<P> Other jet - propelled animals have similar problems in efficiency . Scallops, which use a similar design to jellyfish, swim by quickly opening and closing their shells, which draws in water and expels it from all sides . This locomotion is used as a means to escape predators such as starfish . Afterwards, the shell acts as a hydrofoil to counteract the scallop's tendency to sink . The Froude efficiency is low for this type of movement, about 0.3, which is why it's used as an emergency escape mechanism from predators . However, the amount of work the scallop has to do is mitigated by the elastic hinge that connects the two shells of the bivalve . Squids swim by drawing water into their mantle cavity and expelling it through their siphon . The Froude efficiency of their jet - propulsion system is around 0.29, which is much lower than a fish of the same mass . </P> <P> Much of the work done by scallop muscles to close its shell is stored as elastic energy in abductin tissue, which acts as a spring to open the shell . The elasticity causes the work done against the water to be low because of the large openings the water has to enter and the small openings the water has to leave . The inertial work of scallop jet - propulsion is also low . Because of the low inertial work, the energy savings created by the elastic tissue is so small that it's negligible . Medusae can also use their elastic mesoglea to enlarge their bell . Their mantle contains a layer of muscle sandwiched between elastic fibers . The muscle fibers run around the bell circumferentially while the elastic fibers run through the muscle and along the sides of the bell to prevent lengthening . After making a single contraction, the bell vibrates passively at the resonant frequency to refill the bell . However, in contrast with scallops, the inertial work is similar to the hydrodynamic work due to how medusas expel water - through a large opening at low velocity . Because of this, the negative pressure created by the vibrating cavity is lower than the positive pressure of the jet, meaning that inertial work of the mantle is small . Thus, jet - propulsion is shown as an inefficient swimming technique . </P> <P> Many fish swim through water by creating undulations with their bodies or oscillating their fins . The undulations create components of forward thrust complemented by a rearward force, side forces which are wasted portions of energy, and a normal force that is between the forward thrust and side force . Different fish swim by undulating different parts of their bodies . Eel - shaped fish undulate their entire body in rhythmic sequences . Streamlined fish, such as salmon, undulate the caudal portions of their bodies . Some fish, such as sharks, use stiff, strong fins to create dynamic lift and propel themselves . It is common for fish to use more than one form of propulsion, although they will display one dominant mode of swimming Gait changes have even been observed in juvenile reef fish of various sizes . Depending on their needs, fish can rapidly alternate between synchronized fin beats and alternating fin beats </P> <P> According to Guinness World Records 2009, Hippocampus zosterae (the dwarf seahorse) is the slowest moving fish, with a top speed of about 5 feet (150 cm) per hour . They swim very poorly, rapidly fluttering a dorsal fin and using pectoral fins (located behind their eyes) to steer . Seahorses have no caudal fin . </P>

What part of the fish is used to propel the fish through the water