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The Holothurian Fauna of Cook Strait, New Zealand

[Introduction]

Diagnosis: Tentacles with lateral digits, or claw-shaped. Tentacle ampullae long (reduced in one deep-water species). Spicules derived from triradiate tables with solid three-pillared spire; tail with tables with round to oblong disc, or long fusiform rods. In one species large fusiform plates or rods develop in the skin of the body wall with advancing age. Dark red egg-shaped phosphatic bodies often present. In some species anchors and racquet-shaped plates present in young individuals. Mostly large forms, 6–15cm long (Deichmann, 1960).

The members of this family are unique in that the calcareous deposits of the juvenile may be transformed into phosphatic material with the passage of time. The phosphatic material is deposited as small orange or red concentrically laminated granules. As a consequence of this phenomenon, juveniles, half-grown specimens, and adults of the same species have often been placed in different species, because of the differences in their spiculation. Clark (1907) referred all of the then known species of the family Molpadiidae to the genus Molpadia Risso. Heding (1931) attempted a revision of the family, listing the known species under five genera, one of which (Pseudomolpadia) was a new genus. Later, Heding (1935) erected another new genus (Eumolpadia). Deichmann (1936), pointed out a number of inconsistencies in Heding's reasoning, and suggested a return to Clark's (1907) idea that all of the species be placed under the single generic name Molpadia in the meantime, until the life histories of at least a few typical cases be worked out.

Heding (1931, 1935) used differences in structure and sculpture of the calcareous ring, in certain features of internal anatomy, and (to a lesser extent) in calcareous deposits, as criteria for separation at the generic level. It appears that these criteria may be rather unreliable, and subject to more or less drastic changes with growth. The calcareous deposits of the tail are relatively unaffected by deposition of phosphatic material and these deposits are thus reliable criteria for identification of juvenile and adult specimens alike. The deposits in the body wall should be used only when the life history of the species is fairly well known, and a series of specimens have been examined, or when the body wall deposits page 11are so distinctive as to belong to a certain species. The calcareous ring should supply characters of secondary importance only.

Two genera of Molpadiidae are represented in the Cook Strait collections.

Key to the Cook Strait Genera in Family Molpadiidae
1 (2) Deposits include anchors and three-armed anchor-plates, and spired tables with three perforations Heteromolpadia n.g.
2 (1) Deposits in the form of large fusiform rods and scattered irregular tables Molpadia Risso