OKINAWAN
OPISTHOBRANCH OF THE WEEK

Siraius immonda (Risbec, 1928) 19mm

Opisthobranch of the Week Data

Taxonomy:

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


Species Account:

        Siraius immonda is considered to be very rare in the waters of Okinawa's main island as I have seen and collected a total of only three individuals. On these pages at an earlier date (27 Jul '98) this animal was featured as S. nucleola. I've decided that Bill Rudman's message on the Sea Slug Forum (Rudman, 2000) concerning S. immonda provides a credible reason to change the name S. nucleola to that of S. immonda.

        Siraius immonda is unlike any other dorid that I've seen or collected from Okinawan waters. The orange pumpkin-colored dorsum along with the small dark central triangular patch slightly posterior to the rhinophores, distinguishes this moderately small attractive animal from other Okinawan opisthobranchs. The color and pattern was very similar on the three animals collected here. It should be noted, however, that Brodie & Willan (1993) report that Australian specimens (as S. nucleola) show quite a bit of variation in the mantle color, including khaki, orange, dirty yellow, and blue-black mantles. Baba (1993) reports the first record of these animals in Japan (also as S. nucleola) during 1951 and that they too were variable in color (yellow, green, and dark green). When additional Okinawan specimens are found it's reasonable to assume the same amount of color variation will be found as in other areas of the Indo-Pacific region.

        It's perhaps noteworthy to point out that all three of the Okinawan specimens were collected during the same month, over a four year period (11 March 1991, 20 March 1993, and 5 March 1994). The more recently collected two individuals ('93 & '94) were collected from exactly the same vertical pier partition at Tengan Pier, which was covered with miscellaneous brightly pigmented encrusting poriferans and byrozoans.

        I've added a second page with two images of the 21mm animal collected on 20 March, 1994, as well as an image of an animal collected from the Keramas * by Atsushi Ono.

Literature Cited:


        * Keramas = the Kerama Islands Group, located 30 ~ 40 kilometers west of the Okinawa capitol, Naha.


Page Date: 03 Jan '05
Page Modification Date: 03 Jan '05
Digitally manipulated photo
Copyright © 2005 Robert F. Bolland