Order : SACOGLOSSA
Suborder : PLAKOBRANCHACEA
Superfamily : LIMAPONTIOIDEA
Family : Costasiellidae
Tropical western pacific
Costasiella usagi is very variable in colour.
In Ichikawa’s description the colour is described as: “The head and tail are not pigmented,, and the rhinophores are black. The median cerata are grey, while the outer ones are black. All the cerata possess a white end and a black tip. Moreover, all the cerata bearl longitudinal striations reaching from the base of each ceras to the white apical portion. … The ventral side of the foot is white. …. Smaller specimens have the median cerata coloured green, instead of grey. “Ichikawa doesn’t say what colour the longitudinal striations are but apparently they are white. From photos on the Seaslug forum, the rhinophores range from completely black to black edged, and in some specimens green can be the predominate colour in all cerata.
Ichikawa, M. (1993) Saccoglossa Opisthobranchia) from the Ryukyu Islands. Publications of the Seto Marine Biological Laboratory, 36: 119-139.