Pleurobranchus caledonicus
Risbec, 1928

Order: NOTASPIDEA
Family: Pleurobranchidae

DISTRIBUTION

Described from New Caledonia. Records on Forum from Okinawa, Marshall Ids, Hawaii and sthn Queensland, Australia.

PHOTO

Locality: Old Woman Island, Mooloolaba, Sunshine Coast, 12 m, Queensland, Australia, Pacific Ocean, 08 May 2004, Subtidal. Length: 40 mm. Photographer: Gary Cobb.

Pleurobranchus caledonicus was described by Risbec (1928) from three 50 mm long animals he collected from under a block of dead coral at Noumea, New Caledonia. To my knowledge this species has not been rediscovered until this report from Gary Cobb from southern Queensland. Risbec describes the mantle as deep yellowish with maroon brown patching. The most distinctive feature was a small hole in the centre of the mantle surrounded by a 'clair' ring. I presume the 'clair' can be translated as meaning 'colourless'. As Risbec had three animals, all presumably with this hole, it is unlikely that the hole was a developmental fault or the result of damage to the mantle.

Other points of interest in Risbec's description are that the internal shell is approx 18 mm in length and covers the digestive gland and reproductive organs; the radular teeth lack denticles; and that the jaw elements are cruciform, and lack denticles except for a terminal pair, similar to those seen in Berthella martensi. Gary Cobb's animal fits Risbec's description rather well.

One question we can't answer at present is whether this is really a species of Pleurobranchus? There are not that many characters which conclusively differentiate Pleurobranchus from Berthella, but one external feature is the way Pleurobranchus has a deep cleft in the anterior mantle to acommodate the rhinophores. This is absent in Berthella and certainly not present in P. caledonicus. Internally, the blade of the jaw elements is edged with denticles in species of Pleurobranchus but can be present or absent in Berthella. The absence of denticles, as reported by Risbec, combined with the absence of an anterior mantle notch, would suggest that P. caledonicus is most likely to be a species of Berthella. However confirming the identification and its generic position must wait until the anatomy of this animal  can be investigated.

It has many external similarities to the Atlantic species Berthella africana and was previously identified on the Forum as Berthella cf. africana.

• Risbec, J. (1928) Etude anatomique des gastéropodes Tectibranches de la presqu'île de Nouméa, avec description de cinq espèces nouvelles. Archives du Museum d'Histoire naturelle, Paris, 3, 37-68.

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