Opisthobranch of the Week Data
Pinufius rebus is considered to be very rare in Okinawan waters; only four individuals (5, 10, 12, and 15mm) have been seen by Atsushi Ono (per. comm.) and RFB has seen none directly off Okinawa's main island. The above featured animal was photographed, and collected, from Agenashiku Island, in the Kerama Islands, one of a series of islands in the Kerama Islands Group, which are located 30 ~ 40 kilometers west of the Okinawa Capitol, Naha. This featured animal was found on the underside of a Porites colony by Atsushi during November of 1999 in 15m of water. Atsushi's photograph is used here with his kind permission.
Pinufius rebus is one of several species of nudibranchs which feed on Porites coral and retain symbiotic zooxanthellae from the coral. Rudman (1999) has a description of the animal's association with the zooxanthellae on the Sea Slug Forum.
The following description concerning the morphology and color of Pinufius rebus is taken from Rudman (1981):
Morphology. The animal is fattened, broadly oval in outline with a slight tapering posteriorly. There is a rounded oral veil anteriorly, sometimes slightly bilobed. Around the edge of the dorsum is an irregular double row of cerata or cerata like processes, smaller ones being at the edge and larger ones just inside it. There are also a number of regularly arranged cerata arising from lateral ridges some distance from the edge, usually two on each ridge. The lateral ridges run from a central longitudinal ridge which leads from a bulge at the anterior end down the dorsal mid line. From the central ridge, on each side, these lateral ridges run out to the edge recurving posteriorly about halfway to the edge.
There are two pairs of ridges anterior to the pericardial hump and two or three distinct pairs behind the pericardium. Within the sunken pits formed by these ridges are a series of small rounded tubercles arranged fairly regularly with one in the centre and approximately five in a circle around the central one.
There is a pair of smooth rhinophores, rounded distally, at the anterior end. These are not retractile. There are no oral tentacles arising from the oral veil but the most anterior ceras on each side is slightly larger than the others and gives the appearance of an oral tentacle. The anus opens by a small pore at the posterior end of the body, just to the right of the dorsal mid line.
When crawling, the foot extends slightly behind the dorsum. Ventrally, the oral region is not distinctly separated from the foot. The mouth is surrounded by an oral veil which joins the foot posteriorly. Compared with the width of the body, the foot is narrow, being one third or less of the total width of the animal.
Colour. The general appearance of the animal is pale with brown speckling, the actual colour beautifully matching the colour of the Porites colony the Pinufius is on. The brown can range from yellow brown to quite dark brown and even to reddish purple. In one instance a large Porites colony was yellow brown on one side and reddish purple on the other, and Pinufius on different parts of the colony matched the background perfectly.
In detail, the epidermis is basically translucent, almost transparent. A general background straw colour is caused by body fluids. The brown speckling is caused by pigments (zooxanthellae) in the complex ramifications of the digestive gland which lie just under the surface of the dorsum. The central and lateral ridges appear lighter in colour than the surrounding dorsum, as do the rounded tubercles in the pits. There are scattered small patches of white specks, often, but not always, on the tubercles. There is also a distinct white half ring on the posterior side of the base of each rhinophore. The oral veil is translucent with a. radiating pattern of brown speckling.
The cerata have a transparent epithelium through which the brown of the digestive gland can be seen. There are three regions of white granules, forming white bands at the base, in the middle and just below the tip of the ceras. The underside of the body is whitish translucent, with cream coloured gonad lobules showing through in the posterior half of mature animals.