OKINAWAN
OPISTHOBRANCH OF THE WEEK

Opisthobranch of the Week Data

Taxonomy:

        Phyllodesmium serratum was described by Baba (1949) as Hervia serrata collected from shallow waters of Sagami Bay in 1939 & 1940 (six specimens). The following is taken from Baba's original description (1949, pp. 105-106 & 179):

        Length 2-4cm. Rhinophores simple. Branchial papillae in 6-8 groups on either side, in a single row in each group. They are about 12 in the 1st, 10 in the 2nd, 8 in the third to 5th, thus decreasing in number backwards, the anterior groups standing on horseshoes, the rest forming mere oblique rows. Genital orifice immediately below the fore-leg of the first right horseshoe, anus and nephroproct as usual in position. Foot corners tentaculiform. Body yellowish white, branchial papillae pale rosy, their veins yellow. Masticatory edge of jaw-plates with a row of only 7-8 denticles. Radula formula 14-18 X 0.1.0. The central tooth V-shaped, with a prominent median cusp which is flanked by a series of as many as 16-23 denticles.

Many species of Phyllodesmium, such as Phyllodesmium longicirrum, have evolved a symbiotic relationship with single-celled plants called zooxanthellae which they keep alive in their bodies, and living to some extent from the products of the zooxanthellae photosynthesis. These "solar-powered" nudibranchs are described elsewhere in the Forum. Phyllodesmium serratum does not have such a relationship, and is unusual in feeding on a wide variety of octocorals. In Tasmania and Victoria it feeds on the stoloniferan Clavularia sp and in New South Wales it is often found on the telestacean Carijoa sp. The white animal figured here from the [sic] Darwin was one of a whole cluster of juveniles found feeding on the gorgonian Junceela sp. Interestingly they are the offspring of an animal which was collected feeding on the alcyonacean Steronepthia. As colour is dependent on the colour of the food they are eating, this species is highly variable in colour except for the white median dorsal line.

        Rudman (1991)in making a comparison of both P. serratum and the South African P. horridum mentions that in most anatomical features, the two species are very similar; there are however some consistent differences in both color and the morphology of the radular teeth.

        Phyllodesmium serratum is considered to be very rare on Okinawa's main island as I have seen and collected only the above featured individual. This animal was found in an area of mixed sand and coral rubble at the reef forefront during a mid-morning SCUBA excursion. It was found on the red octocoral as illustrated.

        Also, Jun Imamoto & Nishina Masayoshi have several photos of P. serratum from the waters of mainland Japan.

        As of this date there are eleven described Phyllodesmium species found here in the waters of Okinawa's main island. The eleven species are:

Literature Cited: