Opisthobranch of the Week Data
Ceratosoma trilobatum is considered to be uncommon on Okinawa; I have collected nine individuals to date, but I've seen and photographed perhaps an additional dozen or so which were not collected. This moderately large, attractive chromodorid is variable in color. The tail is approximately the same length as the notum and the foot is quite narrow. The specific name has reference to the three strong lobes at the posterior end of the notum: a median horn and paired lateral wings.
Two other similar appearing species of Ceratosoma found here are C. tenue (which is found in roughly the same frequency as C. trilobatum, but my experience has been to find it more often in deeper water than is C. trilobatum). Although C. trilobatum and C. tenue are similar in appearance they are usually quite readily separated from each other by the pair of prominent lateral projections located near the gills of C. trilobatum. Also, C. gracillimum is a similar appearing chromodorid but its absence of a distinct mantle edge between the head and the well-developed lateral lobes seems to segregate it from the other two species.