Winged thecacera, Thecacera pennigera, is a small, attractive sea slug that is typically translucent white with small orange, yellow and black to blue-black spots or flecks. It has a blunt and broadened head with two lateral extensions and paired rhinophores (olfactory organs) located inside flared, incomplete rhinophore sheaths. The anterior of the body is broad and it tapers posteriorly to a long, slender foot. A u-shaped tuft of gill is located dorsally just forward from a pair of finger-like white-tipped, postbranchial processes. These processes have coloration similar to the rest of the body (NIMPIS 2002). Picton and Morrow (1994) report that the glandular postbranchial processes are defensive in function.The color form of the Atlantic T. pennigera specimens is somewhat different than that of Pacific specimens, with the spots in Atlantic specimens being notably smaller than Pacific variants (Sea Slug Forum 2006).