Sea Slug from Hawaii

March 29, 1999
From: Caleb and Jamie

I found a beautiful red and orange sea slug very similar to mimasa (?), but it will sometimes spread out wings? and swim through the water with a flying motion.  It has large tentacles on its head as well as some anemone looking things on its back near the end of it.  I was wondering if I should give it to the Waikiki Aquarium, or if you could provide some information on its food as well as its scientific name.  Sorry not to much information and no picture.

Mahalo nui Loa (Thanks in Hawaiian)

Caleb and Jamie

Nakacom@aol.com

Dear Caleb & Jamie,
I am pretty sure you are describing Hexabranchus sanguineus, and perhaps the yellow colour form I have illustrated above. Usually this species has a brilliant red and white colour pattern and hopefully someone kind will send us a photo to put on the Forum. Hexabranchus is often called the Spanish Dancer because the frilly edge to its mantle looks like the dress of a Spanish Flamenco Dancer.
    
The "wings" your describe are really part of its back or 'mantle'. In the more common red colour form, the upper side of the mantle is brilliantly coloured in red and white, while the underside is a dull mottled colour of pinks, browns and whites. When Hexabranchus is quietly crawling along, its 'wings' are folded over the rest of its back, covering the brilliant colour pattern with the dull mottled underside. When it is disturbed, however, - poked, prodded, nipped etc, it will quickly unfold the mantle to display its bright colour pattern. It then starts to vigorously flap its 'wings' and swim away.  Its swimming is basically an escape mechanism and the sudden display of the bright colour pattern is thought to have evolved to startle potential predators such as fish. See the pages on the Forum describing the use of colour in defence and especially 'deimatic colour display'

The tentacles at the front are rhinophores, which it uses to 'smell' its food, and the anemone-looking things on its back are its gills which it uses for breathing.

Hexabranchus, feeds on a variety of sponges.

Best wishes,
Bill Rudman.

Rudman, W.B., 1999 (Mar 29). Comment on Sea Slug from Hawaii by Caleb and Jamie. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/730

Factsheet

Hexabranchus sanguineus

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