Distribution and morphology of defensive acid-secreting glands in Nudipleura (Gastropoda: Heterobranchia), with an emphasis on Pleurobranchomorpha

Abstract

Secretion of acidic substances with defence and repellent properties is known in several metazoan taxa, including Gastropoda. Here we investigate and compare defensive acid-secreting cell types of various genera within the heterobranch taxon Pleurobranchomorpha by analysing the sizes and distribution of the secretory epithelial cells and subepithelial glands of the epidermis. Additionally, we investigate the median buccal gland (MBG), which is only known from pleurobranchs and one nudibranch species, Plocamopherus ceylonicus. The present data indicate a high similarity among the epidermal acid glands (EAGs), which consist of highly elongate cells containing a large vacuole with nonstaining contents. When acid-gland cells are concentrated into larger subepidermal acid glands (SAGs), the cells are of cuboidal or globular form, again containing a large nonstaining vacuole. This is also the case for the internal MBGs, although here the epithelial cells are considerably larger. In the latter, overall cell size seems to be related to body size, because specimens of similar size possess acid cells of equal size, whereas larger specimens (e.g. adult Bathyberthella) exhibit much larger cells. In contrast to SAGs, in which cells are often fused, the cells in MBGs are always distinct and fusion was hardly observed. Preliminary results indicate a uniform distribution of EAGs all over the body, whereas SAGs (only present in Berthellina spp.) are more densely distributed along the lateral sides than along the mid-part of the notum. The evolution of acid-gland types within Pleurobranchomorpha is discussed. The MBG has probably evolved twice in Heterobranchia, once within the Pleurobranchomorpha and independently in P. ceylonicus, a member of the nudibranch Euctenidiacea.