A long aeolid with blue-violet cerata, common on shallow rubble and hydroids.
To about 12 cm, long and slender, with cerata in dense clusters along both sides. Colour runs blue, lilac, purple, brown or nearly white and is worth ignoring: it follows the animal’s symbionts and its diet, so juveniles start white and darken as zooxanthellae accumulate. Note that the common name is shared with Glaucus atlanticus, the pelagic blue dragon that washes up on beaches, which is a completely different animal. The scientific name needs a caveat too. What was long called Pteraeolidia ianthina was shown in 2015 to be several species; that name now belongs to a temperate eastern Australian animal and the widespread Indo-Pacific one takes the older name semperi. The split is not finished — work in Singapore alone found at least two distinct lineages living side by side — so the honest label is Pteraeolidia cf. semperi.
The photosynthesis here is genuinely demonstrated, which distinguishes it from a lot of what is said about solar-powered sea slugs. Radiolabelled carbon work in 1986 showed that between a quarter and a half of the carbon fixed by the zooxanthellae in an hour moved into the animal within that hour. The symbionts are taken from the hydroids it eats and then cultured in branches of the digestive gland running out into the cerata, which is why the animal darkens as it grows. Hydroid-encrusted reef, wreck and rubble, from the intertidal to about 30 m, across the Indo-Pacific. It is often said to sting; the mechanism is plausible, since it stores nematocysts from its prey, but there are no clinical reports behind the claim.
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.
