December 14, 2007
From: Dene Banger
Concerning message #21292:
Good Day Bill,
(If I may address you as such.)
I do understand that many nudibranchs use the sugars produced through photosynthesis by retained zooxanthellae. One would expect then that a nudibranch would spend a considerable amount of time in locations that receive light in order to take advantage of the nutrition produced by the algae during the short retention time.
However, this has not been our observations in the case of Aeolidiella stephanieae which prefer to spend 90% of their time in areas which have no light. This being the case then, retained zooxanthellae would be providing very little to no nutrition to this species before being passed through the digestive tract. Which starts to raise the question, "If this species spends so much time in the absence of light, is it dependent on nutrition being generated from retained zooxanthellae, or are the retained zooxanthellae being used for another function?"
email@example.comBanger, D.T., 2007 (Dec 14) Re: Aeolidiella stephanieae - zooxanthella for orientation?. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/21308
I am glad you replied as it made me realised I took the title your gave your original message at face value. Aeolidiella stephanieae does not seem to retain or utilise zooxanthellae at all. As Steve Kempf says in an earlier message [#6364] this species starves if it no longer has Aiptasia to eat. Aiptasia certainly has symbiotic zooxanthellae but not all nudibranchs that ingest zooxanthellae from their food are adapted to kepp them alive for their own use. As I say in the species Fact Sheet is one that has no real adaptations for a zooxanthellae symbiosis. As I discuss on the solar powered pages, we get a whole gradation of species showing the gradual evolution of the physiological and physical adaptataions needed to optimise this relationsghips. The genus Phyllodesmium probably shows this best.
How did the symbiosis evolve? Probably it started with nudibranchs feeding on cnidarians which had symbiotic zooxanthellae. Since the colour of the animal is often related to its food colour, its possible that the first stage in this process wa simply retaining zooxanthellae so that the nudibranch matched the colour of its food. There is an interesting parallel with the retention by sacoglossan opisthobranchs of plastids from their algal food. Again there is a complete gradation from species which rely almost entirely on the products of photosynthesis to those that just digest the chloroplasts along with rest of the algal tissue.
This doesn't really answer your suggestion about orientation, but since Aeolidiella stephanieae doesn't really harbour the zooxanthellae, then any behavioural observations can't really be related to harbouring zooxanthellae. In fact the behaviour you describe here, staying in the shade, is the normal type of avoidance behaviour that many nudibranchs have. It is a protective behavior to protect them from predation. Those nudibranchs which are showy and visible during the day [chromodorids, phyllidiids] usually have nasty chemicals in the skin to deter predators.
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