Phyllodesmium magnum, Rudman 1991
This was one of two species of Phyllodesmium I photographed in the Lembeh Strait, Northern Sulawesi, in October 2001. The larger, better-known, and somewhat more common P. longicirrum was the other. Like P. longicirrum , this aeolid has the telltale brown patches on its body and cerata that identify it as one of the “solar powered” nudibranchs. The patches contain colonies of zooxanthellae--single-celled plants that, like all plants, photosynthesize their own carbohydrate nutrients. These plants, however, produce many more nutrients than they need, then “leak” the excess to their host, usually a soft coral, stony coral or anemone. In this manner they acquire a safe home with good access to light.
This nudibranch is likely a secondary host that acquired its plants by preying on soft corals, as do other members of its genus. Somehow these slugs manage selectively to eat and digest the coral animal itself without harming the zooxanthellae, which they subsequently “farm” in their outer tissues. Coloration in this species is somewhat variable, some individuals having more of the bluish-purple sheen than others. The tips of the cerata are usually light. This branch can be confused with Phyllodesmium macphersonae . The tie breaker can be seen in an image of Phyllodesmium magnum also posted on Bill Rudman's Sea Slug Forum. All is not what it appears to be in the branch identification business!
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