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The Holothurian Fauna of Cook Strait, New Zealand

Molpadia Risso, 1826

Molpadia Risso, 1826

Diagnosis: Molpadids whose calcareous deposits include tables, anchors, and rosettes of racquet-shaped plates and large fusiform rods in various combinations. Tail deposits fusiform.

Type Species: Molpadia musculus Risso.

The single Cook Strait representative of this perplexing genus is Molpadia violacea (Studer), which is probably related to the type species of the genus, but the relationship is not absolutely clear, as M. violacea seems to lack the characteristic rosettes of racquet-shaped plates and anchors which are found in M. musculus.

Molpadia violacea (Studer) Plate III, figs. 4–8
  • Trochostoma violaceum Studer, 1876; Theel, 1886, p. 42, Pl. II, fig. 4; Pl. XI. fig. 1.
  • Molpadia musculus H. L. Clark, 1907, p. 165, Pl. XI.
  • Haplodactyla violacea Heding, 1931, p. 280.
  • Molpadia violacea Deichmann, 1960.

Material Examined: VUZ 87, South of Cape Palliser, 400 fathoms, mud, 1 specimen; VUZ 101, off Palliser Bay, 550 fathoms, mud, 6 specimens.

Diagnosis: Deposits in the form of large fusiform rods with two to three arms, up to 1.1mm in length. No anchors or anchor-plates. No rosettes of racquet-shaped plates. Tail deposits two-armed fusiform rods up to 0.8mm in length. One anterior process on each radial piece of the calcareous ring perforated for the passage of the radial nerve.

Description: The smallest specimen has a total length of 47mm; the largest is 78mm in length. The specimens are approximately cylindrical in shape, elongate, with the posterior end attenuated to form a distinct caudal appendage which occupies up to 20% of the total body length.

Colour in alcohol ranges from a light-brownish red to a dark brick-red. The anterior extremity of the body and the tail are greyish-white. The skin is quite thin and coarse to touch.

The calcareous ring is composed of 10 pieces, five radials and five interradials, joined to form a solid ring. Anteriorly, the radials each have two short and blunt processes, one of which carries a small perforation for the passage of the radial nerve. The interradials each have one anterior process and no perforation. The radials have a forked posterior process, while the interradials have none. The ring is sculptured on its outer surface, and the sculpture varies within the species.

The internal anatomy is similar to that in Heteromolpadia marenzelleri,

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Two types of calcareous deposits are present in the skin:

1. Fusiform Rods: These are found everywhere in the body wall, especially in the tail, where they occur in great numbers, closely aggregated together, lying transverse to the longitudinal axis of the body. The rods vary in length up to a maximum of 1.1mm, and they have an expanded central portion which carries a small number of perforations.

(a) Rods from the tail (Plate III, fig. 6): The tail rods are in general smaller (average length 0.7mm) than those from other areas, and have fewer perforations. The ends of the rods tend to project above the level of the skin, and they can be seen with the naked eye.

(b) Rods from the posterior third of the body, near the tail (Plate III, fig. 8): These are massive deposits, with an average length of 1.0mm. Many of the rods have three arms, while others have two, and there are four to eight central perforations. The rods are grouped into small clusters.

(c) Rods from the middle of the body (Plate III, fig. 7): Mid-body deposits closely resemble those from the posterior third of the body in general features, but even more variability in shape is displayed.

(d) Rods from the extreme anterior end of the body (Plate III, fig. 4): These are similar to those from the tail, and are of a comparable size.

2. Perforated tables (Plate III, fig. 7a): Tables (length 0.3–0.6mm) with short central spires and 3–6 perforations are scattered sparsely in the skin. The spire is composed of a single column, and in many cases it is absent. Developmental stages are occasionally seen (Plate III, fig. 5).

Red phosphatic deposits are present, grouped together in small clumps. They are similar to those in Heteromolpadia marenzelleri (Theel).

Distribution: Theel (1886) described specimens taken from the vicinity of Kerguelen Island at depths between 20 and 120 fathoms, and from about 50 miles east of East Cape, New Zealand in 700 fathoms. The two new localities recorded here, Palliser Bay, 550 fathoms, and south of Cape Palliser, 400 fathoms, indicate that this species may be relatively common in deeper waters about New Zealand, and probably elsewhere, achieving its distribution by spreading across the seafloor in deep water.

Discussion: The specimens described here are similar in most respects to those described and figured by Theel (1886). There appears to be a complete lack of anchors and rosettes, even in smaller specimens. Thus the species is sharply distinguished from M. musculus (Risso). Deichmann (1960) believes that M. violacea is an extreme form of M. musculus with narrow rods and lacking the anchor and racquet stage completely.