Locomotion--Molluscs A. Bivalves 1. Structure of a bivalve a. External shell b. Inside the shell, the mantle, which not only covers the bivalve body, but also secretes the shell from special tissues along the edge of the mantle. c. Mantle at one end is extended into incurrent and excurrent siphons, which may be just thickened regions along the mantle edge, or extensions from the edge. d. Foot extends from the body and contains not only muscle tissue, but also includes fibers that run both laterally and longitudinally e. Shell has at least two sets of muscle at anterior and posterior end that run right to left and can pull the two shells together. f. Where the two valves are joined, there may also be hinge teeth and ligaments that hold the two shells together. 2. Function of the foot a. The fact that the foot contains material (digestive gland, etc.), it acts as a fluid skeleton which the muscles can contract against. b. Thus the foot can be extended and contracted from within the shell. In bivalves that are motile, the foot is large. In bivalves that are sedentary, the foot is small. c. The foot is aided in it ability to pull the creature down into the substrate because, it can cause the substrate to behave more like a fluid than a solid. This happens because the motion of the foot cause solifluction (liquification) of the substrate. The foot is first extended downward, then contracted into an anchor shape at it tips. Then the longi- tudinal muscles contract and pull the whole shell down into the substrate. The fact that the shell is wedge-shaped, aids in this process. 3. Function of the other muscles. a. The paired adductor muscles serve to close the bivalve shell. The question then is how does the shell get opened up when closed. b. The opening of the shell occures in many instances because when closed the shell stretch the ligaments along the hinge line and also compresses a small rubber like structure located in the hinge. Because this material is elastic in nature, when the adductor muscle relax, the shell will slightly gape open. B. Movement in bivalves 1. Movement in scallops is accomplished by the rapid contraction of the adductor muscle. Instead of being located at ends of the shell, the muscles are in one muscle mass at the center of the shell. Water trapped in the mantle cavity can not escaped except by areas lateral to the hinge. This produces two "jet" streams which then propell the mollusc forward. The scallop literally claps its way along the ocean floor, using the shell shape to provide some lift and rise and falling as force is produced and then as the animal glides in the recovery stroke. 2. The mechanics of muscle contraction a. For invertebrates, there are two types of muscle tissue, smooth and striated. This discussion relates to the latter. Striated tissue in a light microscope looks like long fibers with cross bars. Under higher magnification, it is apparent that muscle cells consist of fibers filled with fibrils. Each fibril is composed of sarcomeres. Because sarcomeres in adjacent fibrils are arranged in parallel, the zone that demarcates one sarcomere from the next are aligned to produce the cross striations. b. The fibrils are composed of two contractile proteins, actin and myosin. Myosin consists of thicker filaments. Actin is thinner and has an end that can change its morphology by bending and at the same time can also bond with the myosin to produce acto myosin. Bond formation is facilitated by Ca++ and by ATP. When a bond is formed, the angle at the end of the actin is shortened and thus the actin filament is pulled along the myosin. As bonds are broken and remade, the sarcomeres are shortened and therefore the muscle itself is shortened. The relationship between actin and myosin has been likened to a rachet mechanism. Relaxation of the muscle occurs when the bonds are broken and the antagonistic muscle contracts to stretch the previously contracted muscle. Since the membrane elements of a muscle are also compressed when a muscle contracts, when the bonds are broken, they will return to their previous length and thus back slide to enlengthen the sarco- mere. c. Other kinds of materials--In molluscs in addition to using actin and myosin, they use tropomyosin as a contractile fiber 3. The mode of contraction described above is true of all modes of muscular contraction, both in inverts and verts. The question is often not of movement, but of remaining contracted for long periods of time. Typical muscle will become fatigue if contraction is maintained for too long a time period. Therefore, how can molluscs keep their shells closed for long periods without the constant expenditure of energy. This can take place because the molluscs contain "catch-fibers" in their muscle tissue. These fibers contract and then maintain position, not because they maintain the bonds, but because at the molecular level a "lock" is formed that does not need continuous expenditure of energy for maintenance.