Behaviour of Dendrodoris denisoni in New Zealand

September 24, 2001
From: Liz Buckley

Hi,
I am a student at the Bay of Plenty Polytechnic in New Zealand, and am studying the nudibranchs in the area. I currently have 9 Dendrodoris denisoni (Gem Nudibranch) in my tank, and am studying their social behaviour. I have encountered something a bit strange, and am wondering if anyone one can shed some light on this for me: I observed two nudibranchs nose to tail in the mating position on the 17 September in one corner of the tank, the next day, they had moved around the tank to the other side and were joined by a juvenille. By late afternoon yesterday, there were seven of them in a group, with two pairs mating. Today, all 9 of them are concentrated in one area, and seem to be huddled together. My question is two fold: Is this ordinary mating behaviour, and if they are mating how long before the lay the eggs? If they are not mating, is there something wrong in the tank? They have a huge variety of food ranging from sponges to algae, and do seem to be eating properly. The ph, ammonia, nitrite and nitrate levels are all fine and have not changed since they were put in to the tank over a week ago.

If you are able to help in anyway that would be fantastic!
Kind regards
Liz Buckley
Tauranga

nudibranchchick@hotmail.com

Buckley, L., 2001 (Sep 24) Behaviour of Dendrodoris denisoni in New Zealand. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/5311

Dear Liz,
I am afraid there is little known about the biology of this species. All I can say is that it will feed on a sponge, but what species I am not sure. I think it lays a white typical dorid egg ribbon and has very small eggs which hatch into planktonic larvae.

However anything you can observe will be of interest and would be very welcome on the Forum. Any chance of any photos? Now to your question about their clustering together. We don't know much about nudibranch behaviour, but most workers who have had much to do with living animals will have seen animals clustering together, often mating. In the field you may find a group clustered together under on rock, and yet there will be none under nearby rocks. Sometimes they will be together because they have been attracted by a food sponge but other times there seenms to be no obvious attractant other than other slugs of the same species. This is especially the case in artificial environments like an aquarium. I assume that they attracted to each other by species specific body chemicals. These may attract by being diffused in the water, but I suspect following a mucus trail or direct contact as they crawl around in a confined space are the most likely causes of the aggregations. I certainly don't think they are 'huddled' together 'holding hands' because they are stressed.

How long after mating do they lay eggs? Again we don't know much about this. Some species certainly store their partner's sperm for some time so even if you see them mating soon after mating it's hard to prove the eggs were fertilised as a result of the recent mating you observed or from stored sperm from an earlier mating.

Please keep in touch with your observations, I am sure many vistors to the Forum will be interested in hearing of your studies.
Best wishes,
Bill Rudman

Rudman, W.B., 2001 (Sep 24). Comment on Behaviour of Dendrodoris denisoni in New Zealand by Liz Buckley. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/5311

Factsheet

Dendrodoris denisoni

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