Dmitry L. Ivanov

Morphology of the jaw apparatus in 8 species of Patellogastropoda (Mollusca, Gastropoda) with special reference to Testudinalia tesulata (Lottiidae)

The fine structure of the jaw apparatus was studied by scanning electron microscopy in eight species of Patellogastropoda. The jaw apparatus is an unpaired two-layered dorsolateral structure with anterior and posterior wings attached to the odontophore by muscles. The jaw of Testudinalia tesulata (O.F. Müller, 1776) is a derivative of the cuticle typical for the foregut. The tissue forming the jaw is a specialized foregut epithelium (gnathoepithelium), consisting of a special type of cells called gnathoblasts. The jaw grows in areas of the epithelium characterized by high concentration of electron-dense vesicles, ER and long microvilli that penetrate deep into the jaw plate. This indicates that the gnathoblasts take an active part in jaw growth. In most cases, these areas of the gnathoepithelium are highly folded. The main differences between the species studied are form and thickness of the frontal edge of the jaw. These differences do not correlate with the systematic position of the species studied but likely depend more on the feeding mode. The transmission electron microscopy studies yielded new morphological criteria for comparison between various gastropod species and other members of Trochozoa, in particular, Annelida. The jaws of Annelida are cuticular structures formed on the surface of specialized epithelial cells, often also called gnathoblasts. The jaw of Patellogastropoda can be attributed to the first type of annelid jaw formation characterized by an epithelium with long microvilli and continuous growth.

Prochaetodermatidae of the Indian Ocean collected during Soviet VITYAZ cruises 19591964 (Mollusca : Aplacophora)

Four new aplacophoran mollusc species of Prochaetodermatidae (Prochaetoderma arabicum n. sp., Chevroderma lusae n. sp., Chev. javanicum n. sp., and Chev. vityazi n. sp.) and three new records (Chev. turnerae Scheltema, Chev. paradoxum Ivanov & Scheltema, Claviderma laticarinatum Ivanov & Scheltema) are added here to those described earlier for the western Indian Ocean and Arabian Sea and Atlantic Ocean. One of the species, the abyssal Chev. turnerae, is cosmopolitan throughout the Atlantic Ocean and is here reported from the eastern and western Indian Ocean. Prochaetoderma arabicum n. sp. is the only species in the genus Prochaetoderma that has been collected in the Indian Ocean/Arabian Sea. Two eastern Indian Ocean species, both with a curiously long narrow posterium, are superficially similar; one, Chev. javanicum, occurs at hadal depths in the Java Trench and the other, Chev. vityazi, was collected from the lower continental slope. So far, no prochaetodermatid species has been collected from the Indian Ocean at depths less than 1000 m.

Fine morphology of the jaw apparatus of Puncturella noachina (Fissurellidae, Vetigastropoda)

Jaws of various kinds occur in virtually all groups of Mollusca, except for Polyplacophora and Bivalvia. Molluscan jaws are formed by the buccal epithelium and either constitute a single plate, a paired formation or a serial structure. Buccal ectodermal structures in gastropods are rather different. They can be nonrenewable or having final growth, like the hooks in Clione (Gastropoda, Gymnosomata). In this case, they are formed by a single cell. Conversely, they can be renewable during the entire life span and in this case they are formed by a set of cells, like the formation of the radula. The fine structure of the jaws was studied in the gastropod Puncturella noachina. The jaw is situated in the buccal cavity and consists of paired elongated cuticular plates. On the anterior edge of each cuticular plate there are numerous longitudinally oriented rodlets disposed over the entire jaw surface and immersed into a cuticular matrix. The jaw can be divided into four zones situated successively toward the anterior edge: 1) the posterior area: the zone of formation of the thick cuticle covering the entire jaw and forming the electron‐dense outer layer of the jaw plate; 2) the zone of rodlet formation; 3) the zone of rodlet arrangement; and 4) the anterior zone: the free scraping edge of the plate, or the erosion zone. In the general pattern of jaw formation, Puncturella noachina resembles Testudinalia tessulata (Patellogastropoda) studied previously. The basis of the jaw is a cuticular plate formed by the activity of the strongly developed microvillar apparatus of the gnathoepithelium. However, the mechanism of renewal of the jaw anterior part in P. noachina is much more complex as its scraping edge consists not just of a thick cuticular matrix rather than of a system of denticles being the projecting endings of rodlets. J. Morphol. 275:775–787, 2014.