Parathyone braziliensis Prata & Manso & Christoffersen 2020, comb. nov.

Parathyone braziliensis (Verrill, 1868) comb. nov.

(Figures 4–5)

Thyone (Sclerodactyla) braziliensis Verrill, 1868: 370–371, pl. IV, fig. 8; Rathbun 1879: 141; Ancona-Lopez 1957: 14–17, figs 34–44.

Ocnus braziliensis: Tiago & Ditadi 2001: 379–384; Magalhães et al. 2005: 63; Prata Oliveira et al. 2010: 12, fig. 5f; Gondim et al. 2011: 7; Miranda et al. 2012: 141, fig. 3b; Miranda et al. 2015: 116–117, figs 5g–h.

Material examined. Rio Grande do Norte State, Brazil: 3 spec., Tubarão River, Diogo Lopes, Macau (UFPB. ECH-1619). Paraíba State, Brazil: 1 spec., Cabo Branco Beach, João Pessoa (UFPB.ECH-1063).

Type locality. Abrolhos, Bahia, Brazil (Verrill 1868).

Description. Specimen UFPB.ECH-1063. Barrel-shaped body, 30 mm long, and 20 mm wide in mid-body (Figs. 4 A–B). Soft skin, thin. Color grayish brown, with brown to blue spots scattered, darker in the interradii. Mouth and anus slightly upturned, five anal teeth present. Tentacles 10, bushy, four most ventral reduced. Tube feet short, distributed in two rows in the radii, and more rows in the interradii, covering the body. Calcareous ring simple with short posterior processes, robust. Dorsal radial plates slightly narrowed anteriorly, at the point of attachment of the retractor muscles (Fig. 4C); dorsal interradial plates triangular, notched posteriorly (Fig. 4D). Ventral radial plate bigger than dorsal, with posterior processes more evident (Fig. 4E); ventral interradial plates with deep notch posteriorly (Fig. 4F). Two rounded madreporites, stone canals short, one longer than the other. Two Polian vesicles balloon-shaped, one smaller. Retractor muscles thin, about half body length. Longitudinal muscles thin. Respiratory trees with short branches. Gonads in a single tuft, with tubules unbranched. Body wall ossicles consist of baskets, with four to eight holes, some irregular, usually shallow, about 12 spines on the crown (50 µm long) (Fig. 5G); irregular plates with four or more holes, two central ones bigger (90 µm long); knobbed buttons with four holes, two central ones bigger (90 µm long); knobbed buttons thin, irregular, with four or more holes (80 µm long); large buttons with six holes and large knobs (100 µm long) (Fig. 5H). Tube feet include supporting rods with two central holes elongated, bigger than others (250 µm long) (Fig. 5I), round to elongated supporting perforated plates (100 µm long) (Figs. 5 K–L), and endplate (150 µm long) (Fig. 5J). Introvert with knobbed plates, elongated, with eight or more holes (50 µm long); smooth plates, with four or more holes (Fig. 5E); rosettes of moderate to large size (50 µm long) (Fig. 5F); knobbed baskets of four holes, some elongated (50 µm long) (Fig. 5D). Tentacles have irregular plates (60 µm long); small to large rosettes (40 µm long) (Fig. 5B); irregular rods (100 µm long) (Fig. 5C), and smooth to very knobbed baskets (40 µm long) (Fig. 5A).

Morphological variations. A total of four specimens were analyzed, the size varied from 16–33 mm long, 5–18 mm in breadth at mid-body, 3–4.5 mm in breadth at anterior end, and 2–6 mm in breadth at posterior end. The number of ossicles decrease throughout life, mainly at the dorsal body wall, and the tegument becomes thicker. The tube feet are smaller and more abundant on adult specimens.

Geographical distribution. Brazil, from Cear to São Paulo (Tommasi 1969). This species was found only in the intertidal zone.

Remarks. The diagnosis of the genus Ocnus was given by Panning (1971), being modified by Thandar (2008), and mentions that Ocnus species have body cylindrical or slightly angular or spindle-shaped; Tentacles 10, bushy, two most ventral smaller; tube feet in single to double rows on radii, mostly rigid and non-rectractile, interambulacra naked; calcareous ring simple, narrow and high, with posterior notches and long tapers anteriorly; body wall ossicles represented by baskets with small, knobbed perforated plates/buttons, often with four primary holes, in some species also large multilocular and often multi-layered plates; tube feet with perforated plates and endplates.

Parathyone braziliensis has barrel-shaped body, small retractile tube feet in the radii and interradii, covering the body; calcareous ring simple, without long tapers anteriorly; body wall with plates, buttons and baskets, without multi-layered plates. The buttons and plates are usually plans or curved, some very knobbed, but does not form a second layer. Deichmann (1957) included in Parathyone the species with tube feet spreading out into the interradii, simple calcareous ring, body wall with baskets and regular, strongly knobbed buttons; tube feet with endplate and large supporting rods. All these characters are present in Parathyone braziliensis, thus this species is better classified in the genus Parathyone than in Ocnus. In general, Ocnus is a heterogeneous group (Pawson 1983; O’Loughlin & O’Hara 1992) that needs review, and some studies have been proposing new genera to better delimit its species, as O’Loughlin & O’Hara (1992); O’Loughlin & Alcock (2000); Thandar (2008); Mjobo & Thandar (2016), among others.

Deichmann (1930) considered Parathyone braziliensis a synonym of Parathyone suspecta. However, Ancona Lopez (1957) showed that the two species differ by having tentacles of equal size in P. suspecta, by having the posterior processes in radial pieces in calcareous ring and by having numerous ossicles in P. braziliensis. Although, both species could have similar tentacles with four most ventral smaller than others, the calcareous ring and ossicles are variable. P. braziliensis differs from P. suspecta by the form of calcareous ring (Figs. 2 C–F; 4C–F), the set of ossicles (Figs. 3 A–L; 5 A–L), the softer skin and grayish-brown color with some blue spots.

Ecological notes. Parathyone braziliensis lives in substrates of gravel and algae, usually in crevices or buried, with tentacles exposed. Young individuals were found associated with algae. This species used to be found in tide pools with some frequency, but nowadays they are more difficult to find at least in the northern part from the northeast coast.