Functional morphology of the membranous lobe within the preoral cavity of Thenus orientalis (Crustacea: Scyllaridae)

Abstract

The membranous lobe of Thenus orientalis (Lund) is a broad fleshy protuberance arising midventrally from the sternal skeleton between the second maxillipeds. Its position, morphology and structure are described and its association with the metastome of other crustaceans discussed. Several roles during ingestion can be postulated. The lobe extends anteriorly towards the mandibles and forms the roof of the preoral cavity and posterior boundary of the mouth. It has a flattened strengthening rod extending the width of the aboral surface, which is covered by three types of anteriorly-directed annulate setae and densely arranged clusters of microscales and microspines. Histologically it is characterized by large amounts of longitudinal and circular striated muscle and tegumental glands beneath the oral surface. These glands secrete sulphated acidic mucopolysaccharides which lubricate food during ingestion. Spherical cells are dispersed throughout the connective tissue beneath the aboral surface. These are characterized by a granular homogeneous cytoplasm, a peripheral cell nucleus and few organelles. Their contents stain positively for protein and neutral mucopolysaccharides and their role is most likely storage of these compounds, although the significance of this is not known. During ingestion, food is passed over the lobe surface and its anterior lip retracts ventro-posteriorly, dilating the preoral cavity to allow quick and efficient swallowing of large food items.