Phyllodesmium magnum

Phyllodesmium magnum,Rudman, 1991

This week we feature another Phyllodesium species, Phyllodesmium magnum. Mike Miller's photo shown, from here the Philippine Province of Batangas, is a very typical specimen (see species no. 630 in Coral Reef Animals of the Indo-Pacific ). Phyllodesmium magnum is one of the larger species of this aeolid genus reaching over 150 mm in length. The curved, flattened cerata make this species distinctive. This species is of particular interest to me because of two interesting biological phenomena is employs. First Phyllodesmium magnum is one of the opisthobranch species retains zooxanthellae alive in its tissue so that it can benefit from their photosynthetic activity. P. magnum's flattened cerata and the densely branched arrangement of the digestive glands, perfectly positions the zooxanthellae dorsally, toward sunlight. This species obtains its zooxanthellae, from the soft coral species it feeds upon, and then derive secondary nutritional benefit from the chemical byproducts released by the zooxanthellae, directly into the digestive glands of the nudibranch.

Secondly, not having the benefit of the defensive cnidosac found in other aeolids,Phyllodesmium magnum is an example of a species that displays "autotomy" or the defensive behavior of casting off its cerata when attacked or disturbed. The autotomization of tissue or body parts is best recalled in lizards, who loose their tail when tugged on, brittle stars who drop the legs to facilitate rapid escape and other nudibranch species like Melibe, who cast off their cerata, and swim away leaving the cloud of cerata for the challenger. In Phyllodesmium magnum, the autotomised cerata produce a sticky epithelial secretion and wriggle around vigorously for a considerable length of time after dropping off, apparently causing sufficient distraction to deter further interest in the now even more naked nudibranch. See Bill Rudman's Sea Slug forum , for details.

Phyllodesmium magnum feeds on several octocoral species including Sinularia, Sarcophyton and as seen in Mike's photo, Cespitularia. It is widely distributed throughout the Indo-Pacific, with documented observations from Tanzania, Australia, New Guinea, Indonesia, the Philippines, New Caledonia, Hong Kong, Guam, Enewetak and Japan.

References of interest:

Rudman, W.B. (1981) The anatomy and biology of alcyonarian feeding aeolid opisthobranch molluscs and their development of symbiosis with zooxanthellae. Zoological Journal of the Linnean Society 72: 219-262.
Rudman, W.B. (1991) Further studies on the taxonomy and biology of the octocoral-feeding genus Phyllodesmium Ehrenberg, 1831 (Nudibranchia: Aeolidoidea). Journal of Molluscan Studies, 57(2): 167-203.

Dave Behrens
Danville, Calif
March 2001

Taxonomic information courtesy of Dave Behrens

David W. Behrens

Author: Pacific Coast Nudibranchs
Co-Author Coral Reef Animals of the Indo Pacific
Propriator of Sea Challengers Natural History Books !

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