Asteroschema Örsted & Lütken, 1856 (in Lütken 1856)

Asterias oligactes Pallas, 1788

Four genera are currently recognized within the Asteroschematidae: 1) the type genus Asteroschema erected for the Caribbean species Asterias oligactes Pallas, 1788 (=Asteroschema oligactes); 2) Ophiocreas, also erected for the Caribbean species, Ophiocreas lumbricus Lyman, 1869; 3) the genus Astrocharis, erected for Philippines’ species, Astrocharis virgo Koehler, 1904; and 4) Astrobrachion, erected for the New Zealand species, Ophiocreas constrictus Farquhar, 1900 (=Astrobrachion constrictum). The monotypic genus Ophiuropsis was erected by Studer 1884 for the Southwest African species, Ophiuropsis lymani Studer, 1884. This genus eventually contained one species with two subspecies, Ophiuropsis lymani lymani Studer, 1884 and Ophiuropsis lymani simplex Mortensen, 1933, but the former subspecies was junior synonymised with Asteronyx loveni Müller & Troschel, 1842 (Asteronychidae) and latter subspecies with Astrobrachion constrictum by Baker (1980). Mortensen (1933) erected a subgenus Astrobrachion (Astroscolex) for one of the two species of Astrobrachion, Ophiocreas adhaerens Studer, 1884 (=Astrobrachion adhaerens)but this taxon was also synonymised with Astrobrachion by Baker (1980).

In his key to the genera of Asteroschematidae, Fell (1960) used the following seven characters: 1) relative length of the arms to the disc diameter; 2) absence/presence of oral shields; 3) absence/presence of ventral arm plates; 4) variable covering of the radial shields; 5) shapes and arrangement of epidermal ossicles on the disc and arms; 6) relative length of the longest arm spine in throughout the arms to the corresponding arm segment; 7) and absence/presence of an abrupt increase in arm width between second and third, or third and fourth arm segments. Baker (1980) showed that two of these seven characters, the relative length of the arms to the disc diameter and absence/presence of oral shields were not useful. McKnight (2000) used another character, the degree of separation of the jaws, to distinguish the three Australian genera of Asteroschematidae, Asteroschema, Astrobrachion and Ophiocreas.

Our review of the taxonomic literature and examination of asteroschematid specimens, has indicated to us that several of these characters are not useful for defining genera. We have found that degree of separation of jaws varies in response to animal preservation. The abrupt increase in arm width, supposedly characteristic of Astrocharis (see Fell 1960) varies across asteroschematid species. An abrupt increase in width in basal portion of the arms can be observed in the original figures of the type species, Astrocharis virgo Koehler, 1904 and of Astrocharis ijimai Matsumoto, 1911. However, photographs in the holotype description of Astrocharis gracilis Mortensen, 1918 (in Mortensen and Stephensen 1918), which was synonymised with Astrocharis ijimai by Okanishi and Fujita (2011a), show no abrupt increase in width (Mortensen and Stephensen 1918; Döderlein 1927; Okanishi and Fujita 2011a) and Astrocharis monospinosa Okanishi and Fujita, 2011a also has no abrupt increase in width (Okanishi and Fujita 2011a).

We propose that four characters are useful for distinguishing the existing genera. The genus Astrobrachion has ventral arm plates separating the lateral arm plates on the oral midline throughout the arms, while the other genera have no ventral arm plates at least from the middle to distal portion of the arms. The genus Astrocharis has completely naked radial shields, whereas the radial shields of the other genera are completely covered by thick skin or epidermal ossicles. Therefore, the absence/presence of the ventral arm plates and the covering of the radial shields are useful generic diagnostic characters as Fell (1960) showed (Table 1).

