Ichthyosaurs, porpoises, dolphins, killer whales all have similar shapes. The long, ribbon-like shape of an eel's body shows another shape.This enables them to hide in cracks, springing out quickly to capture prey, then returning quickly to their hiding spot. Most use camouflage: they change colors to match the ocean floor.But they can not swim as fast as fish using their bodies & caudal fins.Fish can swim slowly for many hours using red muscle fibres.However, there are also species which move mainly using their median and paired fins.The latter group profits from the gained manoeuvrability that is needed when living in coral reefs for example.This is because streamlined body shapes makes the water drag less. Fishes with fusiform shapes can chase prey and escape predators quickly.
It may be polished for use, but is always rough in texture and resistant to slipping.The red fibres are usually alongside a much greater number of white fibres.The white fibres get their energy by converting the carbohydrate glycogen to lactate (lactic acid).The scales are usually covered with a layer of slime which improves passage through the water, and makes the fish more slippery to a predator.Fish swim by exerting force against the surrounding water.Sharks and other chondrichthyes have placoid scales made of denticles, like small versions of their teeth.