Parasite or joy ride? on Pectenodoris trilineata

November 2, 2007
From: Heidi Hösel

Dear Bill,
we observed this unusual association during our stay in Lembeh. The tiny "rider" seemed to be sucked on the nudibranch and did not move during the time watched and took pictures of it. Although I think I saw some kind of rhinophores, other anatomical features (gills, foot/mantle) that would identify the rider clearly as another nudibranch are missing. Its size was about 1 - 3 mm maximally, so I assume the rider could be a nudibranch (Gymnodoris?) larval stage. Could you please explain what my husband and I have seen?

Locality: Lembeh strait, TK2, about 18 m, Indonesia, pacific, August 2006, rubble. Length: 10 - 15 mm (for the Pectenodoris ). Photographer: Heidi Hösel.

Thank you for your information in advance
Heidi Hösel

Hösel, H., 2007 (Nov 2) Parasite or joy ride? on Pectenodoris trilineata. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Heidi,
The tiny hitchhiker could indeed be a juvenile Gymnodoris, perhaps G. rubropapulosa. I often wonder what the juveniles of some of the rapacious nudibranch 'hunters' eat. We assume that juveniles feed on smaller prey than adults but many seem to be 'hard wired' to feed on specific prey animals and if they are only available in large sizes perhaps the result is as in your photo, they bite in and hang on for a ride.

It reminds me of Ross Gudgeon's message [#15816] which shows a small Roboastra gracilis beginning to devour a much larger Nembrotha kubaryana.

I'll add it to the opisthobranch feeding page for future reference.

Best wishes
Bill Rudman

Rudman, W.B., 2007 (Nov 2). Comment on Parasite or joy ride? on Pectenodoris trilineata by Heidi Hösel. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

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