Silax Fell 1962

Genus Silax Fell, 1962

Triplodia Turner & Hallan, 2011: 7–8.

Triodia A.M. Clark, 1970: 67.

Amphioplus Verrill, 1899 [part]: 314.

Amphioplus Verrill, 1899 — Paterson 1985: 92.

Diagnosis

Series of oral papillae composed of two stout infradental papillae, tip of buccal scale forms small proximal oral papilla, wider than long middle lateral papilla (second adoral shield spine), distalmost papilla (first adoral shield spine) largest, scale-like, no ventral compartment ossicles. Up to eight arm spines. Tentacle scales absent or up to two, sometimes present only on proximal arm.

Remarks

The latest molecular evidence (Christodoulou et al. 2019) indicates that species of the genus Amphioplus Verrill, 1899 are found within several clades, suggesting polyphyly. The deepsea forms A. verrilli (Lyman, 1879), A. consors (Koehler, 1908) and A. daleus (Lyman, 1879) form a well separated clade, sister to the North Atlantic Amphiura filiformis (O.F. Müller, 1776) and Amphiura borealis (G.O. Sars, 1871). The closest clade to both of these includes Amphiura chiajei Forbes, 1843, the type species of Amphiura. Most other species of Amphiura are placed in other clades, suggesting polyphyly also of this genus. Combining molecular and morphological evidence, we consider the deepsea Amphioplus clade a separate genus from the other clades. Fell (1962) erected the name Silax for A. verrilli and A.M. Clark (1970) agreed with that, but Paterson (1985) considered Silax a junior synonym of Amphioplus. We propose to apply the name Silax to this deepsea clade with the species S. daleus, S. verrilli and S. consors. Based on their great morphological similarity with the former three species, we transfer also Amphioplus cernuus Lyman, 1879, A. magnificus (Koehler, 1907), A. patulus Lyman, 1879 and Triplodia abdita (A.M. Clark, 1970) (see below) to Silax, although we have no molecular data to support that decision.

Fell’s (1962) diagnosis of Silax is here revised to more accurately describe the conditions found in the type species S. verrilli and to accommodate other species. Fell (1962) described Silax as having four to five dissimilar or subequal oral papillae (identical to Ailsaria Fell, 1962, Amphioplus and Unioplus Fell, 1962), which is an error, as the holotype of S. verrilli has exactly four papillae (Lyman (1879) pl. XII fig. 329; A.M. Clark (1970) fig. 8a; Paterson (1985) fig. 36), consisting of the infradental, the tip of the buccal scale, and the two adoral shield spines. No additional papillae have ever been figured or mentioned for S. verrilli. Although S. verrilli lacks tentacle scales, this character is not considered of generic importance in other ophiuroid genera, e.g., Amphiura, and we amend the diagnosis of Silax to allow tentacle scales. Silax differs from Amphioplus in having larger infradental papillae and low, wide adoral shield spines. Amphioplus has smaller infradentals, papilliform adoral shield spines and one to several additional distal papillae that have been labelled as ventral compartment ossicles by Hendler (2018). Morphologically, several of the species included in Silax are either highly variable or may constitute species complexes. We have observed variable numbers of arm spines, and differences in the presence of tentacle scales in S. daleus , which may be evidence of a species complex. Several species of Silax reach sizes of more than 15 mm disc diameter.

The monotypic genus Triplodia is here regarded as a synonym of Silax due to great morphological similarity. It was originally named Triodia, replaced by Triplodia due to homonymy (Turner & Hallan 2011).