A swift slug...
These are pictures of the nudibranch Trapania velox. A nudibranch is a type of marine mollusc (snail) that loses its shell during metamorphoses from a planktonic larvae (veliger) to a baby sea slug. The word velox from the species name means “swift”, though this slug doesn't seem all that swift. This nudibranch lives on the west coast of North America, from Carmel, California to Bahia Tortugas, Baja California, Mexico. Trapania velox feeds on tiny colonial animals called entoprocts that often grow on top of encrusting sponges (epizoic).
Trapania velox is a dorid nudibranch that cannot retract its gill (phanerobranch). The gills are feather-like projections near the top-middle of the body just behind two black and yellow appendages. There are two other black and yellow appendages near the rhinophores. The rhinophores (which means nose-bearer) are white and yellow and are perfoliate. Rhinophores are used as chemical sensors, so “smell” underwater.
The eggs of Trepania velox are in laid in white ribbons.
This species of nudibranch was first described in 1901 by T.D.A Cockerell from La Jolla. The animal in these pictures was photographed at South Casa Reef (Hospital Point), La Jolla, CA.
References
Behrens, D.W. (2005). Nudibranch Behavior. New World Publications, Jacksonville, FL.
Behrens, D.W., and Hermosillo, A. (2005). Eastern Pacific Nudibranchs: A Guide to the Opisthobranchs from Alaska to Central America. Sea Challengers, Monterey, CA.
Cockerell, T.D. (1901). Three nudibranchs from California. Journal of Malacology 8:85–87.
Smith, R.I. and Carlton, J.T. (1975). Light's Manual: Intertidal Invertebrates of the Central California Coast. 3rd. ed., University of California Press, Berkeley, CA.