The shapes and arrangement of epidermal ossicles on aboral surfaces of the discs and arms have been used to distinguish the four genera (McKnight 2000), however, these ossicles vary in shape with location on the body so it is important only to compare ossicles from similar locations. For this study, we compared epidermal ossicles found on the aboral periphery of the disc and aboral basal portion of the arms, which recently appeared to be useful for distinguishing the species of Asteroschema and are expected to be useful for generic taxonomy (Okanishi and Fujita 2009). Although Asteroschema and Ophiocreas cannot be distinguished by this emended character (leaving aside the two species of Ophiocreas, Ophiocreas gilolensis Döderlein, 1927 and Ophiocreas spinulosus, Lyman, 1883, which have additional tubercle-shaped ossicles on the radial shields), the other two genera, Astrobrachion and Astrocharis can be clearly distinguished as follows: species of Astrocharis have only plate-shaped epidermal ossicles, and species of Astrobrachion have no epidermal ossicles (Table 1).

Astrocharis has been distinguished by its short arm spines (Fell 1960), but the longest arm spine is twice as long as the corresponding arm segment in Astrocharis monospinosa (Okanishi and Fujita 2011a). Since the relative arm spine length on Asteroschema and Ophiocreas species is approximately the same as that of Astrocharis monospinosa, these three genera cannot be distinguished from each other by this character (e.g. Döderlein 1911, 1927, 1930; Baker 1980; McKnight 2000). However, although not mentioned by Fell (1960), the length of arm spines on Astrobrachion species is indeed shorter than that of the other genera, being only the same length as the corresponding arm segment. Therefore, the four existing genera can be distinguished by this character as follows: the relative length of the longest arm spines throughout the arms is as long as the length of the corresponding arm segment in Astrobrachion but two times longer in the other three genera (Table 1).

Asteroschema amamiense Okanishi & Fujita, 2009

Squamophis albozosteres sp. n.

Aboral periphery of the disc and aboral base of the arms covered completely by contiguous plate-shaped epidermal ossicles. Single-layered radial shields completely covered by epidermal ossicles. No ventral arm plates on middle to distal sections of the arms. Relative length of the longest arm spines the same length as the corresponding arm segment throughout the arms.

The generic name is a masculine noun in the subjective case, a compound of Latin, squama (prefix, meaning “scale”) referring to the plate-shaped epidermal ossicles on their body and the Greek ophis (masculine noun, meaning “snake”), referring to their snake-like arms.

(Okanishi and Fujita (2009, 2011a) examined internal ossicles of many species of Asteroschema and revealed that Asteroschema amamiense differed in having both single-layered radial shields and contiguous plate-shaped epidermal ossicles, on the aboral periphery of the disc and the aboral base of the arms, that are similar in shape and arrangement to species of Astrocharis. Furthermore, a recent molecular phylogenetic analysis (Okanishi et al. in press), based on mitochondrial (16S) and nuclear ribosomal RNA genes (18S, 28S), also showed that Squamophis albozosteres sp. n. (as Asteroschema sp.) and Astrocharis monospinosa form a clade that was separated from the two other species of Asteroschema that were sequenced, Asteroschema ajax A. H. Clark, 1949 and Asteroschema ferox Koehler, 1904. This new species also had single-layered radial shields and contiguous plate-shaped epidermal ossicles. However, both Asteroschema amamiense and Squamophis albozosteres differed from Astrocharis species in having covered radial shields and relatively short arm spines that are only as long as the corresponding arm segment. This morphological and molecular phylogenetic evidence suggests to us that these two species should be distinguished at a generic level from the other species of Asteroschema and Astrocharis. Therefore, we describe a new genus Squamophis for these two species. The distinguishing characters for the new genus are given in Table 1.

The genus Squamophis currently comprises two species: Squamophis amamiensis (Okanishi & Fujita, 2009) from south-western Japan, 167–168 m and Squamophis albozosteres sp. n. from north-western Australia, 95–108 m. Asteroschema capense Mortensen, 1925 and Asteroschema igloo Baker, 1980 may also be related to Squamophis amamiensis, based on the similarity of shapes and arrangement of epidermal ossicles (Okanishi and Fujita 2009). However, we have not examined the nature of radial shields on their type specimens and hence we refrain from transferring these two species to the new genus at this time.

