesenterial arrangement of eight macrocnemes spanning the body with microcnemes restricted to the base of tentacles in Scolanthus shrimp and Scolanthus celticus clearly place them within superfamily Edwardsioidea and family Edwardsiidae. Molecular data complement the morphological data: the taxonomic position of Scolanthus shrimp and Scolanthus celticus within Edwardsiidae is further supported by the phylogenetic analysis, which places both species within a highly supported Edwardsioidea. In addition, both new species are recovered with high support within a clade of six species corresponding to the subfamily Edwardsiinae, suggesting that nemathybomes are a strong synapomorphy for the group. Gusmão et al. found a close relationship between Scolanthus and Edwardsianthus, which is supported by three morphological features (i.e., discontinuous ciliated tracts, ovoid parietal muscle, and mesoglea of the aboral end equal in thickness to that of the column). Although the limited taxon sampling and lack of resolution within Edwardsiidae in the analyses of Gusmão et al.'s broad data set prevent them from further commenting on the relationships of Edwardsia, Scolanthus, and Edwardsianthus and of those and other Edwardsiids, the analysis of the more focused data set helped clarify relationships within family Edwardsiidae.
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