Asteroschematidae Verrill, 1899

Okanishi, Masanori, O'Hara, Timothy D. & Fujita, Toshihiko, 2011, A new genus Squamophis of Asteroschematidae (Echinodermata, Ophiuroidea, Euryalida) from Australia, ZooKeys 129, pp. 1-15 : 2-4

Family Asteroschematidae Verrill, 1899

Type genus:

Asteroschema Örsted & Lütken, 1856 (in Lütken 1856)

Type species:

Asterias oligactes Pallas, 1788

Remarks.

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).