Marshall Islands Chromodoris decora

September 14, 2007
From: Scott Johnson

Hi Bill,
This is a follow up on my note questioning Chromodoris decora [#20695 ]. I have observed a large number of these animals both here in the Marshalls and in the type locality of Hawaii and I think there are two species here. The photos here were taken of animals that were not found together like this - these two pairs of specimens were each put together to illustrate the similarities and differences. The similarities are obvious. Both have wide orange margins that contain white spots. There are purple spots mostly submarginally, and the dorsum is sort of a milky or yellowish gray with white markings. Gills and rhinophores are very similar in shape. Also, the egg masses of both contain similar extra-capsular yolk.

The differences are also pretty apparent from the photos. One form (call it form 1) has white patches (glands?) mostly restricted to a single line near the edge of the orange margin, while the other (form 2) has discrete white spots scattered throughout the orange margin. Form 1 has a very regular white line bordering the central part of the dorsum and a distinct white line running down the center that bifurcates and surrounds the gills, while in form 2 the white lines are often broken, the central one often has multiple branches, and the central white lines and branches are spotted with purple. The purple spots in form 1 are in two rows, a row of usually large spots just outside the white dorsal oval line and a row of small spots within the orange margin. In form 2, the purple spots tend to be more uniform in size and scattered in less regular rows.

True, these could all be just variations - both forms are variable, but here in the Marshalls at least they do not seem to overlap. And although it is not something I can demonstrate in pictures, one of the most telling differences is that many of the 133 specimens of these two forms I have recorded here in the Marshalls have been paired or sometimes even in groups of 3. None of those were in mixed groups. They always paired up with the same form, even though they both could be found on the same reefs under similar conditions.

Unfortunately, what might be the biggest problem with separating these two forms into different species is that specimens from the type locality do not exactly match either of them, although they seem closer to form 2. I'll send some photos of Hawaiian specimens separately. Form 1 looks like the drawing of the species given the name "setoensis " by Baba in 1938.

Locality: Enewetak Atoll, 4-5 m, Marshall Islands, Pacific, 1983, Under rocks on lagoon reef. Length: 15-20 mm. Photographer: Scott Johnson.

Scott Johnson

Johnson, S., 2007 (Sep 14) Marshall Islands Chromodoris decora. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Scott,

Thanks again for this in-depth look at Chromdoris decora - at least from the central Pacific. To discuss your question about colour variation or separate species I have added a separate message with scans of some of the early relevant illustrations [message #20717 ]. While I would be comfortable if there were only two colour forms, there seem to me to be so many intermediates that I wouldn't know where to draw the line. The common thread appears to be the inverted white Y in the middle of the back.
It is interesting that your examples from Hawaii [#20697] seem to be almost an intermediate between the two colour forms you illustrate here from the Marshall Ids. Some years ago when I first looked at this species I wondered whether there was a geographic cline in colour pattern but your photos here and others on the Forum show that both colour forms occur from the Marshall Ids to the south Pacific. In fact the best example I have seen of Risbec's New Caledonian C. ndukuei  is in your photos from the Marshall Ids.

Your observations about 'look-alikes' pairing up may be signficant, and I could of course be quite wrong, so if anyone wants an indepth project on a single? species this could be an interesting project.

Best wishes,
Bill Rudman

Rudman, W.B., 2007 (Sep 14). Comment on Marshall Islands Chromodoris decora by Scott Johnson. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from


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