Opisthobranch of the Week Data
Pleurobranchus caledonicus is considered to be very rare on Okinawa as I have seen and collected a total of two individuals over a period of more than thirty years of Okinawa diving. A third uncollected animal from Okinawa was photographed by Randy Birt.
The following species description of Pleurobranchus caledonicusis taken from Bill Rudman's Sea Slug Forum (2009):
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.
Photographs of the two Okinawa-collected animals were originally shown on Bill Rudman's Sea Slug Forum (Bolland, 2002). There is also a photograph in Gosliner, et al (2008, p. 97, as Berthella sp.) of an animal similar to the above featured animal. I've added a second page showing a series of both the collected and photo-only individuals.
The following six Pleurobranchus species are currently known from Okinawan waters: