Flabellina riwo Gosliner & Willan, 1991 (10mm)
Photo by Atsushi Ono

Opisthobranch of the Week Data


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

Species Account:

        Flabellina riwo is considered very rare in the waters of Okinawa's main island (only two specimens have been collected {5mm & 18mm}). The above featured animal was photographed, and collected, from the SE Point of Gahi 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 and photographed on a rock of a coral reef by Atsushi Ono on the first of June, 2000 in 5m of water. Atsushi's photograph is used here with his kind permission. Atsushi has seen over ten individuals in the Keramas(per. comm.). With Atsushi's permission, I've included a second page with a photo of an additional animal he's photographed from the Kerama Islands.

        The most similar appearing Flabellina species found in Okinawan waters which loosely resembles F. riwo is F. bicolor, however both internal and external differences between the two are quite evident. In comparison of the two species Gosliner & Willan (1991) point out the following external characteristics:

Discussion: Flabellina riwo differs markedly from F. bicolor. It is characterized by an opaque white network of pigment on the body as compared to a powdering of pigment in F. bicolor. The cerata bear a bluish purple ring rather than an orange one. Specimens of F. riwo generally have fewer cerata per cluster than does F. bicolor. These differences are consistent throughout the extensive geographical ranges of the two species.

        The following description of Flabellina riwo is taken, in part, from the description of external morphology by Gosliner & Willan (1991):

External morphology: The living animals reach a maximum of 20 mm in length. Most of the body is translucent white, adorned with a dense, lacy reticulum of opaque white lines. The oral tentacles are opaque white for most of their length, but possess a translucent basal portion near their junction with the head. The basal portion of the rhinophores is translucent white; the bulbous, lamellate portion is dull peach to light orange and the apex is translucent white. The base of the cerata may be either translucent white or obscured by opaque white pigment. When translucent, the cream, lobate digestive gland is visible. The apical portions of the cerata are covered with opaque white. Near the middle or in the distal third of each ceras is a broad purple ring.
        The body is narrow and elongate. The oral tentacles are three to four times the length of the rhinophores. The bases of the tentacles are terete whereas the distal third is markedly laterally compressed and paddle-shaped. The rhinophores are cylindrical basally and expand into a perfoliate club containing 16-22 densely crowded lamellae. The anterior foot corners are short, tentacular, and recurved. The cerata are arranged in 3-6 pedunculate clusters per side of the body. Each peduncle contains a single row of cerata inserted into an expanded portion of the notal brim. The notal brim is only evident in areas where the cerata are inserted. The peduncles contain 1-4 cerata. The ceratal formula varies considerably from small to large individuals. The first postcardiac row generally contains the largest number of cerata. The gonopore is located ventrally to the precardiac ceratal peduncle, on the right side of the body. The anus is situated between the precardiac and first postcardiac rows, generally closer to the more posterior peduncle. The nephroproct is immediately dorsal or slightly anterior to the anal papilla.

        Flabellina riwo is distributed in the Indo-West Pacific and it is one of six described Flabellina species presented on these pages. These six species known from Okinawan waters are:

  • F. bicolor
  • F. delicata
  • F. exoptata
  • F. macassarana
  • F. riwo - the above featured animal
  • F. rubrolineata
  • Etymology:

    Literature Cited:

    Page Date: 14 Oct '02
    Page Modification Date: 01 Sep '13
    Digitally manipulated photo
    Copyright © 2013 Robert F. Bolland