Trailing behavior in Risbecia nudibranchs

by B. N. Sullivan

You just never know what you're going to find out next about marine creatures. The photo on this page is a case in point.

We are planning to run a series soon on The Right Blue about nudibranchs. In case you are unfamiliar with the name, nudibranchs are marine gastropod snails without shells -- also known as sea slugs. In order to prepare, we have been going through our photos and notes, and looking up certain bits to make sure we have our facts straight. (After all, we don't want to misinform our readers.)

Risbecia pulchellaOne thing I always do is double check that the species name I have recorded for a creature is correct before I publish it. In my notes, I had recorded the species of the two nudibranchs in this photo as Risbecia pulchella. I did a quick search on the name to see what would come up -- hoping to see images that looked like mine, or a detailed description. Sure enough, I found a few authoritative sources that confirmed that the nudibranchs in my photo were correctly identifed. They are indeed Risbecia pulchella nudibranchs. But I learned something else, too.

A page on the Sea Slug Forum -- a wonderful source of information about nudibranchs, run by the Australian Museum in Sydney -- detailed a behavior said to be typical of nudibranchs in the genus Risbecia. The writer called it 'trailing' behavior:
Also known as queueing or tail-gating, all species of the chromodorid genus Risbecia exhibit this behaviour where they seem to play "follow the leader". Perhaps its a behaviour which has evolved amongst relatively uncommon animals to ensure they find each other for mating. When tailing, one animal appears to follow the mucous trail of the other until they actually make contact. Then the following animal, as can be seen in thse photos, keeps contact by touching the 'tail' of the leader. Sometimes 3 or 4 animals can be seen together.
I have to tell you, my jaw dropped. That is exactly what the two nudibranchs in my photo are doing!

For the record, I photographed these two Risbecia pulchella nudibranchs engaging in 'trailing' in the Red Sea. More precisely, they were photographed at Ras Mohammed, near the remains of the wreck of the Jolanda, at a depth of about 13 meters (42 feet).

I took the photo a number of years ago, but only now did I realize the significance of the behavior that I just happened to record. As I said, you just never know what you're going to find out next about marine creatures.


  1. I envy you, getting to dive near a sunken ship in warm waters around beautiful fish...Lucky.

  2. I saw these for the first time too at the aquarium. These I did get great shots of and posted but it must be so much greater to see there in their natural habitat like this. Wow!

  3. That is too neat! Makes you wonder how many times divers with cameras waited to find them doing that, and you unintentionally captured it. Way to go!!

  4. @ ScienceGuy - It is our good fortune to have been able to dive at places like this and to get to see things like this, but I wouldn't exactly call it luck. All of it required planning and determination -- none of it happened by chance.

    @ 2Sweet - Your aquarium shots are wonderful. Every time I see more of them, I am amazed that you could get such a great result through the glass (or plexiglass?) between you and the creatures.

    @ Kathy - Some of those photo captures are 'luck of the draw', I must admit. The successes make up for the many times when I've spent most of a dive lying on my belly in front of some little creature's lair, waiting for it to come out, when it never did!


  5. I wonder if nudibranchs have any idea how exquisite they are?

  6. Hi Lavender - I think it's safe to say that nudibranchs are not self-aware that way. At all.



We welcome your comments and invite your questions. Dialogue is a good thing!

Bobbie & Jerry