Mollusca: Bivalvia

Bivalvia (Lamellibranchiata, Pelecypoda) is a large class of laterally compressed animals characterized by two calcified variably flattened to deeply cupped valves that are attached to each other at the dorsal surface with teeth spanned by a flexible hinge ligament. Bivalve larvae are amazingly similar from fertilization until metamorphosis which makes learning larval anatomy easy, but identification of larvae from different species difficult when samples are collected from the wild environment. Based on their overall morphology, bivalves are separated into five subclasses but are practically placed into the following groups: clams, cockles, mussels, scallops, and oysters. This chapter discusses the anatomy of the clam and then describes important anatomic differences in oysters, scallops, mussels, and cockles. The tubules in scallops have been described as more acinar-like. The absorptive cells in histologic sections often appear enlarged, are highly vacuolated and have been described as adipocyte-like

Association of a Nonmuscle Myosin II with Axoplasmic Organelles

Association of motor proteins with organelles is required for the motors to mediate transport. Because axoplasmic organelles move on actin filaments, they must have associated actin-based motors, most likely members of the myosin superfamily. To gain a better understanding of the roles of myosins in the axon we used the giant axon of the squid, a powerful model for studies of axonal physiology. First, a ∼220 kDa protein was purified from squid optic lobe, using a biochemical protocol designed to isolate myosins. Peptide sequence analysis, followed by cloning and sequencing of the full-length cDNA, identified this ∼220 kDa protein as a nonmuscle myosin II. This myosin is also present in axoplasm, as determined by two independent criteria. First, RT-PCR using sequence-specific primers detected the transcript in the stellate ganglion, which contains the cell bodies that give rise to the giant axon. Second, Western blot analysis using nonmuscle myosin II isotype-specific antibodies detected a single ∼220 kDa band in axoplasm. Axoplasm was fractionated through a four-step sucrose gradient after 0.6 M KI treatment, which separates organelles from cytoskeletal components. Of the total nonmuscle myosin II in axoplasm, 43.2% copurified with organelles in the 15% sucrose fraction, while the remainder (56.8%) was soluble and found in the supernatant. This myosin decorates the cytoplasmic surface of 21% of the axoplasmic organelles, as demonstrated by immunogold electron-microscopy. Thus, nonmuscle myosin II is synthesized in the cell bodies of the giant axon, is present in the axon, and is associated with isolated axoplasmic organelles. Therefore, in addition to myosin V, this myosin is likely to be an axoplasmic organelle motor