Comparative study of salivary glands in carnivorous and herbivorous cephalaspideans (Gastropoda: Euopisthobranchia)

Abstract

The salivary glands of the cephalaspideans Philine quadripartita (Philinidae), Aglaja tricolorata (Aglajidae) and Haminoea navicula (Haminoeidae) were studied by light and electron microscopy to investigate relationships between diet, phylogeny and morphofunctional features of these glands. In herbivores such as H. navicula the posterior end of the long ribbon-shaped salivary glands is attached to the gizzard, whereas in P. quadripartita and A. tricolorata these glands are shorter and their posterior end is unattached. The salivary glands of the carnivorous cephalaspidean P. quadripartita contains three types of secretory cells. Granular mucocytes and vacuolated mucocytes, both secreting acid polysaccharides, were much more abundant than the granular cells secreting proteins associated with neutral polysaccharides. The same cell types were recognized in the salivary glands ofH. navicula, but in this species the granular cells are much smaller and even less abundant. The salivary glands of A. tricolorata contain different secretory cells, which were named cells with dense vacuoles, alveolar cells and granular cells. The first two produce proteins and acid polysaccharides, while granular cells contain proteins and neutral polysaccharides. Based on these results and previously published data, it can be said that histologically the salivary glands of the carnivorous cephalaspidean P. quadripartita are much more similar to the salivary glands of herbivorous cephalaspideans (Bulla striata and H. navicula) and other herbivorous euopisthobranchs (Aplysia depilans), than to the salivary glands of aglajids (A. tricolorata and Philinopsis depicta) in which mucocytes are absent. Thus, having a herbivorous or a carnivorous diet by itself seems not to have been a major driving factor on the histology of salivary glands in cephalaspideans. The digestive system of aglajids differs from that of other carnivorous cephalaspideans, not only in the histology of salivary glands but also in other aspects such as absence of a gizzard, and the lack of a radula in all but one genus. Concerning the morphology of the digestive system, aglajids are highly derived