Ophiacantha pacifica Lütken & Mortensen, 1899: 166–167

Stöhr, Sabine & O’Hara, Timothy D., 2021, Deep-sea Ophiuroidea (Echinodermata) from the Danish Galathea II Expedition 1950 - 52, with taxonomic revisions, Zootaxa 4963 (3), pp. 505-529 : 522-523

Ophiacantha pacifica Lütken & Mortensen, 1899: 166–167 , pl. XV figs 4–6, pl. XVI fig. 11.

? Ophiacantha bathybia H.L. Clark, 1911: 233 . (not examined)

Figure 4A–D

Material examined

EAST PACIFIC OCEAN • 27 specimens; off Nicaragua; 09°23’N, 089°32’W; 3570 m; 06 May 1952; Galathea II stn. 716; dark, muddish, clay; NHMD-867403, NHMD-867167, NHMD-867470.

GULF OF PANAMA • 4 specimens; 05°49’N, 078°52’W; 3270–3670 m; 11 Oct. 1952; Galathea II stn. 726; clay; NHMD-305702, 305712.

Comparative material

EAST PACIFIC OCEAN • 3 syntype specimens of O. pacifica ; off Ecuador; 01°07’00’’N, 080°21’00’’W; 1573 fathoms [2879 m]; 23 Mar. 1891; Albatross stn. 3398; ZMUC OPH-000399 .

Type material

The largest syntype of O. pacifica has a disc diameter of 11.6 mm. The dorsal disc is densely covered by slender multipointed thorns, obscuring the scales. The radial shields are visible as narrow bar-like ribs underneath the disc covering, about 2/3 as long as the disc radius. The up to six long slender arm spines decrease in size ventralwards, the dorsalmost spine is as long as two arm segments. The dorsal arm plates are bell-shaped, about 1.5 times as wide as long, the distal edge is wave-like curved, consecutive plates are widely separated by the lateral arm plates. The ventral disc is covered by shorter multifid spines. The ventralmost tooth is large, almost as wide as long, with straight proximal edge. The three lateral oral papillae are pointed, tapering, the distalmost one scale-like widened at its base, twice as wide as the other two and somewhat shorter. The oral shield is lozenge-shaped, twice as wide as long, bordered by narrow adoral shields that reach wing-like around the oral shield, separating it from the arms. The ventral arm plates are pentagonal, as wide as long, with acute proximal angle, straight distal edge. The smallest syntype, with 7.9 mm disc diameter, agrees in most characters with the largest one, but the oral shield is proximally elongated to a point and there are four pointed lateral oral papillae on several jaw edges, in addition to the wide scale-like distalmost papilla. The third syntype, with 10.3 mm disc diameter, is similar to the largest one in all aspects.

Galathea specimens

Among the Galathea specimens of Ophiacantha , we found several that agreed morphologically with the syntypes of O. pacifica (table 2). The largest of these had a disc diameter of 16.5 mm. In larger specimens (up to 22.3 mm disc diameter), the dorsal disc and arms concur with O. pacifica , but the number of oral papillae varies, with few or many additional ones crowding along the edges and sometimes as a cluster on the jaws, which is more similar to O. cosmica (see above). They usually have large ventralmost teeth, but narrower and slightly pointed ones in some specimens, not correlated with the number of papillae.

Remarks

The syntypes of O. pacifica concur well with the illustrations in Lütken & Mortensen (1899). Particularly the large ventral tooth is a striking character, mentioned also in the description. The size of the largest syntype agrees with the largest size (12 mm) given in the original description, apart from the longest spines being hardly over two segments long, not as long as three segments as stated in the description. The type material of O. pacifica was collected from Ecuador ( Lütken & Mortensen 1899). Ophiacantha bathybia from the Bering Sea shares with O. pacifica the shape of the oral shield, the 3–4 oral papillae, of which the distalmost is wider, the shape of the disc spines, the number and length of the arm spines (up to three segments), and cannot be separated morphologically. It is currently unclear if O. bathybia may be conspecific with O. pacifica , because molecular data are not available for O. bathybia , yet.

Lütken & Mortensen (1899) did not consider the similarities between their new species and O. cosmica , despite figuring both. With increasing size, the Galathea specimens transit from the typical O. pacifica morphotype to the O. cosmica morphology with increasing numbers of oral papillae. The shape of the oral shield and the ventral tooth varies somewhat. We saw no geographic separation in the Galathea data, possibly due to the difficulties with morphological separation of the species. Molecular data suggest a Northern/Central Pacific clade and a Southern Pacific/Antarctic clade ( Christodoulou et al. 2019), based on four DNA samples of Ophiacantha cosmica from Australia, Antarctica and northern Pacific.

We propose to apply the name O. pacifica to the Northern/Central/East Pacific population and O. cosmica to the Southern Pacific/Antarctic/South Atlantic. More molecular data are needed to understand the boundaries between these species. To understand intraspecific and interspecific morphological differences, more material from a wide range of locations should be compared.

Figures

FIGURE 4. A, B, Ophiacantha pacifica, largest syntype (11.6 mm dd), ZMUC OPH-000399. A, dorsal aspect, B, ventral aspect. C, D, Ophiacantha pacifica (originally identified as O. cosmica by Lütken & Mortensen (1899)), Albatross Exp. stn. 3414, NHMD (unregistered), C, dorsal aspect, D, ventral aspect. E, F, Ophiacantha cosmica, Galathea II stn. 601, NHMD-867169. E, small specimen (12.4 mm dd) with few oral papillae, F, large specimen (22.3 mm dd) with many oral papillae. G, Ophiacantha cosmica, Galathea II stn. 599, NHMD-867198, ventral aspect, H, Ophiacantha sociabilis, Galathea II stn. 281, NHMD-867267 or NHMD-86721, ventral aspect. OPa, oral papillae sensu lato, OS, oral shield, RS, radial shield, T, tooth. Scale bars = 1 mm.