Opisthobranch of the Week Data
I've not personally seen Dermatobranchus diagonalis in the waters of Okinawa's main island and Atsushi Ono mentions (pers. comm.) observing in excess of one hundred animals in the Keramas* and of course considers the species to be very common there. Atsushi has also mentioned to me that D. diagonalis feeds on Clavularia inflata (a soft coral: cnidaria, octocorallia, stolonifera) and is frequently found on colonies of that coral. In addition, Atsushi has also found D. diagonalis being fed on by an unidentified Gymnodoris species which resembles Clavularia inflata.
The authors of a recent paper (Gosliner & Fahey, 2011) have now described the species and the above featured animal is one of several designated paratype** specimens. I've also added a second page w/ an additional image of D. diagonalis, also photographed by Atsushi in the Keramas. The above image as well as the one on the second page are kindly furnished by Atsushi. D. diagonalis is also illustrated in Ono (1999, p. 149 as Dermatobranchus sp. 1) and in Gosliner, Behrens & Valdés (2008, p. 308, as Dermatobranchus sp. 3).
The following information on external morphology is taken from Fahey & Gosliner (2011):
External morphology: The body shape of the living animal is elongate, flattened, and narrows at the posterior end. The foot does not project beyond the mantle margin. There is a series of about 30 shallow dorsal ridges diagonal to the mantle edge, giving the dorsum a distinct leaf-like appearance. Many of these ridges bifurcate along their length. The oral veil extends forward and has rounded corners. The narrow, widely spaced rhinophores are behind the oral veil and are almost cylindrical rather than bulbous. They have a series of longitudinal lamellae on the rounded club. The stalk does not narrow noticeably. Marginal sacs are visible along the mantle edge. Hyponotal and branchial lamellae are absent. The genital opening is on the right side of the body, near the anterior quarter of the body. The anus is situated approximately one-third of the way from the posterior end of the body.
The ground colour of the dorsum, the oral veil, and the foot is brown and is much darker along the medial portion of the notum. There are opaque white speckles and spots of various sizes scattered randomly on the dorsum. There is a pale yellow band present along the margin of the notum. The rhinophore stalk is white basally with a brown club with opaque white on the lamellae. Some orange pigment is present just below the opaque white apex. The oral veil is opaque white with a pale orange pigment band along the anterior margin. The dorsal surface of the foot margin is covered with dense reddish brown speckles.
As of the current date (Early-April, 2014) thirteen described species of Dermatobranchus from Okinawa's main island and the Keramas are featured on these pages. These thirteen species are:
D. albus D. caeruleomaculatus D. cymatilis D. dendronephthyphagus D. diagonalis - The featured animal above D. fasciatus D. fortunatus D. funiculus D. gonatophora D. oculus D. ornatus D. primus D. rodmani
* Keramas = the Kerama Islands Group, located 30 ~ 40 kilometers west of the Okinawa capitol, Naha.
** Paratype: See "Type" Dictionary for explanation.