ABSTRACT
Opisthobranchs, in which nudibranchs are included, represent a diverse group of gastropod molluscs. They express all grades of body organizat ion from fully shelled to completely shell-less, and include the most colorful and structurally diversified forms of gastropods. The evolution ary loss or reduction of shell, the most characteristic molluscan organ of mechanical defense, brings with it several problems, but opisthobranchs have few known predators. It is known that other defense adaptations including behavioral, morphological and biochemical strategies may com pensate for their shell loss (Thompson 1960, Ros 1976, Faulkner and Ghiselin 1983, Avila 1995).
