OKINAWAN
OPISTHOBRANCH OF THE WEEK

Melibe minuta Gosliner & Smith, 2003 (10mm)

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 image of Melibe minuta is of the holotype* specimen (CASIZ** 078529), deposited at California Academy of Sciences. M. minuta is considered to be common on Okinawa, but only at a single collection locale, Horseshoe Cliffs, near Onna Village and for only a relatively short period of time (May, 1991 ~ July, 1992). Over this period of time I've collected and sent quite a few specimens to the California Academy of Sciences (Vic Smith & Terry Gosliner have seventy-two paratypes listed in their paper). Specifically, I've collected approximately 237 individuals in 14 separate collections lots). I use the preceding term "approximately" in as much as several of the lots were made up of more individuals than I had patience for counting in a small sorting bowl (notably RFB #2613-F was made up of around 85 individuals). All collected specimens were found in only a narrow area (approximately ten square meters) of a living stony coral reef near the reef drop off. The specific observed habitat was at the bottom of relatively shallow coral rubble-covered pits in the reef, in most cases the rubble was covered with dense brittle colonies of the red alga Acanthophora sp. The animals were initially observed from the underside of collected coral rubble later at home; the small animals were found crawling on the sides of a sorting bowl and a closer inspection found them to be on the underside of the rubble. On subsequent dives the animals were discovered as the rubble was overturned and closely inspected.

        I've added a second page with some additional images of M. minuta, including one of the paratypes*.

        I collected the last specimen in mid-July, 1992. None have been seen since, at least not by myself. Their absence is perhaps due to the enormous reef die-off in the waters of Okinawa's main island in recent years, due to various reasons, quite possibly the elevated water temperatures experienced here during the last few years.

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

EXTERNAL MORPHOLOGY.- The living animals are translucent and are brownish green in color. The preserved specimens range in size from 3-10 mm in length and are firm and fleshy. The general body shape is limaciform, somewhat elongated, and not compressed anterolaterally. Posterior to the oral hood, the body is humped and rounded in the cardiac and visceral region, tapering abruptly into the posterior portion of the foot, which is often tapered and curled. Ventral to the visceral and cardiac area the rounded body flattens somewhat to form the sides of the foot. The body surface is mostly smooth, with some rugose areas around the neck, ceratal bases, and the side of the foot. There may be some low tubercles on the dorsal portion of the neck, and some small opaque, apparently glandular spots or tiny tubercles in the area dorsal to the foot margin. The foot has a narrow, entire, anterior margin, which is rounded in ventral view. There are no associated foot papillae. The foot becomes wider near the middle of the body, with a clearly defined sole and foot margin visible when viewed from above. The foot tapers posteriorly and projects behind the body proper. Posterior to the projecting, entire, anterior portion, the foot margin becomes undulate. The oral hood is produced from a narrowed base, which lies posterior to the projecting anterior foot margin. The hood is small relative to the body, and its entire, circular margin typically bears an inner and an outer row of cylindrical papillae with rounded or bulbous apices. The outer row is significantly longer and larger, and the inner row may be so short in places that it could be described as a series of tubercles. In some specimens additional rows or partial rows of papillae are present, usually in the anterior portion, to a maximum of 4 rows. The hood surface tends to be more translucent than the margin, and in some specimens is covered with fine white glandular dots, which appeared as low rounded tubercles in others. There are no accessory papillae on the hood surface. The widely separated rhinophoral sheaths arise from the hood surface just anterior to the neck. In general, they are cylindrical and simple, with no sail or large projections, widening and thickening at the distal portion to produce a flanged margin that may be entire and simple, or ornamented with bead-like projections, or short crenulations with rounded apices. In detail, the form varies between an elongate cylinder that widens gradually at the apex to a distinct, shorter, somewhat annulate basal portion with a separately defined distal cup or calyx. The rhinophores are perfoliate with three or four lamellae. There are 4-6 cerata on each side of the dorsum, the anteriormost being opposite in arrangement, becoming more alternate posteriorly. The fleshy cerata arise from a cylindrical base and spread and flatten into fan shaped, oval, or spatulate distal portions which tend to become concave on the inner surfaces, the margins thickened and inrolled. The margins bear a complex combination of multifid projections and papillations, single papillae, crenulations, and tubercles. The convex inner surfaces ear similar protuberences, while the outer surfaces range from bearing low glandular opaque spots bear similar protuberences, while the outer surfaces range from bearing low glandular opaque spots to circular raised tubercles creating a nodular appearance. While the cerata of the living animals may be somewhat translucent, it is not possible to externally ascertain the presence of branches of the digestive gland within them. In the preserved specimens the cerata are firmly held nearly vertical or in a "V". It appears that autotomy or dehiscence is less apt to occur in this species as there are few detached cerata present. The anus is located on a distinctly raised papilla located anterodorsal to the second anterior ceras on the right side. The nephroproct is immediately dorsal to the anus. The gonopore is anteroventral to the most anterior right ceras. There are no papillae associated with the gonopore. In many of the specimens a subdermal black eyespot is observable on the left side below the anteriormost ceras. The right eyespot is most often hidden by the genital organs.
        *Holotype & Paratype: See "Type" Dictionary for explanation.
        **CASIZ: California Academy of Sciences Department of Invertebrate Zoology

Etymology:

Literature Cited:


Page Date: 15 Sep '03
Page Modification Date: 25 Apr '16
Digitally manipulated photo
Copyright © 2016 Robert F. Bolland