Chelidonura amoena population explosion in Taipei

April 27, 2002
From: Todd Garthwaite

Hi Bill,
Here are some shots of different Chelidonura amoena, which I came across at a dive site called "Beyond-The-Rocks" which is located in Dragon Cave Bay, Taipei County's N.E. Coast, Taiwan.
Date: 20 April 2002
Time: between 10:34 - 11:17am
Depth: between 10 - 19.5 meters
Temp: 21 degrees C.

Two weeks before taking these pictures, I hadn't seen a single nudibranch around this site, with the exception of one small Phyllidia varicosa (which is the most common species around here). Imagine my surprise, when 2 weeks later I encounter a mind-boggling population explosion of Chelidonura amoena. There were hundreds upon hundreds of this particular species spread out all over the Bay. (Many of them were nestled or curled up next to each other. Although I'm not sure if they were mating, a great many of them seemed to be taking a very close interest in one another). To be honest, it was quite exciting, as I have never seen anything like it! First of all, C. amoena is not seen very often in Dragon Cave Bay, and secondly, in the 4 years that I've been diving at Dragon Cave Bay, I've never seen dozens, never mind hundreds and hundreds, of nudibranchs -- be it one species (as in this situation), or 20 different species combined. I am certainly confused as to why the sudden population explosion. My only speculation is that this spring, here in Taipei, we've been having a drought. There has been no rain for nearly 3 ½ months, which is extremely unusual for this time of year. Without the heavy rains that normally come around this time of year, I suspect Dragon Cave Bay has been suffering a lot less from the usual mud and silt caused by the run-off that comes downstream and into the bay from the nearby mountains. Without the usual amount of silt and run-off going into the water, perhaps something that pertains to the diet of C. amoena has had a chance to grow more abundantly, thus accounting for the enormous population explosion. Whatever the reason, it was exciting to see!
Stay golden.
Life, liberty, & the pursuit of nudis
Todd Garthwaite
(Taipei, Taiwan)

typhoontoddy@yahoo.com

Garthwaite, T., 2002 (Apr 27) Chelidonura amoena population explosion in Taipei. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/6807

Dear Todd,
Population explosions like this are certainly exciting to see, especially when they are an animal which usually seems rare. I have over the years at times seen large populations of a number of different species of Chelidonura in different parts of the Indo-West Pacific. I guess they result from a number of particularly fortuitous events, starting with the survival in a group of a large population of veliger larvae in the plankton, which are able to settle together and successfully survive together to adulthood. This would, among other things, require a good supply of food. We think that all species of Chelidonura feed on small flatworms, so if you are back at the same place it might be worth having a look for a plague of flatworms. Have a look at Haruo Kinoshita's message about an acoel flatworm for a photo of the most likely food of Chelidonura.
Best wishes,
Bill Rudman

Rudman, W.B., 2002 (Apr 27). Comment on Chelidonura amoena population explosion in Taipei by Todd Garthwaite. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/6807

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