A taxonomic review of the genus Asterostegus ( Echinodermata : Ophiuroidea ) , with the description of a new species

A revision of the genus Asterostegus Mortensen, 1933 (Echinodermata: Ophiuroidea: Euryalidae) is based on seven specimens, including the holotype of Asterostegus maini McKnight, 2003. A new species, Asterostegus sabineae sp. nov., is described from off Reunion Island and two other species, A. tuberculatus Mortensen, 1933 and A. maini, are redescribed. A tabular key to the three species of the genus Asterostegus is provided. Some terminology of the taxonomy of euryalid ophiuroids is revised.

The monophyly of this genus is confirmed by a recent molecular phylogenetic study (Okanishi & Fujita 2013), but taxonomic works on Asterostegus species are scarce.Asterostegus tuberculatus was originally described by Mortensen (1933b) based only on the holotype, but a second specimen was recorded by Clark & Courtman-Stock (1976).Asterostegus maini was also originally described by McKnight (2003), based only on the holotype.McKnight (2003) distinguished the aforementioned two species chiefly by the arrangement and size of tubercles on the radial shields and lateral bars, and by the arrangement of plates on the oral interradius.However, these descriptions lacked quantitative accounts of sizes of the tubercles (Mortensen 1933b;Clark & Courtman-Stock 1976;McKnight 2003), making it difficult to distinguish the two species of Asterostegus as well as to compare newly collected specimens of Asterostegus.
In the present study, Asterostegus maini and A. tuberculatus are redescribed based on the holotype of A. maini and one newly collected specimen of A. tuberculatus.A new species of Asterostegus is based on five specimens collected from off Madagascar.In recent descriptions of ophiuroids, photographs and/or drawings of each part of the body, as well as SEM images of each ossicle, provide valuable taxonomic information (e.g., Okanishi & Fujita 2009;Okanishi & Fujita 2011a, 2011b;Okanishi & Fujita 2014;Martynov 2010).To date, drawings and photographs of A. tuberculatus and A. maini have been presented of limited parts of the body (Mortensen 1933b;McKnight 2003) only.Herein we present detailed photographs of many parts of the body and SEM images of separated ossicles.

Material and methods
The seven specimens used in this study are deposited in the National Institute of Water and Atmospheric Research (NIWA), Muséum National d'Histoire Naturelle (MNHN) and the Swedish Museum of Natural History (SMNH).
Ossicles of one paratype of the new species (SMNH-Type-8536) and of A. tuberculatus (SMNH-123461) were isolated by immersion in domestic bleach (approximately 5% sodium hypochlorite solution), washed in deionized water, dried in air, and mounted on SEM stubs using double-sided conductive tape.The preparations were sputter-coated with gold-palladium and examined with a HITACHI S-4300 SEM at the Seto Marine Biological Laboratory (Kyoto University).A part of the mouth frame, aboral shields, and oral interradial plates of one paratype of the new species (SMNH-Type-8536) and A. tuberculatus were exposed by dissolving skin with domestic bleach.
The number and arrangement of plate-shaped ossicles on the distal side of the adoral shields (Figs 2H, 6F, 7F-G) is an important character for the classification of Asterostegus.They were described as "interradial plates" by Mortensen (1933b: 297) and as "ventral interradial plates" by McKnight (2003: 389).McKnight's "ventral interradial plates" is a relatively unambiguous description because it clearly indicates the position of the plate.However, "ventral" is not usually used for ossicles on the oral frame.Here, we suggest that the ossicles should all be referred to as "oral interradial plates".Bar-shaped ossicles on the aboral and lateral side of arms are also an important taxonomic character (Fig. 7I).These ossicles were described by Mortensen (1933b: 298) as "lateral bars" and by McKnight (2003: 389) as "dorsolateral plate".The "dorsolateral plates" is a term easily confused with the "lateral arm plates" of ophiuroids.We propose the use of "lateral bars" in this study.The lateral bars are present on the proximal portion of the arms of Asterostegus.A pair of bars covers the lateral and aboral side of each arm segment and carry tubercles on their aboral surface.

Diagnosis
Arms simple, not branching.Tubercles present on radial shields and aboral arms.Oral interradial plates present on distal side of adoral shields.Teeth triangular.Oral papillae domed, granule-shaped, present laterally on the jaws.Tentacle pores with two (rarely three) arm spines from fourth arm segment.Oral side of vertebrae with an oral bridge.Lamina of distal arm spines smooth.

Remarks
Based on this study, Asterostegus is currently composed of three species, A. sabineae sp.nov., A. maini McKnight, 2003 andA. tuberculatus Mortensen, 1933.A tabular key to the species of Asterostegus is provided (Table 1).
Species of this genus are distributed in the Indian Ocean, off Madagascar (Mortensen 1933b) and in the Western Pacific, south-east of New Caledonia (McKnight 2003), at a depth of 382-500 m (Fig. 1).European Journal of Taxonomy 76: 1-18 (2014) long, 0.7-2.7 mm wide, completely covered by skin and tubercles (Fig. 2B-C).Oral surface of disc covered by skin (Fig. 2D-E).More than three triangular teeth form vertical row on dental plate (Fig. 2E).

