SHELL MORPHOGENESIS OF ALVEINUS OJIANUS (BIVALVIA: KELLIELLIDAE) AND TAXONOMIC SIGNIFICANCE OF THE EARLY FEATURES

The ontogenetic development of hinge, ligament and other morphological features of a paedomorphic bivalve mollusc Alveinus ojianus (Yokoyama, 1927) of 1.8‐2.0 mm shell length have been examined by light and scanning electron microscopy. At a shell length of 200‐220 m, the provinculum of A. ojianus bears eight or nine larval denticles, the primary ligament pit and the cardinal tooth-complex 3a‐3b, which is located anteriorly. Subsequent to the latter the cardinal tooth I forms. The lateral tooth Lp1 develops in the juvenile stage at a shell length of 700‐1150 m. In adult A. ojianus, the provincular denticles are reduced, tooth-complex 3a‐3b and tooth Lp1 develop with litle change, and the ligament pit is broadened due to the hinge plateau. A comparison of morphogenesis of A. ojianus with that of related families shows differences from the Arcticidae but similarities with some Veneridae. The most significant morphological features of the larval and juvenile shell, which suggest taxonomic relationship of A. ojianus (and, possibly, other kelliellids) with the Arcticidae and Veneridae, appear to be: dentition of the provinculum; distinctness and location of the primary ligament pit; topology and sequence of formation of the cardinal teeth.