Re: Tritonia diomedea - big puzzle

January 18, 2003
From: Cynthia Trowbridge

Dear Russell,
I am interested to hear of your "adult Tritonia" issue. As an ecologist, I can think of several potential explanations of the pattern:
1. Low recruitment and long lifespan will result in such a pattern. In Oregon, researchers wondered for a long-time where the juvenile and sub-adult stages were of Pisaster, our common sea star; we have lots of adults but where are the younger age-classes? It turns out that recruitment can be episodic in time and space. We find juvenile sea stars abundantly at some sites in some years. Another example would be the chiton Cryptochiton. In 20 years on the Oregon, I saw my first "juvenile" this past summer during the beginning on an El Nino event. There are high densities of adults but nothing <10-15 cm.
2. Rapid growth will also result in such a pattern. In Oregon, Placida dendritica on Bryopsis grows incredibly fast so unless you are lucky enough to see it a few days post-larval settlement, you would assume that there were no juveniles. Recruitment experiments, however, indicate that there is high recruitment.
3. Differential habitat-use (e.g. ontogenetic changes in feeding habits) can also result in such a pattern.
4. Size-specific predation can also result in such a pattern. Despite the dogma that nothing eats nudibranchs, many fishes, crabs, sea spiders, and even mites do eat juvenile or small opisthobranchs. If there is intense predation pressure within the beds, there could be the apparent exclusion of entire size-classes or an entire local population of nudibranchs.

I have no specific information about Tritonia but would suggest you think about life history parameters (recruitment, growth, and longevity) as well as ecological effects (water flow patterns on recruitment, predation on certain stages) as possible explanations.
Good luck!
Cordially,
Cynthia

trowbric@onid.orst.edu

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