Mediaster tenellus Fisher 1905

Mah, Christopher L., 2016, Deep-sea (> 1000 m) Goniasteridae (Valvatida; Asteroidea) from the North Pacific, including an overview of Sibogaster, Bathyceramaster n. gen. and three new species, Zootaxa 4175 (2), pp. 101-141 : 120-123

Mediaster tenellus Fisher 1905

Figure 6 A–D, Fig. 7 A

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

Comments. Recently collected specimens from the North Pacific (Axial Seamount, Pioneer Seamount and others listed below) described herein are most similar to the M. tenellus described by Alton (1966a), who identified this species as “ M. tenellus ?” stating that his specimens were significantly different based on the greater R:r value (i.e. longer arms) and the number of superomarginals in the type (about 60 per interradius) versus his specimen (30 to 34 per interradius). Alton speculated that one possible reason for the higher number of superomarginals was due to the larger size of the holotype (R= 5.8 cm) versus those of the specimens he described (one of which, USNM E10436 is listed below, R= 3.1 cm). New material of comparable size (USNM 1407950, R=5.24) bears approximately 44–50 superomarginals per interradius which would appear to support Alton’s prediction.

Collected North Pacific specimens herein conform to Fisher’s description of Mediaster tenellus based on a comparable number of slender furrow spines (four or five), subambulacral spines (three), the pointed spinelets on the tabulae (abactinal plates), and the granular covering on the marginal plates. These individuals differ in that actinal and marginal granules are pointed rather than prismatic and pedicellariae were not observed on the body surface. As indicated in the description, specimens of M. tenellus present in the southern Gulf of Alaska are more delicate and much more similar in appearance to the holotype of M. tenellus than those in the Bering Sea, which possess a more calcified skeleton and have a more stout appearance.

Hippasteria tiburoni , from this same depth range and also from this region was recognized as a distinct species using molecular data (Mah et al. 2014) it is similar to the widely occurring species, Hippasteria spinosa . Mediaster tenellus may yet contain further examples of cryptic species.

Similar species. Fisher (1911) argued that Mediaster transfuga Ludwig 1905 intergraded closely with Mediaster tenellus , possibly forming morphological extremes of a single wide ranging species. Maluf (1988) considered M. transfuga to be a synonym of M. tenellus . A.M. Clark (1993) whereas Hendrickx et al. (2011) maintained these two species are separate. Comparison between the specimens described herein and one of the voucher specimens of M. transfuga (USNM E9647) reveals many shared characters including similar numbers of furrow and subambulacral spines (four and three), spine-tipped actinal granules and tabular plates with similar accessory spinelets and the ovate outline of carinal plates. Mediaster transfuga at R<4.0 also possesses far more marginal plates per interradius (approximately 60) than does M. tenellus and it possesses more elongate arms (R:r=3.8).

In situ observations. This species was observed on basaltic rock and soft sediment substrates with a swollen disk. Examination of one specimen (USNM 1407947) following collection showed the disk region filled with dark sediment, which was consistent with in situ observations of this species sitting on the muddy bottom. Observation of this species suggested that it was abundant throughout the Axial Seamount region (Mah pers. obs). A second MBARI observation ( Fig. 7 A, off the coast of Oregon, dive 1010 in Aug 2006) showed four individuals identified as M. tenellus present on the surface of what is either a molt or a moribund lithodid crab, suggesting scavenging behavior. Although individuals were not remarkable, the sheer abundance of this species throughout the region is likely of ecological significance.

Occurrence. 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.

Description. Body variably stellate (R/r=1.9–3.1), arms triangular, interradial arcs broadly curved. Radial regions swollen, interradial areas depressed ( Fig. 6 A, B). Larger sized individuals are more stellate with more elongate arms versus smaller specimens which have shorter more broadly shaped arms. Further variation in body shape is observed between CASIZ 178975 (Bering Sea) and CASIZ 179380 (southern Gulf of Alaska). The former lot contains stout specimens with broader armtips and lacks pedicellariae versus the latter which has individuals with a more delicate skeleton, more tapering armtips and abundant pedicellariae around the oral region.

Abactinal plates tabulate ( Fig. 6 A,B) i.e. elongate with accessories forming “table-shaped” morphology. Carinal plate series are oval in outline, with adradial and other plates round to polygonal in outline. Disk plates plates mostly larger with smaller secondary plates present at each interradius. Plates becoming smaller interradially and more crowded distally adjacent to the superomarginal series. Interradial paxillae forming ordered transverse rows with fasciolar grooves continuous with contact between superomarginal plates. Each plate covered with spinelets and granular accessories. Peripheral spinelets, six to 30, mostly 15–24, well-spaced, each roughly circular in cross-section. Central plate surface with one to 10 (mostly four to seven) round, low, blunt granules; distally short spinelets present adjacent to superomarginals. Fasciolar grooves well developed with six large papulae present around each plate. Papulae present on radial regions but absent from small triangular interradial areas adjacent to superomarginals. Abactinal plate bases with five radiating bar-shaped ossicles with papular pores present between each pair of plates ( Fig. 6 A inset). Madreporite triangular to irregular polygonal, convex and flanked by three to five abactinal plates. No pedicellariae observed in North Pacific/ Oregon specimens.

