FIGURE 9A–F
Tremaster mirabilis Verrill, 1879: 201; 1885: 638; 1895: 141; Dons 1929: 98; Blacker 1957: 34; Nesis 1960: 1886; Jangoux 1982: 152; Leeling 1984: 274; Clark & Downey 1992: 196; Branch et al. 1993: 42, 64; Clark & McKnight 2001: 163; O’Loughlin & Waters 2004: 36; Stampanato & Jangoux 2004: 4; Gale et al. 2013: 32, 33, 35; Clark & Bowden 2015: 169, 171; Ringvold & Andersen 2016: 25.
Tremaster laevis H.L. Clark, 1941: 53–55.
Tremaster novaecaledoneae Jangoux, 1982: 152; A.M. Clark 1993: 228.
Tremaster mirabilis novaecaledoneae Leeling 1984: 274.
Diagnosis
Body pentagonal to sub pentagonal, strongly arched. Arms five, short, confluent with disk. Abactinal plates imbricate, with variable accessories ranging from large conical to small granuliform spinelets (Fig. 9A, B). Actinal spines single, tall, slender, and spatulate. Marginal plates imbricate, indistinct on lateral surface. Actinal plates in oblique transverse series, each with a large, single spine. Four rows of tube feet. Superambulacral, superactinal plates absent. Interradius very wide, thin, supported by interior distal keels on actinal plates meeting abactinal plates. Five plated ducts forming distinct channel between abactinal and actinal interradial openings (Fig. 9D). (Emended, based primarily on O’Loughlin & Waters 2004; Clark & Downey 1992)
Comments
Further occurrence records here add to our knowledge of Tremaster mirabilis at high-latitudes. They had been previously documented from Kerguelen and Branch Island (Jangoux 1982; Branch et al. 1993; Stampanato & Jangoux 1993), the Ross Sea (Clark & Bowden 2015), the South Pacific (Jangoux 1982) and now from the South Atlantic (Scotia Sea), although none seem to be from the Southern Ocean itself. Distribution of this species is widespread, more so than was documented by Jangoux (1982) or Clark & Downey (1992) with NOAA ship Okeanos Explorer observations and other records showing this species from deep-sea settings throughout the tropical Pacific and the Southern Indian Ocean in addition to the Atlantic (Mah, MS in preparation).
Although the presence of a widespread single morphospecies is not unique within the Asteroidea, it is unusual for a species which occurs at the relatively “shallow” depths that Tremaster mirabilis inhabits (150–1050 m). Few other species are known with this distribution pattern. Hippasteria phrygiana was revealed through molecular phylogenetic to be a wide-ranging single species in the Pacific, Atlantic and Indian Oceans (Foltz et al. 2013; Mah et al. 2014) at a comparable depth. Other species with this apparent pattern tend to occur in cosmopolitan deep-sea bathyal or abyssal habitats, including several taxa within the Porcellanasteridae (Madsen 1955)
North Atlantic Tremaster mirabilis have been observed in situ feeding on the gorgonian Acanthogorgia armata Verrill, 1878 (Gale et al. 2013).
Marginal plates in Tremaster mirabilis
The identification of marginal plates in Tremaster mirabilis has not met with full agreement. Marginal plates were not clearly identified in Verrill’s (1879) original description. H.L. Clark’s (1941) description of Tremaster laevis (a synonym of Tremaster mirabilis) described marginal plates as spine-bearing but without designation of upper or lower series. Jangoux (1982: 157) identified superomarginal plates as the outermost and smallest plates of the interradial abactinal columns which alternate in position along the lateral edge with the superomarginals. Clark & Downey (1992: 195) loosely followed H.L. Clark’s (1941) account and omitted a description or interpretation of superomarginals, stating only that inferomarginals were present, that they were very small and that they formed the actinolateral margin and were characterized by the short, papilliform single spinelets. O’Loughlin & Waters (2004: 37) stated that the superomarginal plate series was absent, and that the actinal margin around the body’s edge was formed by a projecting series of inferomarginal plates. This margin was described as “fragile and frequently broken off.” Application of Blake’s (1978) criterion of recognizing marginal plates around the body ambitus from the terminal plate to the interradius permitted identification of a superomarginal and inferomarginal series.Although this was consistent with Jangoux’s (1982) placement of superomarginal and inferomarginal plate series further work is necessary to clearly recognize marginal plates in this species.
Occurrence
Antarctic/High-Latitude. Ross Sea, Kerguelen and Marion Island, Tristan de Cunha, South Georgia Island (Scotia Sea), Scott Island Bank (South Pacific), Cape Horn, Tierra del Fuego (South Atlantic). 223–494 m. Also throughout the Atlantic and tropical Pacific. Broad depth range: 150–1060 m.