Two-tentacled sea cucumber

Ypsilothuria bitentaculata is a small sea cucumber of the order Dendrochirotida and the family Ypsilothuriidae ^[1]. Its body is U-shaped and measured between 5 and 10 mm in the specimens studied in Brazil ^[2]. It lives on the deep seabed: it has been recorded between 225 and 4440 m depth in several oceans ^[3]. In Peru one specimen is known, collected off San Juan de Marcona, in Ica, at 1104–1171 m ^[4].

Ludwig described the species in 1893 as Sphaerothuria bitentaculata; the genus Ypsilothuria had been created by E. Perrier in 1886, and the family Ypsilothuriidae dates from Heding, 1942 ^[2]. Deichmann (1930) used the name Sphaerothuria talismani for specimens of this species, a name that actually belongs to another species ^[2]. The English name, two-tentacled sea cucumber ^[1], suggests two tentacles, but the authors who have described it count 8 digitiform tentacles, two of them longer than the rest ^[2],[3]. The sources consulted do not explain the meaning of the epithet ^[2].

The body is strongly U-shaped, with a short oral siphon and a short anal siphon opposite each other; the oral one is wider than the anal one ^[2]. The crown has 8 digitiform tentacles of very unequal size, and the two lateral ones are the longest ^[2],[3]. The body wall is thorny because of the calcareous plates, which can be seen with the naked eye ^[2],[3].

The plates of the wall are tables with a perforated disc of irregular outline, 500 to 1400 μm across, with a strong spire of 400 to 500 μm near the edge ^[2]. The tentacles bear arched rods of 80 to 150 μm, with one central perforation and one central apophysis ^[2]. The calcareous ring, which is simple, has radial and interradial plates deeply notched at the front and without projections at the back ^[2].

Similar species

• Ypsilothuria talismani: It is separated by the calcareous ring, whose lateral interradial plates have an entire, unnotched front end, and by the tentacle rods, which are straight with few perforations at the ends ^[2].

The species was long regarded as typical of the Pacific, but it is now also known from South Africa, the southwestern Atlantic and the Mediterranean ^[2],[3]. One author treats it as cosmopolitan, with a depth range of 225 to 4440 m ^[3]. In the western Mediterranean, 27 specimens came from between 900 and 1350 m, only on the open slope and not in submarine canyons, during 223 trawls made between 800 and 2845 m ^[3].

Off São Paulo, Brazil, 8 specimens turned up at 505–511 m on silty sand bottoms, the first record for the southwestern Atlantic ^[2]. The slide of the type specimen, dated 1891, carries the position 6°35'S 81°44'W at 782 m depth ^[2]. In Peru, one specimen comes from San Juan de Marcona, Ica, at 1104–1171 m ^[4].

The Brazilian specimens were collected with a grab and a rectangular dredge, in samples of the fauna living in the sediment, sieved through a 1 mm mesh ^[2]. Their authors note that in southeastern Brazil the species is so far found only in the lower bathyal, possibly owing to the influence of colder South Atlantic Central Water, and that this depth lies within the species' previously known range ^[2].

In the Mediterranean, Mecho and colleagues found it only on the open slope and note that its presence has been cited there since the early 1990s ^[3]. The same authors state that little is known about this group in that sea and that its taxonomy must wait for further sampling ^[3].

The only reference for Peruvian waters we have consulted is a single specimen, collected at 1104–1171 m depth off San Juan de Marcona (Ica) and published in 2025 ^[4]. It is a deep-bottom animal for which the sources consulted describe material collected in scientific sampling ^[4],[2].