The smallest of the ceratosomas, with the same horn behind the gill and the same chemistry.
A slender dorid to about 6 cm, translucent white to pale cream and usually flushed with pink or lavender, with a ridge running down the back that rises into a single curved horn behind the gill plume. Purple-red spots are scattered along the mantle margin and down the sides. Three Ceratosoma are regular on these sites and they separate mostly on size and proportion: Ceratosoma tenue reaches 15 cm with a broad flared skirt on an orange ground, Ceratosoma trilobatum is as large and red-brown with the mantle drawn out into lobes, and this one stays small and narrow with the lateral lobes barely developed. Half-grown animals of the other two are where the mistakes are made, and at that size the ground colour is the more reliable character.
Feeds on sponges and concentrates the terpenoids it takes from them in the mantle margin and above all in the horn. The horn is held towards a disturbance, so the part of the animal a fish is most likely to take is the part carrying the deterrent, and the loss of a piece of mantle costs the slug little. Egg masses are white to pale pink ribbons coiled on or beside the food sponge.
Recorded from about 5 to 35 m on sponge-covered rubble, rock and slope. Seen less often than its two larger relatives, though a 3 cm translucent animal on a rubble slope is also a good deal easier to swim past.
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.
