Structure of the lateral pedal defensive glands of Trimusculus costatus (Gastropoda: Trimusculidae)

Abstract

When attacked by predators or subjected to mechanical disturbance, trimusculid limpets, like many other marine pulmonates, produce white sticky mucus from glands located in the lateral regions of the headfoot. Histology and electron microscopy were used to describe the structure of the glands of Trimusculus costatus. Based on the histochemistry and appearance of their secretory products, three types of unicellular gland cell were identified. Type 1, which was the most prevalent and largest (up to 200 m � 30 mm), stained positively for neutral mucins only and had a secretory product that was electron-lucent. Types 2 and 3 were smaller (up to 50 mm) and stained positively for acidic, neutral and sulphated mucins as well as protein. The secretory product of Type 2 glands was homogenously electron-dense, whereas that of Type 3 was vesicular and granular in appearance. Each gland releases its contents by holocrine secretion through an opening between the epithelial cells. Unlike other marine pulmonates (Siphonariidae and Onchidellidae), T. costatus does not have large multicellular glands encapsulated in a well-developed muscle layer. Its glands are more similar in structure to those of some terrestrial pulmonates.