Asteromorpha rousseaui ( Michelin, 1862 )

Okanishi, Masanori, Olbers, Jennifer M. & Fujita, Toshihiko, 2013, A Taxonomic Review Of The Genus Asteromorpha Lütken (Echinodermata: Ophiuroidea: Euryalidae), Raffles Bulletin of Zoology 61 (2), pp. 461-480 : 468-472

Asteromorpha rousseaui ( Michelin, 1862)

( Figs. 6–9 )

Asterochema rousseaui Michelin, 1862: 6 ; Hoffman, 1874: 53

Astroschema rousseaui von Martens, 1869: 129 ; Lyman, 1880: 45; 1882: 278

Asteroschema rousseaui Lyman, 1872: 4 ; de Loriol, 1893: 55–56; – Döderlein, 1911: 111

Asteromorpha perplexum . A. M. Clark, 1976: 111, 112, 117, fig. 1. Non Asteromorpha perplexum ( Koehler, 1930)

Asteromorpha steenstrupi Lütken, 1869: 60–63 , one plate

Asteromorpha rousseaui Lütken, 1872: 96–98 ; Mortensen, 1933: 57–60, figs. 42–44, pl. VI figs. 6–9.

Astroschema steenstrupi Lyman, 1875: 26

Asteroschema steenstrupi Brock, 1888: 538

Type material examined. — Six dry syntypes of Asteromorpha steenstrupi, ZMUC OPH-479 , off Reunion Island .

Other materials examined. — One dry specimen, USNM E5956 , off Port Louis, Mauritius, 200 m, Dec.1929: three ethanol preserved specimens, MNHN IE-2013-4010 , IE-2013-4002 , IE-2013-4008 , collected by R / V MARION DUFRESNE, MD32 CP172, north of Reunion Island, east of Madagascar, 20°52.S, 55°37.E 105–120 m, 8 Sep.1982: one ethanol preserved specimen, MNHN IE-2013-4011 , collected by R / V MARION DUFRESNE, station MD 32 FA92, north of Reunion Island, east of Madagascar, 19°45.S, 54°07.E 75–125 m, 28 Aug.1982: one ethanol preserved specimen, respectively, MNHN IE-2013-4012 , IE-2013-4006 , collected by R / V MARION DUFRESNE, MD32 DC176, west of Reunion Island, east of Madagascar, 21°01.S, 55°10.E 165–195 m, 8 Sep.1982: one ethanol preserved specimen, MNHN IE-2013-8007 , collected by MIRIKY, CP3260 , between Majunga and Cape Saint-Andre, north-western Madagascar, 15°35.S, 45°45.E 179–193 m, 10 Jul.2009 ( Fig. 1) .

Diagnosis. — Two types of external ossicles on aboral surface of body, one white, domed and round plate-shaped, while the other brown, flat and polygonal plate-shaped. Brown ossicles of disc forming radiating straight rows interradially and/or regularly arranged on radial shields, while the basal portion of arms (aborally and lateral surfaces), bears brown ossicles forming three transverse rows on each arm segment. White ossicles tessellated between these rows. No tubercles on radial shields. Five or six arms, non-fissiparous.

Description of USNM E5956. — Disc diameter 6.1 mm, arm length ca. 52 mm ( Fig. 6 ).

Disc circular with no fission plane ( Fig. 6A ). Aboral surface tumid, covered by both white, slightly domed and round plate-shaped external ossicles and brown, flat and polygonal plate-shaped external ossicles ( Fig. 6B, C ). Brown external ossicles forming five straight rows radiating from center of disc interradially, and patches of two or three brown external ossicles scattered at regular intervals among white external ossicles on radial shields ( Fig. 6A ). White external ossicles ca. 80–120 μm long and 70 μm thick and brown external ossicles ca. 70–100 μm long and 30 μm thick, respectively. Radial shields triangular, contiguous and completely covered by external ossicles, ca. 2.7 mm long and 1.3 mm wide, ( Fig. 6A ).

Oral surface of disc entirely covered by only white, flat and polygonal plate-shaped external ossicles ( Fig. 6D ), ca. 60–90 μm long and 30 μm thick on periphery ( Fig. 6F ) and ca. 100 μm long and 40 μm thick on oral plates ( Fig. 6E ). Four teeth forming vertical row on dental plate. Oralmost tooth triangular ( Fig. 6E ), remaining teeth square, domed oral papillae on each side of the jaws ( Fig. 6E ).

Lateral interradial surface of disc nearly vertical, covered by white, flat and polygonal plate-shaped external ossicles similar to those on oral surface ( Fig. 6G ). Two genital slits in each interradius, 1.0 mm long and 0.40 mm wide. Gonads visible inside each genital slit ( Fig. 6G ). No distinct ossicles suggesting presence of madreporites visible on oral interradius.

