OKINAWAN
OPISTHOBRANCH OF THE WEEK

Hexabranchus sanguineus (Ruppell & Leuckart, 1828) approx. 180mm

Opisthobranch of the Week Data

Taxonomy:

Frequency on Okinawa:
Collection Data:

Species Account:

        This commonly encountered beautiful dorid is probably the most widely recognized, large nudibranch, seen by divers within the Indo-Pacific. The common name, accepted by most English-speaking divers, is "Spanish Dancer"; the name has reference to the frequently seen brightly festive red and white colors of the animal’s mantle and to its frequent habit of performing a "head to tail bucking" action along with the simultaneous flapping of the lateral margins of the upper body when disturbed. Juveniles smaller than about 35mm are often times misidentified by divers as being Chromodoris species; the most readily identifiable field characteristic of the juveniles is the animal’s lateral unfolding of the mantel edge when disturbed, displaying the brightly colored inner edge.

        These exquisite animals are unique in as much as the six gills ("hexabranchus") can be retracted into pockets located at the dorsal posterior portion of the animal’s mantle. The animal is frequently seen to harbor a commensal shrimp, Periclimenes imperator, which can often be seen by a close inspection of the animal’s upper notum and gills.

        Most Okinawa divers have seen the beautiful pink egg ribbon of these spectacular animals at the reef edge and the reef flat. Apparently these animals deposit ova during all times of the year on Okinawa as the current investigator has seen their egg ribbons during all months of the year. These egg ribbons contain multiple thousands of eggs, which hatch into free drifting veliger larvae.

        I’ve collected relatively few of the many adults seen on Okinawa but have collected twelve juveniles ranging in size between 12 and 30mm. These juveniles were found between 1.5 and 48.8m, with most being found in three meters. On the 26th of July, 1990, I observed a large individual feeding on a 50cm3 demospongian located in 51.8m of water; the sponge was noted to be missing perhaps 5% ~ 10% of its mass which was noted as the animal was lifted from the sponge. This H. sanguineus was found to measured 45.7cm (18 inches), which is certainly the largest I've ever seen.


Copyright © 1999 Robert F. Bolland
Digitally manipulated photo