Shell morphology of larval and post-larval mytilids from the north-western atlantic

Comparison of early shell morphological features of six mytilids from the north-western Atlantic revealed differences useful for species identification and classification. Brachidontes exustus, Ischadium recurvum, Geukensia demissa, Amygdalum papyrium, Mytilus edulis, and Modiolus modiolus larvae and post-larvae were cultured in the laboratory. Scanning electron micrographs of the shell and hinge during early ontogenetic stages showed that all species had a long provinculum with taxodont dentition. In addition, provinculum length and number of teeth increased during the larval period in the six species. The small, numerous provincular teeth of Mytilus edulis and the bold, comparatively few provincular teeth of Amygdalum papyrium clearly differentiated these two species. Most species had a low umbo, round posterior margin, and more pointed anterior margin, although Amygdalum papyrium was distinguished by a high, prominent umbo. Distinction of Geukensia demissa and Ischadium recurvum larval shells was difficult due to similarity in their shapes and hinge dentition. However, discriminant analysis using larval shell length, shell height, provinculum length, and number of teeth aided in classification of these and other sympatric species. Presence and type of lateral teeth plainly distinguished the six species during post-larval development. Brachidontes exustus had all three types of mytilid lateral teeth, including primary lateral teeth, which formed immediately posterior to provincular teeth; secondary lateral teeth, which were posterior to the primary lateral teeth and were part of the dissoconch; and dysodont teeth, which formed on the anterior margin of the dissoconch. Modiolus modiolus had primary lateral teeth; Ischadium recurvum had dysodont teeth; and Mytilus edulis had secondary lateral and dysodont teeth; whereas no lateral teeth formed in Geukensia demissa or Amygdalum papyrium during early post-larval development. The provinculum increased in size and complexity during post-larval development in Amygdalum papyrium, Brachidontes exustus, Geukensia demissa, Ischadium recurvum, and Modiolus modiolus, but not in Mytilus edulis.