Mediaster tenellus Fisher 1905

FIGURE 8A–E, 10B

Mediaster tenellus Fisher 1905: 307; 1911: 202; Alton 1966 a (as M. tenellus?): 1680, 1688, 1702; Maluf 1988: 34, 118; A.M. Clark 1993: 263; Hendrickx et al. 2011: 808; Mah 2016: 120;

Mediaster transfuga Ludwig, 1905: 120; Mah 2016: 120–121.

Ceramaster gen. inconclusive. Seid et al. 2025: 159, Figs 61A–B.

Diagnosis. Based on Mah (2016). Body variably stellate, R/r=1.9–3.1, arms triangular, interradial regions weakly curved. Abactinal plates tabulate, topped with spinelets and granular accessories. Peripheral spinelets, 6–30, mostly 15–24, well-spaced, lobate. Central plate surface with 1 –10 (mostly 4–7) round, low, blunt granules; distally short spinelets present adjacent to superomarginals. Marginal plates 30–60 (arm tip to arm tip), both marginal plate series covered with granules, 60–200 (most with about 150). No pedicellariae. Actinal plates round to polygonal in shape, in 3–4 full series arranged in chevron formation. Each plate covered by granules, 2–12, typically 6–8, polygonal, widely spaced granules. Furrow spines slender, 3–6, most commonly four or five, quadrate in cross section, blunt tipped, approximately 10–20% longer than subambulacral spines Subambulacral spines two or three, each about twice as thick as a furrow spine and twice as long.

Comments. The specimen described herein represents the southernmost occurrence for this already widely distributed species and a significant extension to Central America. Individuals observed from farther north were white when alive, whereas individuals figured here are orange.

Fisher (1911) was the first to suggest that Mediaster transfuga Ludwig 1905 intergraded closely with Mediaster tenellus, possibly forming morphological extremes of a single species. Mah (2016) briefly summarized the possibility that these two species could be synonymous. Based on specimens herein, furrow spine number, abactinal tabular granules and number of marginal plates per interradius are all similar with that of M. tenellus, thus it is argued that Mediaster transfuga is synonymous with Mediaster tenellus Fisher 1904.

Ecological Comments. Mah (2016) made prior observations of this species from the North Pacific in the Axial seamount region (> 1000 m depths) on soft sediment and basalt rock bottoms. Observations herein showed this species on soft sedimented bottoms from Costa Rica, consistent with these observations. Based on gut content analyses and in situ observation, this species appears to be a sediment feeder/detritivore (Mah 2016).

Occurrence/Distribution. Aleutian Islands & Gulf of Alaska. Pioneer Seamount and west to Axial Seamount, Co-Axial Seamount. North Pacific region, SW of mouth of Columbia River, Oregon, Mulberry Seamount, Catalina Island (Southern California). Acapulco. 380–1829 m.

Costa Rica, 997 m.

Material Examined. SIO-BIC E7048 Mound 12 methane seep, Costa Rica, 8.9304°N, 84.3126°W, 997 m. Coll. Greg Rouse & Ben Moran, HOV Alvin dive 4917, R/V Atlantis, 1 June 2017. 1 wet spec. R=7.9 r= 3.4 cm.

GenBank. PX400629