Arms five, simple, basal third and/or fourth arm segments thickened (2.0 mm wide and 2.0 mm in high), with flattened aboral and oral surfaces. Remaining segments 1.6 mm width and 1.45 mm height, with arched aboral surface and flattened oral surface. Arms tapering gradually from middle to distal extremities.

Aboral and lateral surface of basal portion of arms covered by white, slightly domed and round plate-shaped external ossicles, ca. 100–150 μm long and 80 μm thick, and brown, domed and round plate-shaped external ossicles, ca. 100–180 μm long and 40 μm thick ( Fig. 6H ), similar to those on aboral disc.

Basal arm segments (both aboral and lateral surface) covered entirely by white external ossicles interrupted by three transverse rows of brown ossicles. Basal-most row contains only brown ossicles while other two rows contain regularly scattered white ossicles ( Fig. 6H ). Oral surface of arms covered only by flat, polygonal plate-shaped external ossicles, ca. 50–80 μm long and 50 μm thick ( Fig. 7A ), similar to those on oral disc. In middle portion of arms, aboral and lateral surface also covered by white and brown external ossicles similar to those on basal portion of arms, ca. 100–130 μm long and 100 μm thick, and ca. 70–100 μm long and 50 μm thick, respectively ( Fig. 7B ). Similarly, on arm segments, brown external ossicles form two transverse rows, with basal rows being continuous and distal rows fragmented ( Fig. 7B, D ). Oral surface covered by white, flat and polygonal plate-shaped external ossicles, similar to those on basal portion of oral arms, ca. 50–80 μm long and 50 μm thick ( Fig. 7C ). Distal portion of arms entirely covered by uniform flat and round granule-shaped external ossicles, ca. 80 μm long and 30 μm thick ( Fig. 7E, F ). Each arm segment with row of brown external ossicles on aboral and lateral surface ( Fig. 7F ).

First to third tentacle pores lacking arm spines; from fourth pore, two arm spines ( Fig. 7A ). In first third of arms, arm spines ovoid and minute ( Fig. 8F ), with inner arm spines ca. one-third of length of corresponding arm segment and outer arm spines four-fifth of length of inner ones ( Fig. 8F ). In second third of arms, arm spines bearing fine spinelets at tips ( Fig. 8G ). Inner arm spines two-thirds of length of corresponding arm segment and outer arm spines half length of inner ones ( Figs. 7C, D , 8G ). In distal third of arms, arm spines hook-shaped with smooth lamina on distal side ( Fig. 8H ). Inner arm spines half length of corresponding arm segment with outer arm spines almost same length as inner ones ( Figs. 7E , 8H ).Lateral arm plates concealed by external ossicles, with one or two pairs of a muscle and a nerve opening, and each of them associated with an arm spine articulation ( Fig. 8E ). Vertebrae in middle to distal portion of arms with oral bridge ( Fig. 8I ).

Colour: Aboral disc surface, five brown lines radiating interradially from disc center. Radial wedges are defined by scattered brown spots that form dashed concentric triangles ( Fig. 6 A–C). Aboral and lateral surface of arms white with brown transverse rows. Configuration of brown rows outlined above ( Figs. 6H , 7B, D ). Oral surface uniformly white.

Variation. — Some colour variations were observed as Mortensen (1933) indicated. The specimens MNHN IE- 2013-4006 , IE-2013-4002 , IE-2013-4008 , IE-2013-8007 show similar colour to USNM E5956 described above and have radiating rows of brown plate-shaped external ossicles on aboral disc ( Fig. 6A ). However, syntypes of A. steenstrupi and specimens of MNHN IE-2013-4010 , IE-2013-4011 , IE- 2013-4012 show no such rows or scattered brown ossicles on the aboral disc ( Fig. 9A ). Brown transverse rows appear on arms of all examined specimens ( Fig. 9A, B ) .

Distribution. — REUNION: around Reunion Island, 75–195 m ( Lütken, 1869; present study). MAURITIUS: off Port Louis, 200 m (present study); northwest of Majunga, 179–193 m (present study).

Remarks. — According to Mortensen (1933) and Baker (1980), A. rousseaui can be distinguished from other species by: 1) absence/presence of oral bridge of vertebrae in the basal portion of the arms, 2) fissiparous/non-fissiparous, 3) absence/presence of tubercles on radial shields, and 4) absence/presence of transverse rows of external ossicles on aboral and lateral surface of the arms.

Mortensen (1933) found that A. rousseaui possesses an oral bridge only in distal portion of the arms but the other species of Asteromorpha possess it throughout the arms ( Mortensen, 1933). We refrain from using this character to distinguish the species because it might be variable depending on growth stage. Mortensen (1933) examined specimens of A. rousseaui that were much larger than those of A. perplexum ( Mortensen, 1933) . To determine the reliability of this character, examination of a series of smaller specimens of A. rousseaui is required.

