Ultrastructure and histochemistry of the digestive gland of the giant predator snail Adelomelon beckii (Caenogastropoda: Volutidae) from the SW Atlantic

Abstract

This study deals with the structure of the digestive gland of the carnivorous gastropod Adelomelon beckiiin Mar del Plata area (Argentina) and discusses the function of its cell types and compare with othergastropods. According to histological and transmission electron microscopy observations the epitheliumis composed of two types of cells that are subject to cyclical changes, involving three phases: absorption,digestion and fragmentation. The majority of the cells, called digestive cells, have a basal nucleus with acytoplasm filled by spherical digestive vesicles in different stages. The apical pole of the cell is coveredwith microvilli and cilia and shows evidence of endocytotic activity during the absorption phase. Theintracellular digestive process passes through: (1) fusion of endocytotic vesicles; (2) transformation inheterolysososmes by fusion with enzyme-rich vesicles; (3) formation of residual bodies after digestion,which are pinched off to the lumen gland during fragmentation phase. The second type of cell in theacini are called vacuolated cells, which occur in clusters, the nucleus is in the mid-basal region andthe cytoplasm is basophilic, filled with stacks of RER. During the absorption phase the vacuolated cellssecrete neutral mucins that probably serve as lubrication to facilitate transport of food particles. During allphases, a large amount of lipofuscin, an insoluble pigment that is accumulated after lysosome digestion, isaccumulated. The functional morphology of the different cell types found in this carnivorous gastropod isvery similar to that found in herbivorous and deposit-feeding gastropods, which suggests that the feedingtype does not influence the micro-morphology of the digestive gland.