Known only from Hawaii
UPPER: Animal with little red pigmentation. LOWER LEFT: Head and right side of body. LOWER RIGHT: Another animal showing scattered red pigmentation. Photos: Scott Johnson
"Translucent white background, with opaque maculations; oral tentacles banded with opaque white and red, with a single band of each near center and a long band of white at apex; apical portion of rhinophores with large band of opaque white; varying amounts of red-orange often present around head and medially; cerata brown internally with opaque white pigment concentrated at apices. Rhinophores conical with two to five slight annulations..." Gosliner, 1979.
Grows to 12mm in length. It is a common in the intertidal and down to 5m during most of the year. This species has been considered synonymous with Phidiana anulifera on the similarities in penial anatomy, rhinophore annulations, and general colour similarities. However the anatomical similarities are not exclusive to these two species and the differences in colour, now that good photos of both are available, suggest they should be retained as distinct species. The orange-red on P. semidecora is very translucent, and its placing is very irregular compared with the distinct opaque orange-red lines on P. anulifera. The cerata in P. semidecora are also much shorter and their upper half is heavily pigmented with opaque creamy white compared with P. anulifera where the cerata are transparent except for fine white spots arranged in longitudinal lines except for a small white apical cap.
• Pease, W.H. (1860) Descriptions of new species of mollusca from the Sandwich Islands. Proceedings of the Zoological Society of London 28: 18-36.
• Gosliner, T.M. (1979) The systematics of the Aeolidiacea (Nudibranchia: Mollusca) of the Hawaiian Islands, with descriptions of two new species. Pacific Science, 33(1): 37-77.
Rudman, W.B., 2001 (August 12) Phidiana semidecora (Pease, 1860). [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/phidsemi