Fissiparity and the absence/presence of tubercles on radial shields were useful taxonomic characters for distinguishing A. rousseaui . All examined specimens in this study of Asteromorpha capensis (N = 4) and A. rousseaui (N = 14) have five or six arms that are uniform in width and have no fission plane. On the other hand, 12 of 18 (67%) examined specimens of A. koehleri and 12 of 16 (75%) of A. tenax have fission planes and six arms with a different width (see Remarks of these two species). Twelve of 16 (75%) examined specimens of A. tenax (including specimens both with/without fission planes) have large tubercles on the radial shields, which are absent in A. rousseaui (see Remarks of each species; Table 1).

Presence/absence of transverse rows of brown external ossicles on the aboral and lateral surfaces of the arms also proved to be a useful taxonomic character but may require more rigorous investigation. Of the four Asteromorpha species, the transverse rows only occur in A. rousseaui and A. koehleri . In the basal portion of the arms, A. koehleri has two rows and A. rousseaui three ( Table 1). In this study, the number of rows of external ossicles was also a useful diagnostic character that can distinguish A. rousseaui from A. koehleri .

Our study of 52 specimens of Asteromorpha revealed that body colour is also a useful diagnostic character. Asteromorpha rousseaui has brown spots at regular intervals and/or brown interradial radiating lines on aboral disc and brown bands on arms. Asteromorpha koehleri is similar to A. rousseaui in colour but lacks brown interradial radiating lines on aboral disc ( Table 1).

Mortensen (1933) recognised two colour variations in A. rousseaui (see Variation above). These variations are distinct and could possibly be distinguished as different species or subspecies. However, we have not examined the type specimens of A. rousseaui and we refrain from describing these variations as different (sub)species here. If the type specimens of A. rousseaui have no radiating interradial lines like the syntypes of A. steenstrupi , then A. steenstrupi should be retained as a synonym of A. rousseaui , and specimens with the radiating lines should be described as a new species. However, if the type specimens of A. rousseaui have radiating lines, then A. steenstrupi could be revived. It is unfortunate that the colour pattern was not sufficiently detailed in the original description of A. rousseaui ( Michelin, 1862) . Therefore, examination of the type specimens of A. rousseaui is required for determining the taxonomic status of these two colour variations. Jangoux (1985) noted that the type specimen(s) were deposited in Museum d’histoire naturelle de Lyon. However, the whereabouts of the type specimens are unknown at present (Sabine Stöhr, pers. comm.).

Figures

Fig. 6. Asteromorpha rousseaui (USNM E5956): A, aboral disc; B, aboral periphery part of disc; C, aboral central part of disc; D, oral disc; E, jaws; F, oral periphery of disc; G, lateral interradial part of disc; H, aboral basal portion of the arm. Double arrow indicates an arm segment. Abbreviations: BTR, basal transverse row; DTR, distal transverse row; Go. Gonad; GS, genital slit; OP, oral papillae; T, teeth.

Fig. 7. Asteromorpha rousseaui (USNM E5956):A, oral basal portion of the arm; B, aboral middle portion of the arm, double arrow indicates an arm segment; C, oral middle portion of the arm; D, aboral distal portion of the arm, double arrow indicates an arm segment; E, oral distal portion of the arm; F, lateral distal portion of the arm. Double arrows indicate arm segments. Abbreviations: BTR, basal transverse row; DTR, distal transverse row; IAS, inner arm spine; OAS, outer arm spine; TR, transverse row.

Fig. 8. Asteromorpha rousseaui (USNM E5956), SEM photographs of internal ossicles: A, B, white and domed plate-shaped external ossicles at aboral basal portion of the arm, external (A) and lateral (B) views; C, D, brown and flat plate-shaped external ossicles at oral basal portion of the arm, external (C) and lateral (D) views; E, lateral arm plate at basal portion of the arm, external view; F–H, arm spines from basal portion of the arm (F), middle portion of the arm (G) and distal portion of the arm (H); I, vertebrae at distal portion of the arm, oral view. Arrows indicate orientations (B, D, I): bas, basal side; dis, distal side; ext, external side; int, internal side. Abbreviations: L, lamina; MO, muscle opening; NO, nerve opening; OB, oral bridge.

Fig. 9. Asteromorpha steenstrupi Lütken, 1869, one syntype (ZMUC OPH-479): A, aboral disc and basal portion of arms; B, aboral middle portion of arm. Double arrows indicate arm segments. Abbreviations: BTR, basal transverse row; DTR, distal transverse row.