Thyone Oken 1815

Thyone ?n. sp.

Figure 10

Material examined: USNM E51306, North Pacific Ocean, Gulf of California, Mexico, Sonora; Choya; sta 67149 (31° 19’ N, 113° 41’ W), 65 ft, 21 Oct 1967, Col: Burch, B. & Burch T., Acc. no: 300062, det. Thyone sp.?, C. Gust Ahearn, 1 spec.

Description. Specimen small, perhaps juvenile, previously dissected, with introvert and tentacles expelled but still attached to body. Length 10 mm, breadth in mid-body about 5 mm, preserved colour whitish. Body somewhat barrel-shaped with mouth and anus at opposite ends with the mid-ventral ambulacrum appearing as a sole lacking tube feet. Tube feet elsewhere well-developed, mostly confined to the remaining four ambulacra with a few also scattered in the inter-ambulacra, short, suckers more or less of the same diameter as tube feet. Anal teeth present. Calcareous ring (Figure 10A) well calcified with deeply cleft radial plates and both radial and interradial plates anteriorly bifid with the latter slightly fused to radial plates. Tube of calcareous ring short, fragmentation of the ring and the processes not discernible perhaps because of the presumed juvenility of the specimen. Polian vesicle single, sac-like; stone canal short, straight, free; madreporite kidney-shaped. Other internal organs lost, perhaps due to evisceration or previous dissection.

Ossicles of body wall comprise tables (Figure 10B) with disc of usually four holes, spire short or elongated, ending in two teeth, some with a spire diverging into two denticulate arms (note by first examiner Gust Ahearn). Tube feet tables (Figure 10C) typically of the Thyone type with the disc straight or slightly curved and the spire, when present, reduced or elongated, terminating in two diverging teeth; disc with four central holes and one at each extremity. End plates present (Figure 10F), well-formed with few minute central holes and several concentric circles of larger holes around these. Introvert deposits (Figure 10D) comprise well-developed tables with multilocular disc and short bidentate spire, spire sometimes absent, then tables resembling multilocular plates. Tentacle deposits as minute plates with several holes and a jagged margin (Figure 10E).

Remarks. Because of the singularity and presumed juvenility of the specimen it was not possible to determine its exact identity. The calcareous ring is clearly like all East Pacific species of Thyone in being deeply cleft but the tentacle and introvert deposits indicate that the current specimen perhaps belong to a species not yet described. Since many holothuroids can lose or modify their deposits with age (see Thandar 1987, 1991, Massin 1992, Martins 2019), describing the single, perhaps juvenile individual as a new species is not here exercised. Although the body wall table discs may match those of Thyone bidentata Deichmann, 1941, known from California to Columbia, it differs in the spire of the tables and the absence of rosettes in the tentacles, unless by some stretch of imagination the minute plates were considered to be rosettes by Deichmann (1941) but regrettably they were not illustrated. The tube feet table discs are also less curved in the current material and their spire short and not bidentate. The specimen also comes quite close to T. benti Deichmann, 1937 from the Californian coast but differs in the absence of any bifurcations to the anterior tips of the plates of the calcareous ring, the presence of a taller spire to the body wall tables and the absence of rods in the tentacles. One anomaly in the current specimen is the occurrence of the solelike ventral ambulacrum. Whether this is a consistent feature or an abnormality will have to be determined from new material. It does not seem likely that it is due to preservation.