Ypsilothuria bitentaculata bitentaculata ( Ludwig, 1893 )

Martins, Luciana & Tavares, Marcos, 2018, Ypsilothuria bitentaculata bitentaculata (Echinodermata: Holothuroidea) from the southwestern Atlantic, with comments on its morphology, Zoologia (e 24573) 35, pp. 1-6 : 2-5

Ypsilothuria bitentaculata bitentaculata ( Ludwig, 1893)

Figs 1–19

Sphaerothuria bitentaculata Ludwig, 1893:141 , pl. XII, figs 16–17, pl. XIV, figs 5–14.

Sphaerothuria talismani – Deichmann 1930: 154, pl. 9, fig. 3 [not Ypsilothuria talismani Perrier, 1886 ]

Ypsilothuria bitentaculata – Panning 1949: 455, Pawson 1965: 6–7, text fig. 1, figs 1–5; Thandar 1999: 373–375, fig. 3 d–f; Massin and Hendrix 2011: 422, fig. 7; Mecho et al. 2014: 294, 295, fig. 13.

Diagnosis. Body strongly U-shapped. Tentacles crown of eight digitiform tentacles, the two lateral ones longest. Calcareous ring simple, radial and interradial plates deeply notched, without posterior projections. Plates of body wall forming a test, arched rods in tentacles. Knobbed plates in introvert.

Description. Body strongly U-shaped ( Fig. 1), length 5–10mm. Tentacles crown of eight digitiform tentacles, the two lateral ones longest ( Fig. 2). Oral and anal siphons short, opposite to each other, oral siphon wider than anal ( Fig. 3). Body wall thorny due to presence of intricate tables, visible with naked eye ( Fig. 4). Calcareous ring simple. Radial and interradial plates with curved base, without projections on the posterior surface, bifurcated anteriorly, the radial slightly higher than the interradial ( Figs 5, 18).

Body wall tables with circular disc perforated, irregular in outline. Each disc consisting of a laminar stereom with holes ranging between 20 and 50 µm all over the disc plate (plate size between 500 and 1400 µm; Figs 6–7). High spire strong (400–500 µm; Fig. 8), placed near edge of disc. Tentacles with arched rods (80–150 µm, Fig. 9), with one central perforation, one central apophysis and perforated edge (2–4 holes). Introvert with knobbed perforated plates, elongated, irregular in outline (160–180 µm, Figs 10, 11). Plates of base of oral siphon with undeveloped spires (400–800 µm, Fig. 15). Oral and anal siphons with similar rods, anal rods smaller, strongly arched (40–70 µm, Figs 12–14). Anal siphon with irregularly perforated plates (300–600 µm, Fig. 16).

Material examined. MZUSP 1306 , Brazil, REVIZEE, off coast of São Paulo, (25°43’S, 45°09’W), 13/Jan./ 1998, 511m, 6 spms. Ibidem , (24°20’S, 43°46’W), 9/Jan./ 1998, 505m, 2 spms .

Comparative material examined. MNHN-IE-2005 6332, Slides of the type [body wall tables] Pacific, (6°35’S, 81°44 W), 24/Feb./ 1891, 782m. MNHN 3693, Pacific, (34°6’S, 119°8’W), 1301m: 20 mm long [all originally labeled S. bitentaculata Ludwig, 1893 ]. USNM 15689, United States, northwestern Atlantic, Virginia, RV Albatross, stn 2723, (36°46’N, 73°09’W), 23/ Oct./1886, 3082m: 10 spms 3–10 mm long [originally labeled Y. bitentaculata ( Ludwig, 1893) ].

Distribution. Ypsilothuria b. bitentaculata has been regarded as a Pacific species (Oshima 1915, Pawson 1965, Gage et al. 1985). This is the first record of Y. b. bitentaculata from the southwestern Atlantic. According to Thandar (1999) Y. b. bitentaculata (as Y. bitentaculata ) occurs in green mud sediments and between 225 and 4440 m ( Cherbonnier and Feral 1978). Off the coast of São Paulo, Y. b. bitentaculata was found in silty sand bottoms around 500 m deep, within the previously known bathymetric distribution of the species. In southwestern Brazil Y. bitentaculata bitentaculata is so far lower bathyal in distribution possibly owing to the influence of the colder waters from the South Atlantic Central Water (SACW) over the southwestern Brazilian shelf. It is not uncommon for deep-water species to occur in depths shallower than usual owing to colder upwelling waters ( Kowalewski et al. 2002).

Remarks. Heding (1942: 26) split Ypsilothuria talismani E. Perrier, 1886 , Y. bitentaculata ( Ludwig, 1893) and Y. attenuata E. Perrier, 1886 into two subspecies complexes, in which he included two additional subspecies: Y. talismani talismani E. Perrier, 1886 , Y. talismani elegans Heding, 1942 , Y. bitentaculata bitentaculata ( Ludwig, 1893) , Y. bitentaculata attenuata E. Perri- er,1886, and Y. bitentaculata virginiensis Heding, 1942 .

