Detailed comparative anatomy of the Pinnidae (Mollusca, Bivalvia) reveals further unusual mantle specializations

Abstract Pinnids are semi-infaunal bivalves with several unusual morphological features, some of them regarded as adaptations to environments with high concentration of suspended particles. The presence of a waste canal on the mantle wall, for example, helps rejecting surplus particles from the mantle cavity. Despite being such anatomically intriguing bivalves, detailed comparative morphology studies on pinnids are still scarce. To gain further insights into the functional anatomy of pinnids, we have thoroughly investigated the mantle of Pinna carnea Gmelin, 1791 by means of scanning electron and light microscopy, including the application of histochemical staining methods. Additionally, the mantle was analyzed in ten pinnids from the genera Atrina, Pinna, and Streptopinna obtained in museum collections. Comparative morphology revealed a uniform structural organization of the mantle in Pinnidae, but a pronounced variation in pigmentation, presence of commarginal folds, and papillae. Our results also revealed intense secretory activity in the inner mantle fold, inner mantle epithelium, and waste canal, including at least three secretory cell types. Secretions are diverse and comprise mainly acidic mucosubstances on the mantle margin, while the waste canal secretes abundant acidophilic material apparently composed of basic proteins, and also possibly lipid content. Based on these results, we hypothesize that efficient mucociliary transportation is achieved in a hydrophobic environment provided by the waste canal, rapidly agglutinating and removing large amounts of particles from the mantle. Discoid glands secreting protein-rich content were found on the middle mantle fold of Pinna carnea and Streptopinna saccata (Linnaeus, 1758). Surprisingly, these mantle glands have hitherto not been recorded in the Bivalvia, representing, therefore, a new morphological feature in the Pinnidae.