Umbraculum umbraculum from New Zealand

December 8, 2006
From: Kent Ericksen

Dear Bill

I found this at about 12 metres inside Blue Maomao Archway, Poor Knights Islands, NZ, during the day 4 December 2006. It is approximately 50 mm in length and appeared to be headed for two clown nudibranchs about 300 mm away.

Locality: Blue Maomao Archway, Poor Knights Islands, 12 metres, New Zealand, Pacific Ocean, 04 December 2006, Mid-morning, archway 16 - 6 metres depth. Length: 50 mm. Photographer: Kent Ericksen.

It looks like a nudibranch - maybe some kind of Dorididae - but I've exhausted all my references. It seems to be 'carrying' something odd on its back.

Can you ID it for me?
Kind regards
Kent

kentericksen@hotmail.com

Ericksen, K.H., 2006 (Dec 8) Umbraculum umbraculum from New Zealand. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/18952

Dear Kent,

This is Umbraculum umbraculum, a strange primitive sea slug with few close living relatives. It is not a dorid nudibranch and was long thought to be related to the pleurobranchs or side-gilled slugs because like them it has a large single gill on the right of the body. However recent studies suggest it and Tylodina are more closely related to the nudibranchs than the pleurobranchs.

The structure which puzzles you on its back is its umbrella-shaped shell, from which it gets its name. Have a look at an ealrier message from New Zealand [#12844] where I label some of the basic parts of this animal. And of course look at the species Fact Sheet and the other messages attached to that page.

The orange 'blob' you can see on the back of the animal, behind the shell, is faecal material. Not perhaps a 'polite society' topic but a good clue to its diet of a group of sponges of a similar colour. Although very obvious when out in the open, Umbraculum can squeeze itself into some most unlikely crevices, and sometimes into cavities it has eaten in a large sponge colony. In these situations, its dull coloured warty back, and the shell, covered in plant growths, render it almost invisible. In the photo alongside it looks as though there is a second half-hidden animal at the bottom right corner, looking a bit like a contracted sea anemone.

Best wishes,
Bill Rudman

Rudman, W.B., 2006 (Dec 8). Comment on Umbraculum umbraculum from New Zealand by Kent Ericksen. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/18952

Factsheet

Umbraculum umbraculum

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