Commensal fish on Ceratosoma trilobatum

June 27, 2005
From: John Weinberg

Dear Bill,
I have attached a scan of one of my slides of Ceratosoma trilobatum. After getting my slides processed I noticed that along with the shrimp, Periclimenes imperator, there is also a small fish apparently living happily. It is on the posterior edge of the left lateral lobe, slightly longer than the shrimp in a similar, head down pose. The fish is almost entirely transparent, the eyes being the most visible part, if you 'zoom in' it is possible to make out pectoral fins, the spine and a few bits of organs.It has gone to a lot of 'trouble' not to be noticed. Have you come across this before, and can you kindly tell me what fish it is? Is it commensal, the eyes could be said to match the yellow spots on the slug and it doesn't seem to be 'worried'? by any toxins from C. trilobatum.

Locality: North Sulawesi, Indonesia, Celebes Sea. Depth: 15 m. Length [of Ceratosoma]: 6 inches. June 2004. current swept black sand. Photographer: John Weinberg

Thanks for your help.
John Weinberg

Weinberg, J., 2005 (Jun 27) Commensal fish on Ceratosoma trilobatum. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear John,
It's amazing what you can find when you look through your photos. I can't say I have noticed a fish like this before, but as you say it does a good job at staying unnoticed. I have included a copy of your photo [lower: far right] where I have altered the colour balnace to show your fish more clearly.

Your fish has caused great excitement amongst my fish colleagues here at the Australian Museum. I showed it to Mark McGrouther, the collection manager, and Doug Hoese, and expert on gobies who suggested it was probably a species of the genus Bryaninops, but suggested we contact Helen Larson, Museum & Art Gallery of the Northern Territory, Darwin, who is an expert on this group. Helen Larson has replied saying:

" - that's neat! I've collected Pleurosicya off holothurians, seapens and Tridacna but never nudibranchs. The fish may be a Pleurosicya, yes, but it has very wide-set eyes like Bryaninops. Mind you it's just a child, so is not easy to identify."

So what we can say is that it is a goby, a family of fish in which many have commensal relationships with other animals. It is also a juvenile, which may mean it will not stay with the nudibranch as it matures, but basically, like so much in the sea, we have a lot to learn.

Best wishes,
Bill Rudman

Rudman, W.B., 2005 (Jun 27). Comment on Commensal fish on Ceratosoma trilobatum by John Weinberg. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

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