Plocamopherus imperialis? in Sydney

June 23, 2000
From: P.Zylstra & H.Rothenfluh


We found this nudibranch at Clifton Gardens Wharf in Sydney harbour on 13 May 2000 [New South Wales, Australia]. We thought it might be Plocamopherus imperialis? We saw it during the day scrunched up -- when we came back at night it had starting moving about. It had a slimy trail stretching out behind it -- the water movement washed it about but it stayed 'stuck' to the silt. It was about 10cm long.


Paula Zylstra
Harry Rothenfluh

Zylstra,P. & Rothenfluh,P., 2000 (Jun 23) Plocamopherus imperialis? in Sydney. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Paula & Harry,
Yes your animal is Plocamopherus imperialis, and at 10cm is about as big as it gets. The slimy trail you mention is often quite noticeable when nudibranchs crawl over fine muddy sediments. I'm not sure whether this is beacuse it is easier to see on mud or whether they have to produce more mucous secretions to deal with crawling over mud.

I have pointed an arrow at one of the large posterior 'knobs' that Plocamopherus has on each side of its mantle. If you have a look at the top of this page you will see that these organs produce light when they are disturbed, a bit like a firefly or glow-worm. If you come across one again on a night dive, a bit of vigorous (but gentle) prodding may produce a fascinating display for you.

In your lower photo the dorsal keel behind the gills is well displayed. This is used as a paddle to help the animal swim when it is disturbed.

Best wishes,
Bill Rudman.

Rudman, W.B., 2000 (Jun 23). Comment on Plocamopherus imperialis? in Sydney by P.Zylstra & H.Rothenfluh. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

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