Trachythyone flaccida Thandar, 2013, n. sp.

Trachythyone flaccida n. sp.

(Figures 1 & 2)

Diagnosis. A plump, soft-bodied, barrel-shaped species of Trachythyone, up to 50 mm in length when preserved. Colour in alcohol light greyish brown. Mouth and anus terminal. Anal teeth absent. Tube feet non-retractile, scattered, slightly more numerous ventrally. Body wall ossicles comprising a superficial layer of irregular, cup-like baskets, often accompanied by cross-shaped deposits, and a deeper layer of smooth, elongated to irregular, multilocular plates, aggregated at base of tube feet. Tube feet and tentacles with rods; introvert without ossicles; end-plates absent.

Etymology. The specific name refers to the soft, limp body.

Material examined. Holotype SAM A28116, Dr. Fridtjof Nansen Demersal Survey, Stn. 3924, off southern Angola, 16º 11´S, 11º 46´E, 6 III 2006, 36 m.; Paratypes —SAM A28117, same data as holotype, 4 spec.

Description of holotype. Holotype slightly damaged, deflated, tentacles retracted. Body soft, flabby; form barrel-shaped (see paratype—Figure 1A), bent upwards at both ends but ends never attenuated. Length about 50 mm, breadth in mid-body 23 mm. Preserved colouration a dull greyish brown. Mouth and anus terminal. Tentacles bushy, 10, ventral two much reduced, arising close together. Tube feet small, rigid, non-retractile, scattered throughout body with no regular arrangement but slightly more numerous ventrally and in the radii than elsewhere, suckers much reduced, of equal diameter to tube feet. Anal teeth absent.

Calcareous ring (Figure 1 E) simple, no posterior bifurcations or processes on radial plates; each radial plate with long anterior projection and a notched posterior margin; interradial plate triangular, also anteriorly prolonged. Polian vesicle single, saccular; stone canal short, free, madreporite bean-shaped. Gonad (testis) mature, tubules unbranched. Respiratory trees well branched, left shorter than right, both arising from cloaca independently. Retractor muscles arise at mid-body. Body wall thin, soft except in region of tube feet where it is strengthened by accumulation of ossicles.

Body wall ossicles a superficial layer of cups/baskets and a deeper layer of plates, accumulated without order, mostly at the bases of tube feet, apparently causing the latter to stand erect. Baskets irregular, cup-like (not as common as in some paratypes), with some crosses, present in addition to baskets, usually at posterior end. Baskets (Figure 1 C), 30–60 µm (mean 42µm), base smooth, cross-shaped, with usually four holes but sometimes up to eight holes but then base multi-armed; rim of baskets with few, short spine-like projections but not spiny. Crosses (Figure 1 D) few, and represent developing baskets resembling the incomplete baskets (crosses) of some Pseudocnella species. Plates (Figure 1 B, Figure 2 A&B) smooth, 200–500 µm (mean 300 µm) long, usually elongated but sometimes irregular, perforated by numerous small holes, some plates appear slightly twisted, and the elongated ones often with one end slightly serrated, dentate or irregular but never knobbed or spinous. Tube feet with perforated rods (Figure 2 C), 110–240 µm long (mean 182 µm); end-plates absent. Tentacle rods (Figure 2 D), 180–310 µm long (mean 230 m), of same form as tube feet deposits. Introvert without deposits.

Remarks. In its body form, soft texture, scattered non-retractile tube feet, irregular cup-like baskets, crosses often present in addition to baskets, and absence of introvert deposits and end plates, this species is rather distinctive. Among the Atlantic species of Trachythyone the new species appears close to T. bouvetensis (Ludwig & Heding, 1935) from the Antarctic-Subantarctic and T. crassipeda Cherbonnier 1965 from Brazil. It also shares several features with T. corbicula Cherbonnier, 1965 and T. fallax Cherbonnier, 1958 from the East Atlantic. It comes closest to T. bouvetensis but differs in the scattered distribution of tube feet, absence of long, spine-like projections to the rim and base of the cups, plates without a spinous end and the absence of four large central holes in the plates. It resembles the Brazilian T. crassipeda in its barrel-shaped body, scattered distribution of tube feet and shallow cup-like baskets but differs in the absence of anal papillae, end-plates in the tube feet and rosettes in the tentacles. Perhaps all three species (T flaccida, T bouvetensis and T. crassipeda) represent relics of the Gondwanaland rift. Recently Primo & Vàsquez (2004) stated that the results from their study of the biogeography of the southern African ascidian fauna were in line with those of other southern African marine invertebrates and that the fauna of Namibia (as far as southern Angola, the limit in their study), also showed a clear relationship to that of the Antarctic region. This may be due to similar temperature regimes or a Gondwana conection. Hence a distant south-west African and east South American link is therefore also possible. T. corbicula and T. fallax, on the other hand, have an elongated, curved body with tube feet restricted to the ambulacra, although crosses are present in the former species.

Species of Trachythyone differ from each other in the combination of several characters such as the shape of the body, distribution of podia, completeness or otherwise of baskets, presence or absence of X-shaped deposits, the form of the plates and tentacle and introvert deposits. Except for the poorly known T. glaberrima, the key given below separates the remaining 19 species of the genus.