Swimming Underwater
Freestyle, breaststroke, butterfly... people swim with different strokes. There are even more diverse swimming styles for ocean creatures. The principle involved varies according to body structures and survival needs. We can indeed experience the wonder of nature through considering swimming styles!
Among the common marine organisms, most move about in the water by jet propulsion and undulatory swimming. The jet propelled creatures can change their body shape and swiftly squeeze water out of certain parts of their bodies, and hence move in the opposite direction to the water jets. This is similar to the principle of a rocket launch. Typical examples include jellyfish, squid and scallops. Yet they cannot move continuously like a rocket. They can move each time they squeeze out the water, so jet-propelled creatures are not good at swimming in general. However, if threatened, they can squeeze out a high speed water jet, and promptly escape!
Most fish, dolphins and whales adopt undulatory swimming, and use fins to assist with propulsion and direction control. There are three types of undulatory swimming, according to the parts of body involved: anguilliform, carangiform and thunniform. The anguilliform types have longer bodies in general, and the whole body moves in an S-shape. Eel is an example. They swim relatively slowly, but their efficiency is high. That is, they can move for a certain distance, expending minimal energy.
In carangiforms, fish propel themselves forward by moving the hind parts of the body and caudal fin (tail fin). Examples include trout and seabass. Some fish swim in thunniform style, with only their caudal fin moving, such as tuna and mackerel. Dolphins and whales swim in thunniform style, but their caudal fin is horizontal instead of vertical.
The mystery of dolphin swimming
Scientists have encountered interesting problems in studying locomotion of aquatic animals in water. For example, in 1936, British biologist James Gray estimated the friction dolphins encounter in seawater, according to their bodyweight, and calculated the energy they require to swim. At the same time, he recorded dolphins' food intake. He concluded that "the energy required in swimming by dolphins is even higher than that they take in". This is contradictory to the laws of physics; the conundrum was only solved in recent years. Researchers calculated water friction using modern equipment and computers, and identified many factors that were unknown in previous studies, such as vortices created by caudal fins and turbulence created by nearby objects, which can help the dolphins swim over long distances and at high speed. Salmons also make use of turbulence among rocks in rivers to swim faster, and even "hop about" as they go upstream.
The water world embraces all kinds of magic. Many swimming modes are yet to be discovered by scientists. We humans have created airplanes by mimicking birds. Maybe one day we will invent ways to cross the ocean more easily, through studying ways marine creatures swim.