Asterostegus sabineae
One oral interradial plate present on distal side of adoral shield (Fig. 2H).Lateral interradial surface of disc nearly vertical, covered by skin and granule-shaped external ossicles, ca.150-400 µm long (Fig. 2H).Two genital slits in each interradius, 3.0 mm long and 0.4 mm wide (Fig. 2H).One madreporite present on distal side of oral interradial plate (Fig. 2H).3A), on each lateral bar.If two tubercles present on lateral bar, both are smaller, ca.800 µm long (Fig. 3A).From middle to distal portion of arms, tubercles gradually decrease in size distally, becoming absent near arm tips (Fig. 3B-C).Oral surface of arms covered by skin (Fig. 3D-E).First to third tentacle pores lacking arm spines; two arm spines from fourth pore (Fig. 2F).In proximal third of arms, outer arm spines ca.one quarter to half as long as corresponding arm segment, and inner arm spines slightly shorter than outer spines (Figs 2F; 3D).In middle of arms inner and outer arm spines subequal, ca.two-thirds as long as corresponding arm segment (Fig. 3D).In distal third of arms, arm spines hook-shaped (Fig. 3F).Inner and outer spines subequal, ca.two-thirds as long  5A).Arm spines club-shaped, both 650 µm long and wide on proximal portion of arms (Fig. 5D), 450 µm long and 500 µm wide on middle portion of arms (Fig. 5E), hook-shaped, ca.230 µm long on distal portion of arms, with one secondary tooth and smooth lamina on proximal portion (Fig. 5F).

Distribution
Only known from off Madagascar, 409-473 m depth (type locality, present study).

Remarks
The new species falls within Asterostegus by virtue of having simple arms, tubercles on radial shields and aboral arms, oral interradial plates, triangular teeth and granule-shaped oral papillae, an oral bridge on oral side of vertebrae, and smooth lamina of distal arm spines (Fell 1960;McKnight 2003).
Asterostegus sabineae sp.nov.can be distinguished from the other two species as follows: two to four tubercles on the radial shields of the present new species form a line on each radial shield (Fig. 2A), whereas the tubercles of A. maini number ca. 10 and are scattered on the periphery of the radial shields only (Fig. 6C).Those of A. tuberculatus number 5-15 and are scattered over the entire radial shields as well as on the interradial areas and periphery of the aboral disc (Fig. 7A).The size of tubercles of the new species on the periphery of the radial shield is ca.1.0-1.2mm long (Fig. 2B).In contrast, those of A. maini and A. tuberculatus in the same position are ca.250-300 µm (Fig. 6C) and ca.350 µm long (Fig. 7A), respectively.
The new species has one or two tubercles on the top of each lateral bar in the arms proximally (Fig. 2H), while A. maini and A. tuberculatus have three to five (Fig. 6H), and two to three tubercles (Fig. 7I), respectively.
The number of oral interradial plates of the new species is one on each interradius (Fig. 3F) while there are three to five plates in A. maini (McKnight 2003) and five to eight in A. tuberculatus (Fig. 7G) (Mortensen 1933b).The oral interradial plates of the latter two species form rows (see also the description of A. tuberculatus) (McKnight 2003).Disc.Disc circular in shape with notched interradial margins (Fig. 6A-B).On aboral surface, radial shields and their surrounds tumid (Fig. 6A, C).Aboral surface of the disc covered by skin and clubshaped tubercles on periphery of radial shields, ca.250-300 µm (Fig. 6C).Radial shields, ca.10.0 mm long and 1.7-2.1 mm wide, and completely covered by skin (Fig. 6D).Oral surface of the disc covered by skin (Fig. 6E).Seven to eight spear head-shaped teeth forming a vertical row on dental plate (Fig. 6E).Domed and granule-shaped oral papillae lying on each side of jaw, but can not be seen when wet (Fig. 6E).Two or three oral interradial plates form a row on proximal side of adoral shields but cannot be seen clearly when wet (Fig. 6F).The oral interradial plates projecting from lateral side of the disc (Fig. 6G).Lateral interradial surface of the disc nearly vertical, covered entirely by skin (Fig. 6G).Two genital slits in each interradius, 1.6 mm long and 0.2 mm wide (Fig. 6G).

