FISH DIVERSITY

With more than 29,000 species, fishes are the most diverse group of vertebrates on the planet. Of that number, more than 12,000 species are found in freshwater ecosystems, which occupy less than 1 percent of the Earth’s surface and contain only 2.4 percent of plant and animal species. But, on a hectare-for-hectare basis, freshwater ecosystems are richer in species than more extensive terrestrial and marine habitats. Examination of the distribution patterns of fishes in these fresh waters reveals much about continental movements and climate changes and has long been critical to biogeographical studies and research in ecology and evolution.

All fishes are vertebrates (Subphylum Vertebrata), which means that they have a backbone. Fishes are a very diverse group, but the major characteristics of fishes are that they 1) live and grow in water, 2) swim with fins, and 3) use grills for gas exchange (breathing). There are three classes of fishes; the jawless fishes, the cartilaginous fishes, and the bony fishes. As their name suggests, jawless fishes do not have lower jaws, and typically suck onto their prey using hook like teeth. The cartilaginous fishes are the sharks and rays. They do not have a calcified bony skeleton like ours, but rather a more flexible skeleton made of cartilage, like what our ears and noses are made of. Sharks have a very large oil-filled liver that helps them to remain buoyant in the water column. The bony fishes are the most diverse and abundant class of fishes. They have a calcified bony skeleton like ours and use a special gas-filled organ, the swim bladder, for buoyancy. The more gas the fish pumps into the bladder, the more buoyant it is. This is analogous to a human taking a deep breath of air, the more air in the lungs, the better you float in the water. Of course, fishes do not want to float on top of the water, but the idea of having greater buoyancy after more air is the same. Some fast swimming fish, like mackerel, that may move up and down in the water column very quickly, do not use a swim bladder, but rather use oil for buoyancy (if you have ever eaten mackerel you may have noticed that it is a particularly oily fish). The external anatomy of a fish is very different from our own, because fishes are adapted to move and live in water, and we are adapted to live on land. Therefore, locomotion and sensory structures may look very different, although their general functions are very similar. For example, fishes have “noses” (called nares) that don’t look anything like our own, yet their purpose is to smell chemicals in the water. Likewise, the internal anatomy will look very different from our own, however, most of the major organs are the same (e.g., heart, stomach, liver, spleen) and have the same basic function. A few internal structures, like the swim bladder, are of course unique to fishes. In this lesson we will be examining the external and internal anatomy of a bony fish and comparing this to a human. Fishes do have a few specialized structures that have no counterpart to humans. The lateral line is one example. The lateral line detects physical vibrations in the water that allows the fish to sense other animals and objects in the water, even if they can’t be seen. Many types of fishes use inner ear stones, called otoliths, to detect changes in body position. Because of their unique characteristics and growth patterns, scientists often use otoliths to classify fish, as well as determine their age. These stones rest on a bed of sensory hairs, that send messages to the brain about the orientation of the fish. Sharks have an organ in their snout, called the impullae of Lorenzini, that detects weak electric fields. It is thought that they use this structure to detect prey, perhaps being able to distinguish the weak electrical signals given off by injured animals.