Chemical Defence in Cephalaspidean Gastropods: Origin, Anatomical Location and Ecological Roles

Abstract

Within the opisthobranchs, the cephalaspideans are traditionally considered a transitional group between typical testacean prosobranchs and shell-less opisthobranchs. The cephalaspidean anatomy, including the presence of a cephalic shield, is related to burrowing through soft sediment. Recent studies have shown that some herbivorous and carnivorous cephalaspideans contain secondary metabolites. The micro-herbivorous bubble snails of the Bullidae and Haminoeidae families are known to have secondary metabolites which have different ecological roles. The polypropionates isolated from Bulla gouldiana and B. striata were deterrent to fishes while the secondary metabolites of Haminoea callidegenita, H. fusari, H. hydatis, H. navicula, H. orbignyana and H. orteai were alarm pheromones employed during cross copulation. In Bulla gouldiana and B. striata, the defensive secretion was located mainly in a white gland along the margin of the mantle. In Haminoea species, alarm pheromones were located in external parts (cephalic shield, parapodial lobes and posterior pallial lobe). The carnivorous cephalaspideans Navanax inermis and Philinopsis depicta employ chemotaxis to follow the slime trail of their prey, which include other cephalaspideans or even congeneric individuals. N. inermis and P. depicta sequester alarm pheromones and allomones from their cephalaspidean prey, which are ejected when N. inermis and P. depicta are disturbed. The specific metabolic patterns of Mediterranean cephalaspideans suggest that these patterns can be used as chemotaxonomic markers. We propose the use of a single Thin Layer Chromatography to differentiate among Mediterranean Haminoea species.