MV F 162657, holotype, stn SS05/2007 116, off Broome, northwestern Australia, 16°45.09'S, 121°02.48'E – 16°44.36'S, 121°02.12'E, 100–108 m, rocky bottom, 23.3 °C, 30 Jun 2007, epibenthic sled. MV F162658, two paratypes, stn SS05/2007 188, off Ashmore Reef, northwestern Australia, 12°26.42'S, 123°36.03'E – 12°26.58'S, 123°36.35'E, 95–96 m, rocky bottom, 24, 8 °C, 7 Jul 2007, benthic dredge (Fig. 2).

Epidermal ossicles conspicuous white, slightly domed and round plate-shaped, irregularly placed on aboral side of disc and forming two transverse bands on aboral and lateral sides of each arm segment. Rest of the aboral surface uniformly brown except light purple near the tips of arms and finally without color at the tip.

MV F162657: disc diameter 3.4 mm, arm length approximately 50 mm (Fig. 3).

Disc. Disc five-lobed with slightly notched interradial edges: lacking evidence of fission (Figs 3, 4A). Aboral surface almost flat, but radial shields and their surrounds slightly tumid, covered by white, slightly domed and round plate-shaped epidermal ossicles and brown, flat and polygonal plate-shaped epidermal ossicles (Fig. 4A–C). Epidermal ossicles covered by a thin skin. White epidermal ossicles forming transverse rows at the aboral disc (Fig. 4A), almost uniform in size on aboral disc, 70–120 µm long, approximately 100 µm thick. Brown epidermal ossicles obscured by skin and cannot observed externally (Fig. 4A–C); relatively large near the periphery, 150–250 µm long, approximately 50 µm thick, and relatively small at the disc center, 100–150 µm long, approximately 50 µm thick. Radial shields completely covered by epidermal ossicles, oblong, approximately 1.2 mm long and 0.6 mm wide, not reaching the center of the disc.

Oral surface of disc entirely covered by only white, flat and polygonal plate-shaped epidermal ossicles, 50–100 µm long and approximately 50 µm thick (Fig. 4D–G). Four to five triangular teeth forming a vertical row on dental plate. Domed granule-shaped oral papillae lying on either side of jaw (Fig. 4F).

Lateral interradial surface of disc nearly vertical, covered by epidermal ossicles similar to those on oral surface (Fig. 4H). Two genital slits (0.6 mm long and 0.3 mm wide) present in each interradius. No distinct ossicles suggesting existence of madreporites or oral plates observed on any oral interradius, and only epidermal ossicles covered these surfaces (Fig. 4H).

Arms. Arms simple, five in number, no abrupt change in width near the arm base (Fig. 3). The basal portion of the arm 1.4 mm wide and 1.5 mm high, with an arched aboral surface and flattened oral surface. Arms tapering gradually toward the arm tip (Figs 3, 5A, D, G).

The aboral and lateral surface of the base of arms completely covered by white epidermal ossicles, 150–300 µm long, approximately 60 µm thick, and brown epidermal ossicles, 100–300 µm long, approximately 50 µm thick (Fig. 5A, B), similar to those on aboral periphery of disc. Epidermal ossicles on basal portion of arms covered by thin skin. Brown epidermal ossicles obscured by skin, similar to those on aboral surface of disc. Oral surface of the base of arm covered by white epidermal ossicles, 50–100 µm long, approximately 50 µm thick, similar to those on oral surface of disc (Fig. 5C). From basal to middle portion of the arms, the size of plate-shaped epidermal ossicles decreasing on both the aboral and lateral surfaces (Fig. 5D, F), the white domed ones to 100–200 µm and the brown polygonal ones to 100–150 µm. Brown polygonal ones on oral surface decreasing to 50 µm (Fig. 5E). The distal sections of arms covered by scattered granule-shaped epidermal ossicles of 30 µm, finally disappearing near arm tip (Fig. 5G, H).

