Stolus cognatus

Stolus cognatus (Lampert, 1885)

Figures 4 & 5

Semperia cognata Lampert. 1885: 67.

Thyone cognita Deichmann, 1930: 169, pl. 15, figs. 1–4 (lapsus calami).

Thyone cognata H.L. Clark, 1933: 115; Deichmann, 1938:134.

Stolus cognatus Panning, 1949: 462 (passim); Pawson & Miller 1981: 393. Prata et al, 2017: 7, fig. 9 a–f; Prata et al. 2020: 417, fig. 8.

Thyoneria cognata Deichmann 1954: 398; Deichmann 1963: 110; Tikasingh 1963: 97, figs.70–72, Caycedo 1978: 165, pl. 4, fig. 4. Diagnosis (according to Deichmann 1954, amended herein). A species of Stolus up to 140 mm long, fairly delicate, spindle or U-shaped with the oral and anal ends often turned up. Tentacles 10, of equal or unequal size, mid-ventral pair sometimes reduced or even absent. Tube feet in double rows, a few also scattered in interambulacra. Calcareous ring tubular (Figure 4E), with or without any obvious fragmentation; radial plates shorter than interradials, both fused forming a distinct tube with the radial plates carrying short posterior processes broken into a few pieces. Body wall ossicles comprise numerous elongated, smooth plates with mostly a double series of holes (Figures 4A, 5A) plus a varying number of small plates with several, often reduced holes (Figure 5B). Tube feet with perforated rods with or without the third arm (Figure 4B); end plates mostly absent, when present small, almost rudimentary, except in young individuals. Introvert with rosettes (Figure 4D & E). Tentacles with heavy rods in stalk and delicate ones in branches.

Material examined: USNM- E22532, North Atlantic Ocean, Caribbean Sea, Venezuela, Isla Margarita, 14 I 78, Jones M.L., det. Miller, J., 1 spec.; USNM- E22528, North Atlantic Ocean, Caribbean Sea, Venezuela, Isla Margarita, Jones M.L., date not recorded, det. Miller, J., 2 spec.

Habitat. Soft bottom to eel grass.

Distribution. Caribbean region, shallow-water.

Remarks. This species, well characterized by smooth, multilocular plates in the body wall, is unique amongst the Caribbean holothuroids. Of the three specimens here examined, all from Venezuela and determined by J. Miller, the two smaller ones, on the basis of their ossicles, are undoubtedly referable to Solus cognatus. They look identical, fusiform to Ushaped, uniformly beige without any blotches or other markings and with the skin rough to the touch. The remaining specimen is barrel-shaped with dark blotches on a rather smooth skin. Its calcareous ring is simple, cucumariid-like and its ossicles are slender rods, quite different from the robust ones of the former two specimens. It is obvious that this third specimen is a cucumariid but was not determined any further. While other writers record equal tentacles in S. cognatus, Deichmann (1954) records both equal and unequal tentacles, the latter as four large, four medium and two small (mid-ventral), or only four large and four small with the mid-ventral absent.

Deichmann (1954) was the first to recognise that this species stands apart from other species and hence erected the genus Thyoneria to accommodate it but without diagnosis or comments, nor designation of the type species. Since then, the combination Thyoneria cognata was used a few times but also without any comments [(see Caycedo (1978), Deichmann (1963) and Tikasingh (1963)] – the latter perhaps on Deichmann’s advice). Pawson & Miller (1981) rejected the new genus and accepted Panning’s (1949) combination Stolus cognatus. All subsequent authors from Hendler et al. (1995) to Prata et al. (2020), amongst others, continued to use the combination Stolus cognatus. Although Thyoneria Deichmann, 1954, is rejected by WoRMS (732056, accessed 16 June 2021) the writer was tempted to revive the name because of the presence of both equal or subequal tentacles, with the ventral two rarely reduced or altogether absent, and the occurrence of only smooth plates in the body wall. However, this action was not taken because of the writer’s dislike for monotypic genera [(see Thandar & Samyn (2004)] and the fact that holothuroid ossicles are reputed to be very homoplastic. It is here opined that this species is perhaps still experimenting with the reduction of its two ventral-most tentacles, consistently present in its congenerics (i.e. 8+2 arrangement), with the subdivision of its calcareous ring and perhaps also in the modification of its body wall ossicles since the differences in size and shape of these deposits and their perforations vary considerably within the species. Therefore, while the genus Stolus, characterized by its type species (S. buccalis) should be restricted to include only forms with the two mid-ventral tentacles consistently reduced, a complex subdivided calcareous ring and body wall ossicles as usually knobbed, single-layered buttons/plates with/without a handle on one side, S. cognatus is still retained within it.