Giant nudies mating - Thailand

May 13, 2010
From: Ingrid Dolensky

Please find attached some photos of the "giant nudies" of Koh Haa Lagoon mating. They are the size of a ladies forearm (25-35cm).
We think they are Jorunna rubescens, and they seem similar to the photos of these that you have so far, though none of them seem as fond of branching coral as these.

A local dive instructor friend of ours first noticed these last year amongst the fire coral beds of Koh Haa lagoon in March/April 2009. As this is a very popular dive site we were all suprised that we had not noticed them before, as many of my divemaster and instructor friends have been diving there for years.
There were many of them on 2009, with many pink egg ribbons giving away their camouflage (10 or more nudies found in 10 minutes). They were all sitting on the top of the fire coral at this time (the ones we could see).
When I returned later in the year, I could not find them from October 2009-January 2010. Returning to dive in the lagoon in Feb/March 2010 they had returned. These were the only two I found on the dive where I had my camera.

My instructor friends that stay all year advise me they only see them at this time of the year for a few weeks. We think maybe they are underneath the fire coral or somewhere else the rest of the year, as they only appear when their eggs are around (although their camouflage is very good against the coral here).
Their rhinospores are a soft pink-rose colour when out, which they are not in these photos (maybe because they are mating?).

Locality: Koh Haa Lagoon, 8m, Thailand, Andaman Sea, 25 February 2010, branching coral area of lagoon with sand underneath. Length: 30 cm. Photographer: I. Dolensky.

Ingrid Dolensky

Dolensky, I., 2010 (May 13) Giant nudies mating - Thailand. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Ingrid,

Yes these are Jorunna rubescens. Although this species grows to quite a large size I suspect it has an annual life cycle which means you are only likely to see it as it grows in size and perhaps like sea hares congregate to mate. Certainly in Tanzania I have seen it in huge congegrations in shallow reefal lagoons laying great numbers of their typical pink egg ribbons.

Best wishes,
Bill Rudman

Rudman, W.B., 2010 (May 13). Comment on Giant nudies mating - Thailand by Ingrid Dolensky. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from


Jorunna rubescens

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