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.)

One 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.