Not a volute or nudibranch?

January 19, 2010
From: Mike Burnett

This animal was found in about 17m of water on an underwater rock island west of Bare Island (Sydney), water temp about 19c, 10 Jan 2010.

It has a siphon like a volute, but no shell, and is about 6 cm long. It is doesn't look like a nudibranch either and looks so different to anything I've seen before I don't know where to start looking to try and identify it.

Locality: Bare Island, Sydney, 17m, NSW, Australia, Tasman, 10 January 2010, Silt covered rock outcrop. Length: 6 cm. Photographer: mike burnett.Can you help please?

Thanks.
Mike Burnett

mrburnett@tpg.com.au

Burnett, M.R., 2010 (Jan 19) Not a volute or nudibranch?. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/23105

Dear Mike,

This is a primitive sea slug called Umbraculum umbraculum. If you have a look at the Fact Sheet, and the messages attached to that page, you will see photos which show many aspects of its anatomy and biology. It does have a shell - it is round and flattened and sits on the top of the animal. In your photo it is covered in sea weed and associated mud and silt. There is a large gill hidden under the right side of the shell and until recently Umbraculum and its relative Tylodina were considered the most primitive of the side-gilled slugs or pleurobranchs, which also have a gill on the right side, but we now think they are two unrelated groups.

Also the siphon you can see is in fact one of a pair of head tentacles. You are right to compare it with the siphon of carnivorous snails like the volutes, because like their siphon it is a hollow enrolled tube used to suck water into the mantle cavity and gills. While their primary function is to provide oxygen rich water to the gills, the water also passes over sensory regions so that both animals can 'smell' chemicals in the water which can lead them to their food.

Best wishes,
Bill Rudman

Rudman, W.B., 2010 (Jan 19). Comment on Not a volute or nudibranch? by Mike Burnett. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/23105

Factsheet

Umbraculum umbraculum

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