Colour variation in Halgerda dichromis

November 24, 1999
From: Valda Fraser

Dear Bill
To answer your question about colour variation, to date all specimens of Halgerda dichromis I have seen have seemed very similar in colour. Over the weekend, however, I saw a specimen that has made me stop to think. This one has no black spots. Are they just missing, or is this another kind of Halgerda?

I also wonder if it is possible for this nudibranch to adapt its colour to the immediate environment - like a chameleon. I ask this as I have noticed that when the animal is on sand it is less vividly coloured. Refer to the two lower photos for comparison.

You will note that the pair in the lower right photo are mating.

Locality: South Coast KwaZulu-Natal, SOUTH AFRICA. Scottburgh / near Port Shepstone.
UPPER RIGHT: 60mm long, 24m depth,
LOWER LEFT: 55m long, 36m depth,
LOWER RIGHT: 50mm long, 20m depth.
Date: October - November 1999

Valda Fraser

Fraser, V., 1999 (Nov 24) Colour variation in Halgerda dichromis. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Valda,
This is certainly the sort of colour variation I was hoping for. The orange lines in the upper photo look very like those illustrated by Fahey & Gosliner, although your specimen lacks any black lines. The other two photos, and your first photo seem to suggest the three are the same species.

Very few sea slugs have been reported to show any chameleon-like ability to change colour although some can gradually change colour if their colour is derived from food pigmentation. The difference in the yellow/orange colour may be genetic or it is possible that it could be a food-based pigment from their sponge food. If the sand-dwelling ones are feeding on a less intensely coloured sponge than the rock dwelling ones then I guess a locality related colour difference could develop. But I have no evidence to back that up.

The other possibility is that the yellowish animal (lower left) is another species. It definitely has more complete ridges than the orange lined animals, looking more like Halgerda formosa in shape, if not in colour.

All very interesting,
Best wishes,
Bill Rudman.

Rudman, W.B., 1999 (Nov 24). Comment on Colour variation in Halgerda dichromis by Valda Fraser. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from


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