First to third tentacle pores lacking arm spines; 4th and more distal pores with one arm spine. Arm spines on basal one-third of arm ovoid, minute, approximately one-third to one-half the length of corresponding arm segment (Fig. 5C). Arm spines in middle one-third of arm the same length as corresponding arm segment, bearing fine thorns at their apex (Fig. 5E, F). Arm spines on distal one-third of arm hook-shaped with conspicuous lateral secondary teeth along inner edge (Fig. 5G, H). Length of hook-shaped arm spines gradually decreasing to two-thirds of the corresponding arm segment on distal third of arm, and number of secondary teeth decreasing from two to one. All tentacles pores lacking a sheath around the cylindrical, narrow tube feet (Figs 4E, 5G).

Lateral and ventral arm plates completely concealed by epidermal ossicles over basal to middle portion of arms, but lateral arm plates visible in distal portion of arms (Fig. 5G).

Color.Aboral surface of disc brown, with white spots highlighting the domed epidermal ossicles. Pigmentation on aboral distal portion of arms lighter and appearing purple, finally disappearing at the tip (Fig. 5H). Oral side entirely white (Fig. 3).

MVF162658: Disc diameter 5.3 mm, arm length at least 200 mm.

Flat and polygonal plate-shaped epidermal ossicles at aboral periphery of disc, approximately 236 µm long and 43 µm thick (Fig. 6A, B), the white, round and domed plate-shaped epidermal ossicles approximately 136 µm long and 40 µm thick (Fig. 6C, D). On aboral surface at base of arm, domed ossicles slightly oblong, approximately 226 µm long and 34 µm thick (Fig. 6E, F), whereas the other ossicles flat and round, granule-shaped, 64 µm long and 20 µm thick (Fig. 6G, H).

The radial shields flat and oblong, single-layered, approximately 1.15 mm in length and 0.57 mm in width (Fig. 1B).

Arm spines on basal one-third of arm ovoid (Fig. 6I), in middle cylindrical, bearing fine thorns at tip (Fig. 6J), and distally, they hook shaped with conspicuous secondary teeth along inner edge (Fig. 6K). Number of secondary teeth decreasing gradually to one along distal quarter of arm.

Each lateral arm plate associated with one arm spine and has separate muscle and nerve openings (Martynov 2010) (Fig. 7A). Oral side of each arm vertebra with a longitudinal groove along midline, no oral bridge (Okanishi et al. in press) formed to surround the radial water vessel and nerve (Fig. 7B, C).

Although only three specimens have been collected, some morphological variation was observed. The smaller holotype (3.4 mm in disc diameter) has no abrupt reduction in arm thickness, but the basal portion of the arm on the two larger paratypes (5.3 mm and 5.6 mm in disc diameter) are slightly widened. The difference between the three specimens may be due to a difference in their sexual maturity or reproductive state similar to the congener, Squamophis amamiensis (Okanishi and Fujita 2009).

North-western Australia; 95–108 m. Type locality: off Broome, 100–108 m (Fig. 2).

The specific name is a masculine noun in apposition formed as a compound of Latin words, albus (adjective, meaning “whitish”) and a plural form of zoster (masculine noun, meaning “ring”), referring to the rings of white plate-shaped dermal ossicles of arms.

Squamophis albozosteres sp. n. and its congener, Squamophis amamiensis, are similar to each other, however, they can be distinguished by the morphology of the epidermal ossicles on the aboral body and by pigmentation. Squamophis albozosteres has conspicuously white, domed and plate-shaped epidermal ossicles on the aboral side of the disc and basal to middle portion of the arms, forming two transverse rows on the lateral and aboral surfaces of each arm segments (Fig. 5F). Whereas Squamophis amamiensis has only uniform coloured, flattened and plate-shaped epidermal ossicles on the corresponding surfaces. The aboral body surface of Squamophis albozosteres is basically brown with white spots and the tips of the arms are light purple, finally with no color, but that of Squamophis amamiensis is uniformly orange or pinkish brown (Okanishi and Fujita 2009).