OKINAWAN
OPISTHOBRANCH OF THE WEEK

Melibe engeli Risbec, 1937 (12mm)
Photo by Atsushi Ono

Opisthobranch of the Week Data

Taxonomy:

Frequency on Okinawa: Collection Data:
[brackets indicate range for all Okinawa-collected specimens of the species]


Species Account:

        The above featured animal, Melibe engeli, was photographed and collected by Atsushi Ono off Ama Beach, Zamami 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 by Atsushi during May of 1999 in 3m of water on a brown alga. I've not personally seen M. engeli in the waters of Okinawa's main island but Atsushi reports (per. comm.) this to be a common species in the Kerama Islands. There are several images of M. engeli found on both the Sea Slug Forum as well as on Jun Imamoto's Japanese Sea Slug Site (photos of an approximate 15mm animal from Amami-Ooshima Island, Japan).

        Melibe engeli is unique in comparison with other Melibe species in as much as it has a transparent body as well as a flattened, leaf-like posterior process associated with the rhinophoral sheaths (Atsushi's above image readily shows this leaf-like process). Rudman (2000) reports that it can swim by lateral flexion of the body, and that it readily fed on small crustacea, both crabs and shrimps, as observed in aquaria.

        The following description of external morphology is taken from Smith & Gosliner, 2003:

EXTERNAL MORPHOLOGY. - The living animals are virtually transparent, with a slight greenish yellow coloration and white and cream colored internal organs. The preserved specimens reach a maximum of 30 mm in length and are white to pale yellow, extremely transparent, with the internal organs and muscle fibers clearly visible. The body is limaciform and elongate, somewhat compressed anterolaterally, tapering posteriorly into a narrow rounded posterior portion of the foot. The body surface has few to many conical papillae, tapering to acute or acuminate apices. There are scattered fine white dots or tubercles on or near the body surface, which are more numerous on the papillae. The foot is narrow and linear, opaque white, with the anterior margin rounded and entire. There are 2-4 small conical papillae tapering to acute or acuminate apices on or proximal to the anterior margin of the foot. The circular oral hood is small compared to the rest of the body. The margin of the hood is entire in the majority of the specimens observed, although in some it is indented slightly, usually basally. In all specimens there is an inner and an outer row of long, tapering, conical, tentacular papillae with recurving tips. These are most often found in an opposite arrangement, but may also be found alternating. Some specimens exhibit up to 5 rows, most often near the basal margin. These rows are more or less equal in length, and have a visible axial fiber extending from the hood margin out to the tip, and a concentration of fine white spots. There are additional papillae on the dorsal surface of the hood, generally resembling those on the body surface, and more concentrated towards the anterior margin. The rhinophores are sheathed and born within separated processes that arise from the dorsal surface of the oral hood. The sheaths are somewhat inflated and cylindrical with spreading circular margins that produce a flattened, leaf-like posterior process. There is a great deal of variation among specimens, but a "cockscomb" of 3-5 obtuse to acutely pointed papillations is typical [sic] present, creating an effect that resembles the outline of a typical ceras. The cerata are inflated, and variable in outline, ranging from oval, saccate, or pyriform, to elongate and cylindrical. The surface may be smooth, or covered with low tubercles that give it a broadly warty look, or that may give the margins a lobed or undulating appearance. In the living animals these may appear more papillate. The distal margins range from plain and oval to papillately ornamented. The ornamentation may range from low obtuse papillae to one to three flattened processes bearing 3-6 elongate triangular pappilae [sic] in a "cockscomb" pattern. This wide range of variability may be due in part to the dehiscence or autotomy of the cerata at the basal end. Specimens have been observed with few to many missing cerata, and with cerata in different stages of apparent regeneration and growth. The cerata are transparent, and the branches of the digestive gland within them are visible. There are fine white spots or tubercles, possibly glandular in nature, visible on the surface of the cerata, and concentrated on the margins and the papillae. There are 3-6 cerata on each side of the dorsal midline. The anteriormost left and right cerata are opposite, with the remaining cerata most often alternating. The anus is located on the right side in the gap between the first and second anterior cerata, midway or slightly more proximal to the anteriormost ceras. The nephroproct is immediately dorsal to the anus. The gonopore is well ventral to and on a line with or just anterior to the anteriormost right ceras. There are no papillae associated with the gonopore.

Literature Cited:


Page Date: 22 Sep '03
Page Modification Date: 22 Sep '03
Digitally manipulated photo
Copyright © 2003 Robert F. Bolland