The largest of the Phyllodesmium, with long translucent cerata holding symbiotic algae.
The largest aeolid nudibranch known — regularly over 12 cm — and unmistakable at that size: a huge translucent cream-to-golden animal whose cerata are broad and flattened into leaf-like paddles, each traced by white digestive gland ducts fanning out into fields of brown zooxanthellae near the surface. Smaller individuals might be taken for another Phyllodesmium; a large one could not be mistaken for anything. Note that WoRMS now places it in the Myrrhinidae, where guides and older literature say Facelinidae.
The paddles are what the name refers to and the symbiosis is real: whole dinoflagellate cells, not chloroplasts, housed in branches of the digestive gland running out into the cerata, and maintained for over six months. That is worth distinguishing from the kleptoplasty of the sacoglossans, which is a different mechanism. What has not been measured for this species is how much carbon actually moves from symbiont to animal, so “it photosynthesises” is supported and “it lives on sunlight” is an extrapolation. The second point of interest is defensive. It feeds on soft corals that carry few stinging cells and it has consequently lost functional cnidosacs — it cannot steal nematocysts the way other aeolids do. Instead it stores the soft corals’ own chemistry: nineteen secondary metabolites were isolated from a single animal, three of which deterred pufferfish in trials. Indonesia to the Great Barrier Reef, on and around soft-coral reef and rubble.
Ratings are per location: 1 means found on most dives that look for it, 5 means a handful of records. Where enough dives have been logged the figure is calculated from them; otherwise it is an editorial estimate, and says so. We publish habitat and destination, not the dive site.
