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A Self-propelled motion is a fundamental ability in many organisms. From the beating flagella of tiny plankton keeping afloat near the ocean surface to the playful perfection of dolphins surfing the bow wave of a ship, marine creatures have adopted a huge variety of styles, speeds, and methods of movement. Each species has its own particular need for the evolutionary developments that have taken place, but the basic requirements are the same-finding food, avoiding predation, seeking a mate or a safe place to have young, or migrating to an area with more favorable conditions.
B The particular physical properties of water that most affect movement are density, viscosity (stickiness), and buoyancy. Seawater is about 800 times denser than air and nearly 100 times as viscous. Consequently, there is much more resistance to movement than on land, as anyone will know who has ever tried to wade through waist-deep water. However, with density comes much greater buoyancy, so that organisms need to spend relatively little energy to stay afloat. As they move through the ocean environment, organisms seek to make wave motion, currents, and natural turbulence work to their advantage, not detriment.
C Most fishes have swim bladders to help them offset the density of their bodies and so maintain neutral buoyancy with minimal effort. These small, gas-filled chambers contain specialized networks of blood vessels that can add or remove gases such as oxygen and carbon dioxide. The ability to remain indefinitely at a constant depth without expending energy is especially important for slow-moving fishes that seek food in the shallows, or for those that hunt and scavenge in kelp forests. Active swimmers, such as mackerel, skipjacks, and sharks, do not have swim bladders because they need to change depth more rapidly than they could regulate the gas content. These fishes must swim forever or they will sink.
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