The world's biggest nudibranch

September 5, 2000
From: Bruce Wight

Hi Bill,
I know you want the pictures of the Melibe, but I had to send you these photos I just took off the Island of Beqa in Fiji last week. These were the biggest branchs I have ever seen. I have seen some large Aplysia vaccaria diving around here in our local waters of Southern California, but these branchs made the Aplysia look small. I think they are a large form of the species Hexabranchus sanguineus . I have seen Hexabranchus in other tropical Pacific areas before, but they are usually 6 to 8 inches long. These two were 18 or more inches long and 12 inches across. I included a photo of them with some divers so you could get a sense of scale. Have you ever seen any this large? These two were found on the sand at 130 feet, but after we got out of the water I talked with one of the dive guides and he told me that he has seen even larger ones in the shallower water around 30 feet. This was the first time I have ever used a wide angle 24mm lens to photograph nudibranchs instead of some type of macro lens. Look close and you can clearly see a Periclimenes shrimp on one of them. Will get those shots of the Melibe to you sometime over the weekend.
Take care
Bruce Wight

Wight, B., 2000 (Sep 5) The world's biggest nudibranch. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Bruce,
Firstly thanks again for the beautiful shots of Melibe leonina.

Concerning your large nudibranchs. Although I don't think there has been any anatomical study undertaken, the prevailing view is that they are very very large Hexabranchus sanguineus. I have previously mentioned on the Largest, heaviest, etc page of reports of Hexabranchus growing to at least 520mm in length off Djibouti in the Red Sea (Double, 1992). I have also seen photos of very similarly sized animals from deepwater off Fiji. Interestingly these large animals all seem to have a very similar colour pattern. On a rough conversion your 18 inches equals about 45cm, so your animals are definitely of similar size to those from the Red Sea give or take a bit of stretching.

Thanks for these great shots Bruce. I had been thinking that since in 2 weeks time Sydney is going to be swamped by the Olympic Games this would be a good time to make a call for some messages about the biggest, shortest, longest etc sea slug. If anyone has any candidates I'll of course need some photographic evidence so no fishermen's tales please.

Best wishes,
Bill Rudman.

Reference: Double, T. (1992) Here be Giants. BBC Wildlife, 10(5): 34-40.

Rudman, W.B., 2000 (Sep 5). Comment on The world's biggest nudibranch by Bruce Wight. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

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