Valve ultrastructure in the Order Chitonida (Polyplacophora) from the South West Atlantic

URTEAGA, D.; PASTORINO, G.

Ambéres, Bélgica

Congreso; World Congress of Malacology; 2007

UNITAS Malacologica

The ultrastructure of the intermediate valves of Plaixiphora aurata (Spalowsky, 1795) (Mopaliidae), Chaetopletrra isabellei (dtOrbigny, 1841) (Ischnochitonidae, Chaetopleurinae), Cnllochiton puniceus (Gould, 1846) (Ischnochitonidae, Callochitoninae) and Tonicia lebruni (Rochebrune, 1887) (Ischnochitonidae, Lepidochitoninae), collected along the Argentine coast, was studied based upon different sections, i.e., transversal, sagittal, exsagittal, diagonal and parallel to the surface. Four main layers were found: tegmentum, articulamentum, p. hypostracum and myostracum, according to terminology described by Hass (1972). The tegmentum is the outermost layer. It covers the dorsal surface ofthe valves and eventually folds at boundaries. This layer is made up of micro crystals without evident order, although in the upper portion it seems to form spherulitic sectors. The p. hypostracum shows a greater development in the jugal area, occupying most of its thickness. It has a crossed lamellar arrangement with its long horizontal axis normally to the sagittal axis of the chiton, although this orientation becomes somewhat disorganized when thin prismatic layers pass through the p. hypostracum. Muscles are inserted in the myostracum, which is made up of crystals arranged prismatically with their longer axis almost normal to the ventral surface of the valve. The articulamentum, which is imbedded into the soft tissues of the chiton, is made of micro crystals that diverge from the centre towards the dorsal and ventral sides, becoming almost parallel to each other. Differences among the species studied are discussed. In addition, these features are compared with those observed by Böggild (1930) and Haas (1972) for other chiton species.