Marginal plates 30–42 (armtip to armtip). Superomarginals wide (W>L) forming broad border around abactinal surface ( Fig. 6 A). Superomarginals offset from inferomarginals forming zig-zag contact between plates. Both marginal plate surfaces covered with granules. Superomarginals covered with closely but evenly arranged spine-tipped granules, 60–200 (most with about 150). Inferomarginals with differently sized granules. Larger, coarser granules, four to 60 on upper lateral surface of inferomarginals adjacent to superomarginal contact, widely spaced. Larger granules becoming more abundant more distantly along arm. Granules, 10–20, smaller, gradually decreasing in size, on ventral surface of inferomarginals becoming smaller adjacent to actinal plate surface. A large, spine-like granule present among the large sized, widely spaced granules on the inferomarginals beginning at approximately 50% of arm distance to armtip. Fasciolar groove present between marginal plates and abactinal/ actinal surfaces. Terminal plate hexagonal, surface smooth. No pedicellariae observed.

Actinal surface composed of three or four series of plates in chevron formation ( Fig. 6 C, D). Each plate round to hexagonal in outline, with especially prominent overlapping arrangement adjacent to inferomarginals. Plates covered by two to 12, typically six to eight prismatic, widely-spaced granules, these triangular to quadrangular in cross-section on each plate surface. Fasciolar groove contiguous with marginal plates.

Furrow spines slender, three to six, most commonly four or five, quadrate in cross section, blunt tipped, approximately 10–20% longer than subambulacral spines ( Fig. 6 D). Subambulacral spines two or three, each about twice as thick as a furrow spine and twice as long. When three spines are present, central subambulacral tallest with two flanking spines, approximately 25% to 50% of central subambulacral length. These shorter subambulacral spines become more consistent with other spination on adambulacrals more distally along arm. Three to four prismatic granules on adambulacral plate surface adjacent to actinal plates. Adambulacral granules continuous with those on actinal plates. Nine or ten furrow spines on oral plates with large paired blunt spine directed into mouth.

Color in life is white or white with dark orange highlights.

Material. Alaska. CASIZ 178975 Aleutian Islands , 55.96017, -168.9409 to 55.96304, -168.9908, 804–811 m.

Coll. D.E. Stevenson aboard F/ V Arctic Ocean. 7 wet specs. R=5.2, r=2.9; R=4.9, r=2.3; R=5.1, r=2.2; R=4.0, r=1.8; R=5.0, r=2.6; R=5.7, r=2.7; R=5.1, R=2.4; R=5.1, R=2.4; CASIZ 179380 Gulf of Alaska, 55.74377, - 135.3743 to 55.74862, -135.3535, 661–667 m. Coll . NMFS staff aboard F/ V Sea Storm. 5 wet spec. R=3.9, r=1.3; R=4.3, r=1.4; R=3.6, r=1.1,R=4.9. r=1.6, R=4.0, r=1.4. North Pacific / Oregon . USNM E10436, SW of mouth of Columbia River , 46º2.7’N, 124º57.3’W, 915 m, Coll. M. Alton aboard R/ V Commando, 30 May 1964, 1 dry spec. R=3.1, r=1.4 ; USNM 1407945 , Axial Seamount , 45.946369, -129.98085, 1513.7 m, Coll. Clague et al . MBARI D79-A3. 1 wet spec. R=4.0 r=1.3; USNM 1407946 , Axial Seamount , southeast rim, 45.947746, -129.97676, 1510.2 m, Coll. Clague et al. , MBARI D79-A5. 1 wet spec. R=2.1, r=0.9; USNM 1407947 , Axial Seamount , southeast rim, 45.950191, -129.97488, 1522.9 m, Coll. Clague et al. , MBARI D79-A7, 1 wet spec. R= 2.1 r=0.6; USNM 1407948, President Jackson Seamount C, 42.748909, -128.0661, 1373.9 m, Coll. Clague et al., MBARI, D82-A19. 1 wet spec. R=4.3, r=1.97; USNM 1407949 , Axial West central Caldera. 45.9694, -130.0399, 1450.6 m. Coll. Clague et al. , MBARI D71-PC41, 1 wet spec. R=2.7, r=1.1.

California. USNM 1407950 , Pioneer Seamount , 37.369960, -123.41022, 1002.6 m, D84-A2. 1 wet spec. R=5.24, r=2.75 ; USNM 1407951 , Pioneer Seamount , 37.370402, -123.41072, 986.5 m, Coll. Clague et al . MBARI, D84-A4. 1 wet spec. R=4.7 r=2.2; USNM 1407952 , Pioneer Seamount , 37.373054, -123.41494, 856.6 m, Coll. Clague et al. , MBARI, D84-A8. 1 wet spec. R=2.43, r=1.35; CASIZ 117614 Mulberry Seamount , 37°26.5'N, 123°28.7’W 1280–1828 m (700–1000 fms), Coll. U.S. S. Mulberry. 1 wet spec. R=3.9, r=1.6 ; CASIZ 117538 , Point Sur , Monterey County, California. 380 m (208 fms), Coll. R.L. Bolin. 1 wet spec. R=4.8, r=1.5.

Figures

FIGURE 6. Mediaster tenellus USNM 1407951. A. Abactinal surface. Scale bar = 10.0 mm. Inset shows stellate radiating ossicles on plate base. Scale bar = 1.0 mm. B. Closeup of abactinal plates. Scale bar = 2.0 mm. C. Actinal surface. Scale bar = 10.0 mm. D. Close up of actinal surface showing furrow spines. Scale bar = 3.0 mm.

FIGURE 7. In situ images of M. tenellus and N. diomedeae. A. Mediaster tenellus present on molt or moribund crab. B. Nymphaster diomedeae in feeding aggregation. C. N. diomedeae approaching dead gonatid squid.