Asterostegus maini
arMs.Arms simple, five in number, with no abrupt gap in width due to presence of gonads in proximal portion of arms.Arms tapering gradually distally.Proximal portion of the arms 6.7 mm wide and 7.1 mm high, oblong in cross-section.Aboral surface arched and oral surface flattened from middle to distal portion of arms.Proximal portion of the arms covered by skin and three or four (rarely five) club-shaped tubercles, ca.170-340 µm long, on each lateral bar (Fig. 6H).Tubercles gradually decrease in size and number distally, becoming absent near arm tips (Fig. 6I).Entire oral surface of arms covered by skin (Fig. 6J).First to third tentacle pores lacking arm spines; fourth pore with two (rarely one or three) arm spines (Fig. 6J).All arm spines on each tentacle pore subequal throughout arms (Fig. 6J, K).In proximal third of arms, arm spines ovoid and minute (Fig. 6J), ca.one-third to two-thirds as long as corresponding arm segment (Fig. 6J).In middle of arms, arm spines club-shaped, as long as corresponding arm segment.In distal third of arms, arm spines hook-shaped, as long as corresponding arm segment (Fig. 6K).Lateral arm plates concealed by skin.
colour.McKnight (2003) described the colour of the holotype as "In ethanol, areas between radial shields as well as dorsal surfaces of arms light brown, rest of body creamy-white".However, by our observation, the holotype is uniformly creamy white.The colour might be diluted in long term ethanol preservation.Colour in life is unknown.

Remarks
Asterostegus maini is related to A. tuberculatus in sharing relatively small tubercles and multiple oral interradial plates on each interradius, while A. sabineae possess relatively large tubercles and only one oral interradial plate.
Asterostegus maini and A. tuberculatus can be distinguished by the size and arrangement of tubercles on the disc, the number of tubercles on lateral bars, and the number and arrangement of oral interradial plates (Table 1, see also remarks for A. sabineae).

Diagnosis
Tubercles present on periphery of radial shields as well as interradial periphery of disc, ca.350 µm long.
On proximal portion of arm, two or three tubercles on top of each lateral bar.Five to eight oral interradial plates forming two rows.
arMs.Arms simple, five in number, with no abrupt gap in width due to presence of gonads in proximal portion of arms.Arms tapering gradually distally.Proximal portion of arms 6.85 mm wide and 5.5 mm high, oblong in cross-section.Aboral surface arched and oral surface flattened from middle to distal portion of arms.Proximal portion of arms covered by skin and two or three club-shaped tubercles, ca.400 µm long and 750 µm height (Figs 7I; 8J-K), on each lateral bar.On middle portion of arms, tubercles granule-shaped, ca.450 µm long (Fig. 9A), one or two on each lateral bar.On oral surface, four to five granule-shaped external ossicles, scattered on each arm segment, ca.150-300 µm long on proximal portion of arms (Fig. 9C) and ca.100-150 µm long on middle portion of the arms (Fig. 9D).
Tubercles and external ossicles on oral surface gradually decrease in size distally, becoming absent near European Journal of Taxonomy 76: 1-18 (2014) arm tips (Fig. 9B, E-F).First to third tentacle pores lacking arm spines; two arm spines from fourth pore (Fig. 7D).In proximal third of arms, arm spines ovoid and minute (Figs 8G; 9C).Outer arm spines ca.two-thirds as long as corresponding arm segment, inner arm spines slightly shorter, ca.two-thirds as long as outer spine (Fig. 9C).In middle of arm, arm spines club-shaped (Figs 8H; 9D).Inner and outer arm spines of equal length, ca.half as long as corresponding arm segment (Fig. 9D).In distal third of arms, arm spines hook-shaped with smooth lamina on distal side (Fig. 9E).colour.Uniformly white, slightly grayish on aboral disc surface except radial shields (Fig. 7A).Black spots appear on aboral and lateral arms randomly, every 2-20 arm segments (Fig. 7I).Colour in life is unknown.

Remarks
Asterostegus tuberculatus can be distinguished from the other two species in the arrangement of tubercles on the disc, numbers of tubercles on lateral bars, and the arrangement of oral interradial plates (Table 1, see also the Remarks for the other two species).

Fig. 1 .
Fig. 1.Known distribution of Asterostegus maini McKnight, 2003, A. sabineae sp.nov., and A. tuberculatus Mortensen, 1933 are indicated by square, circle and star shaped symbols respectively.Solid symbols indicate the type locality of each species.

on aboral surface of the disc Numbers and arrangement of oral interradial plates Number and arrangement of tubercles on aboral disc Number of tubercles on top of each lateral bar
DescriptionMeasureMents. Holotype (MNHN IE-2013-8012): disc diameter 11.7 mm, arm length 118 mm.One paratype (SMNH-Type-8536): disc diameter 13.3 mm, arm length at least 55.4 mm (arms convoluted).Species Tubercles

Table 1 .
A tabular character key to the species of Asterostegus.
European Journal of Taxonomy 76:1-18 (2014) DiagnosisAbout 10 tubercles present on periphery of radial shields, ca.250-300 µm long.On proximal portion of arm, three to five tubercles on top of each lateral bar.Three to five oral interradial plates forming one row.