Opisthobranch of the Week Data
Pectenodoris trilineata, a small and highly attractive chromodorid, was originally described by Adams & Reeve as Goniodoris trilineata and later by Bergh as Chromodoris virgata. Rudman (1984) moved the animal into a newly erected genus, Pectenodoris and the following is taken from his 1984 description of the species:
The animal is small, seldom exceeding 8 mm in length, elongately oval in outline with a relatively wide mantle overlap. In preserved specimens a series of large single mantle glands, opening ventrally, can be seen arranged around the mantle just in from the edge. The gills are simple and form an arc around the anus, open posteriorly.
The body and mantle are purple with a thin white border to both the foot and the mantle. There are also three white longitudinal lines on the mantle developed to different degrees in different specimens. Usually there is a median white line which runs from in front of the rhinophores back to the gill pocket where it divides to encircle or partially encircle the gill pocket. In some specimens this line is broken in two. There is a pair of white lines, one on each side midway between the median line and the border. In most specimens this is a straight line running from just behind each rhinophore back to just in front of the gills. In some specimens this line is absent and in others it is broken into a series of dashes. Sometimes small orange marks are present on the white lines.
The rhinophore stalks are purple and the clubs are white with an orange band at the base and the tip of each club. The gills are similarly coloured with a purple base, white mid region and orange tip.
The specific name, trilineata of course means "three lines" and has reference to the three, frequently prominent, longitudinal white lines adorning the dorsal aspect of the notum. It should be noted that the above Kerama animal, although lacking the outer pair of lines, is well within the range of variation known for the species. Rudman (1984, 1986) mentions the three white dorsal lines are variable and sometimes the outer pair are completely absent. Rudman also quotes the following from the original species description by Adams & Reeve: "This, though a small species, is very elegantly coloured, and is among the most beautiful of a group which contributes, by its variety of form and colour, to enliven the solitudes of the ocean". Very well said indeed.
Pectenodoris trilineata is reported from the western margins of the Pacific, from the South China Sea to the Great Barrier Reef (Johnson, & Gosliner, 1998) and it's considered to be very rare in the waters of Okinawa's Kerama Islands. I've not personally seen any of these animals in the waters of the main island of Okinawa, and Atsushi Ono reports seeing only the above single uncollected Kerama animal. Ono mentions finding this beautiful little chromodorid beneath dead coral in 5m of water surrounding Gahi Island (pers. comm.), one of a series of islands in the Kerama Islands Group, which are located 30 ~ 40 kilometers west of the Okinawa capitol, Naha.
The only other member of the genus currently known from Okinawan waters is Pectenodoris aurora, also found and photographed by Ono off Gahi Island, as well as the holotype specimen from Seragaki, Okinawa (main island).