Favorinus

Aeolid nudibranchs of the family Facelinidae, mostly 15 to 30 mm, slender, with the cerata in clusters along the back and colours running from translucent white through cream and orange to dark brown, frequently banded. The genus character sits on the rhinophores: each carries one to three distinct bulbous swellings along its length, so the rhinophore reads as beaded rather than smooth or lamellate, and that is visible in an ordinary macro photograph. It is what separates Favorinus from the other slender aeolids on the same ground.

Beyond the genus the position is less comfortable. Favorinus tsuruganus, white with dark and orange banding and three swellings on each rhinophore, can usually be named. Several of the other animals photographed in the region belong to species that are undescribed, or that overlap in colour with described ones closely enough that a photograph will not settle it, and colour in this genus is affected by what the animal has been eating. This entry is pitched at the genus for that reason.

The diet is what makes them findable. Favorinus feeds on the egg masses of other sea slugs, on occasion on the eggs of its own kind, and sometimes on other nudibranchs outright. The animal is therefore normally found sitting on or immediately beside an egg ribbon — the white, cream or pink coils laid on sponge, algae, hydroid stems and rubble — and a ribbon with a section eaten out of one edge often still has the slug on it. Searching egg ribbons rather than searching open substrate is the difference between finding them regularly and finding them by accident, and the ribbons are considerably more conspicuous than the animal. Recorded across the tropical Indo-Pacific from shallow water to about 25 m, and present wherever the sea slug fauna is dense enough to keep a supply of eggs on the bottom, which means they turn up where and when other slugs are spawning rather than on any pattern of their own.