Chelidonura amoena population explosion

May 24, 2003
From: Todd Garthwaite

Hi Bill,
There’s been another Chelidonura amoena population explosion in Dragon Cave Bay, off the NorthEast Coast of Taiwan, this spring. While this explosion comes about a month later than the one I reported back in April 2002, this population explosion is just as large if not larger. How so many Chelidonura amoena can just appear out of nowhere is truly amazing. You can’t go for more than a meter or two and not spot a cluster of Chelidonura amoena lying next to each other or entwined together. They are at all varying depths, perched on varying types of underwater terrain. One thing that this month’s Chelidonura amoena population explosion has in common with the one last year is the water temperature. Currently the temperature is 21 degrees C.; the same as last year. Maybe that’s of some significance. I wasn’t able to identify any flatworms near any of the clusters of C. amoena, though. I remember you telling me that they are the most likely source of food for this species.

While not necessarily related to the phenomenon I mentioned above, one other exciting find that I came across was one 12mm specimen of Chromodoris kuniei. I spotted it at a depth of 20 meters, along “Nudibranch Alley” in Dragon Cave Bay. I had never encountered one in Taiwan before, (although I’ve encountered dozens of them in the Philippines. Especially on the wrecks off Sabang in Puerto Galera). I watched it for about ten minutes and for a nudibranch it managed to cover quite a bit of distance rather quickly. I was most surprised. It also exhibited, as you have described it, “the characteristic mantle waving behaviour, in which all the mantle edge is raised and lowered together.” It also lifted its head up and swayed it from side to side quite a few times. It's not often you get to see nudis do that. The contrasting colors of this species of nudibranch certainly made it a joy to be seen! What a shame I didn’t have my camera handy.
Stay golden, Bill.
Peace, love, & nudis
Todd Garthwaite

Garthwaite, T., 2003 (May 24) Chelidonura amoena population explosion. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Todd,
Thanks for keeping us informed. I've got no idea why these population explosions are occurring. In Sydney, we sometimes get population explosions of one or two tropical species, which seem to be associated with eddies of warm water which drift down from the tropical north in late winter-early spring. Perhaps you have similar bodies of warm water travelling north with larvae. I suspect hese animals can grow remarkably quickly so its likely that your population explosions involve animals growing very quickly on the spot rather than migrating to an assembly point in Dragon Cave Bay. I may be wrong. I guess the only way to find out is to try and be there before they 'arrive' next year and try and find some little juveniles - not that easy I am sure. Keep an eye out for feeding events and if you can find any colour variants - perhaps even a white one - I would be very interested.
Best wishes
Bill Rudman

Rudman, W.B., 2003 (May 24). Comment on Chelidonura amoena population explosion by Todd Garthwaite. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from


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