When we jump into the water to view the fishes we have to adopt some of their anatomical strategies in order to function in their environment.
If we want to dive we need weights to counteract the bag of gas - our lungs - which keeps us afloat. Once we have achieved neutral buoyancy we find we can fine-tune our position in the water by simply inflating or deflating our lungs. Similarly, most fishes have an internal bag of gas which they are able to adjust by diffusing gas in and out as they move between different depths. This allows them to hover, their energies and fin design devoted to maneuverability rather than fighting gravity. Sharks and rays lack these swim bladders so their fins are designed to provide lift as well as propulsion. This means that they have had to sacrifice fine control of their movements. Although it may not seem that way to us, compared with an agile bony fish, sharks are relatively clumsy movers.
Swim bladders probably started as lungs. Early in their evolution, some fishes are thought to have moved from open seas to warm coastal waters and swamps. Warm water hold less oxygen than cold water so these fishes are thought to have coped with the deficiency by gulping air from the surface into a pouch in the gut where the extra oxygen could be absorbed. A pouch full of air is a useful design feature and it caught on in a big way. Eventually a number of fishes developed a capacity for filling their bladders with gas from the gut. Most bony fishes now have swim bladders with gas from their blood which made trips to the surface unnecessary. In time, the bladder became separated from the gut. Most bony fishes now have swim bladders unless they are bottom dwellers such as hawk fishes, blennies and many gobies and scorpion fishes.
Our wet suits protect us from sharp corals and other abrasive surfaces. Reef fishes generally have strong scales, which serve to protect them. On top of the scales is a delicate layer of skin, which is laid out in a spiralling pattern around the fish's body, sometimes in a left hand pattern and sometimes wrapped to the right. Some fishes, such as manta rays and a number of eels, gobies and blemmies, have no scales at all. Sharkskin is covered not with scales but with tiny teeth, which give it a texture of sandpaper.
A fish's skin is fairly waterproof, like ours, but not delicate surfaces, notably the gills, where the membrane is thin, allow water to pass through. Because the sea is saltier than fish blood, water is drawn out through these membranes by osmosis. This means that marine fish is constantly losing water from it's body which it has to replace by drinking lots of sea water. The salt taken in with this water is expelled by special cells on their gills.
Our primary difficulty with life under water is our inability to obtain oxygen. To get around this problem we have to take a tank of air with us to provide ourselves with a link to the surface. There is oxygen in the water - but just one-thirteenth the amount present in the air. Fishes, therefore, have to be extremely efficient at extracting it and, indeed, manage to remove up to 95%. They do this by taking water in through their mouths and forcing it out through the gill slits where a curtain of filaments, rich in thin blood vessels, remove oxygen and expel carbon dioxide. Associated with the gills are grids - gill rakers - which act as screens preventing debris in the water from damaging the delicate gills. In some species, particularly plankton feeders, these have become fine combs which trap food.
It is all but impossible for us to catch up with even the smallest fish when it decides to get away from us. Swimming muscles make up 40-65 percent of the weight of a fish. Each segment of muscle contracts a fraction of a second later than the one in front, the body relaxing at the same time, so that contractors pass along the side of the fish in an undulating wave. In very fast fishes like tuna, however, the muscles are devoted to moving just the powerful and efficient sickle-shaped tail. By cutting down on undulations, these fishes decrease water resistance.
There are two types of muscle - red muscle which requires less energy and is used for normal swimming and white muscle which is used for energy-expensive rapid bursts of speed. High-speed cruisers like mackerels have more red muscle for sustained strong swimming than less active fishes. The tail of a flying fish contains over 90 white muscle, which suggests that flying is an energy-expensive method for avoiding predators, not a strategy to save energy as has been suggested.
We have to wear a mask underwater to compensate for the fact our eyes need a layer of air next to them for light to be refracted in the correct way. Light travels at different speed in water and air. The change in speed as it enters the air in the mask from the water causes a shift in its angle which magnifies what we are looking at. Objects therefore appear 25% larger and closer than they actually are.
To propel ourselves through water, we often borrow another fishy feature - fins. By adding them to the end of our bodies, we are imitating the first fishes which had only tail fins and, without swim bladders, were able to swim far above the sea floor. Since then fishes have sprouted a great variety of fins which, apart from propelling them forwards and backwards, give them precision control, acting as keels, stabilizers, props, brakes, oars and paddles while providing lift as well as a means of defence and attack in some species. It is fins which differentiate fishes from other vertebrates.
