On the biology of Berthellina citrina (Gastropoda: Opisthobranchia) and its defensive acid secretion

The opisthobranch gastropod Berthellina citrina (Ruppel and Leuckart, 1828¹) was collected from shallow water in the Gulf of Elat (Red Sea), where it is encountered only on sandy substrata overlaid with stones. The mollusc is nocturnal, feeds on various types of sponges, and hides during the day under stones. It releases an acid secretion (pH 1) containing sulphate and chlorine ions. This secretion is released, as a response to stimulation, from the entire body surface. In histological sections stained by standard dyes, the secretory cells remain colorless. In controlled laboratory experiments, it was found that this acid secretion protects B. citrina against all tested sea anemones, fishes, and crustaceans.