Some authors have suggested that the morphological differences between the subspecies of Ypsilothuria are due to ontogeny only ( Gage et al. 1985, Billet 1988). The shape of the calcareous ring, for example, changes with growth and the notches in the calcareous ring may not always be present, especially in juveniles. However, in the four specimens dissected for this study (maximal length: 6.0 mm; 7.9 mm; 8.2 mm and 10.0 mm), the shape of the calcareous ring was constant and the notches in the calcareous ring were always present.

Recent observations are not consistent with Billet’s (1998) claim that the average size of the dermal plates varies with the size of the specimens. Specimens from the Indian Ocean with about 20 mm had two-spired dermal plates ranging between 1500 and 3000 µm ( Samyn and Van den Spiegel 2016), whereas in the southwestern Atlantic a half-sized specimen (10 mm) had single-spired dermal plates ranging from 500 to 1400 µm. Contrary, specimens from South Africa, although more than twice as large as the southwestern Atlantic specimens (25 mm), had single-spired, unexpectedly small dermal plates from 700 to 1270 µm ( Thandar 1999).

Several authors (e.g. Ludwig 1893, Deichmann 1930, Heding 1942, Pawson 1965, Massin 1996) have referred to the shape of the plates at the base of the tentacles to differentiate between Y. t. talismani and Y. b. bitentaculata . However, as not- ed by Thandar (1999) and in agreement with our results, the knobbed plates are in fact present in the introvert (not in the tentacles). The plates of the introvert cannot be found neither in the type specimen of Y. bitentaculata (MNHN 6332) – since all that remains of it is a slide with the body wall plates –, nor in the in the specimen MNHN 3693 which is incomplete. In the specimen MNHN 3693 (20 mm) the size of the anal plates ranged from 400 to 600 µm, whereas in the Brazilian specimens (5–10mm) the anal plates ranged from 300–600 µm. The northwestern Atlantic (USNM 15689; 3–10 mm) and the southwestern Atlantic specimens from Brazil agree well with each other.

Ypsilothuria t. talismani actually differs from Y. b. bitentaculata in the morphology of the lateral interradial plates of the calcareous ring, whose anterior ends are entire in Y. t. talismani and deeply notched in Y. b. bitentaculata . Additionally, the tentacle rods are straight and have few perforations at the ends in Y. t. talismani (viz. Heding 1942), whereas the rods are arched and have one central perforation and one central apophysis in Y. b. bitentaculata . Of noteworthy is that Billett’s (1988: fig. 16B) illustration shows curved tentacle rods in Y. talismani similar to Y. bitentaculata bitentaculata ( Fig. 9).

Ypsilothuria b. bitentaculata can be separated from Y.b. attenuata sensu Heding (1942) from the Northern Atlantic in having: (i) the calcareous ring with narrow radial and interradial plates (versus broad radial and interradial plates in Y. b. attenuata ), (ii) the anterior end of the radial and interradial plates deeply notched in the radial and interradial plates ( Figs 5, 17, 18) (versus notched only in the interradial plates in Y. b. attenuata ) and (iii) arched tentacle rods (versus irregular tentacle rods in Y. b. attenuata ).

However, the differences between Y. b. bitentaculata and Y. b. attenuata sensu Billett (1988) are subtler. In Y. b. attenuata from South Africa both the radial and interradial plates of the calcareous ring are narrow and notched just as in the southwestern Atlantic specimens of Y. b. bitentaculata ( Figs 5, 18). In the southwestern Atlantic, however, the size of plate holes range between 20 to 50 µm, compared to 20 to 30 µm in South Africa. Additionally, in Y. b. bitentaculata the tentacles rods are provided with one central perforation and one central apophysis ( Fig. 9), whilst in the South African Y. b. attenuata the tentacle rods are perforated near the end only and lack the central apophysis. In that respect, Billet’s and Heding’s Y. b. attenuata are similar to each other ( Heding 1942: fig. 30: 1, 2; Billet 1988: fig. 20B).

Ypsilothuria b. bitentaculata and Y. b. virginiensis share similar shape of the calcareous ring ( Figs 18, 19), but can be distinguished by slight differences in the morphology of the tentacle rods, which are arched and provided with one central perforation and one central apophysis in Y. b. bitentaculata versus arched and perforated only at the ends in Y. b. virginiensis.

The specimens from the southwestern Atlantic (São Paulo, Brazil) are herein assigned to Y. b. bitentaculata on the account of the morphology of their tentacle rods and the size of the holes on their body wall plates.