Final answer:
Aquatic bilateral animals often have a fusiform shape to decrease drag, allowing for faster and more efficient swimming in the water's higher viscosity. These adaptations are essential for navigating aquatic environments, as seen in the streamlined shapes of sharks and dolphins that enable high speeds.
Step-by-step explanation:
Aquatic bilateral animals often have a fusiform shape to reduce drag. This streamline form is a tubular body tapered at both ends, enabling animals to swim efficiently through water. For instance, sharks can reach speeds of up to fifty kilometers per hour, and dolphins 32 to 40 kilometers per hour, thanks to this fusiform shape which is vital for reducing drag in the water's higher viscosity compared to air. On land, however, organisms are more affected by gravity, with drag playing a less pivotal role. Birds, for example, have adaptations tailored more towards overcoming gravity than reducing drag.
The shape of aquatic animals is crucial for survival and efficient movement. Flattened bodies, modified claws, and other adaptations help these creatures to withstand fast currents and optimize oxygen absorption. For instance, freshwater trout in fast-moving streams demonstrate adaptations that reduce drag while still allowing effective predation. Unlike terrestrial animals, where drag is minimal, aquatic species have evolved with body plans, including the arrangement of fins and skin textures, to navigate through water efficiently. Even the shared streamlined body shape of dolphins and fish is a result of analogous evolution, where similar environmental pressures lead to similar adaptations.