Diversity
The bony fish comprise the largest section of the vertebrates, with over 20,000 species worldwide. They are called bony fish because their skeletons are calcified, making them much harder than the cartilage bones of the condrichthyes. The bony fishes have great maneuverability and speed, highly specialized mouths equipped with protrusible jaws, and a swim bladder to control buoyancy. The bony fish have evolved to be of almost every imaginable shape and size, and exploit most marine and freshwater habitats on earth. Many of them have complex, recently evolved physiologies, organs, and behaviors for dealing with their environment in a sophisticated manner.
 Jaws
One of the most defining features of fish is their jaws. In the history of life on Earth, the evolution of jaws gave fish tremendous advantages and they have become on of the most dominant groups in the ocean. The size and shape of a fishes jaws, along with the size and shape of its teeth dictate what a fish eats. Some fish have large jaws with sharp teeth like this grouper. Other fish, like parrot fish have highly specialized teeth and jaws for feeding on very specific prey (coral, in the case of the parrot fish.)
Reproduction
In fishes, ovoparity (laying under-developed eggs) is most common; the eggs are inexpensive to produce, and as eggs are in the water, they do not dry out (oxygen, nutrients are not scarce). The adult can produce many offspring, which they broadcast into the plankton column. When the offspring settle out of the plankton, they may be in totally new environments, allowing for a great area in which the young may survive. This mode also comes with its disadvantages; when born, the fish must first go through a larval stage for growth before they transform into the adult stage. In this larval stage, they must fend for themselves in obtaining food and avoiding predation. They may not find a suitable environment when they settle out of the plankton column. The survival of individual eggs is very low, so millions of eggs must be produced in order for the parent to successfully produce offspring. Text taken from: A quick course on Ichthyology.
There are other strategies. Some fish invest more energy in fewer eggs. A few fish, like the male leafy sea dragon to the left, actually carry their eggs . Male gobies guard the eggs in a nest until they are born. The male yellowhead jawfish actually guards the eggs by holding them in his mouth!
 Movement
The spotted hand fish, seen to the right ,is very unusual. Its hand-like fins are adapted to better allow it to walk along the bottom. For most fish, their fins are adapted for swimming not walking. Most fish use their back fin called a caudal fin for propulsion. Stiffer, vertically elongated caudal fins like those of a bluefin tuna are designed for speed. Rounder softer caudal fins like those of a butterfly fish are better for close-cornering and maneuvering. Some fish will use exaggerated pectoral fins to move.
Body shape also had a great impact on fish movement. Just like in boats and submarines, a longer more fusiform shape like that of a barracuda makes a fish faster. A flatter, rounder shape like that of a trigger fish, may make give a fish an advantage when navigating the cracks and crevices of a coral reef.
Avoiding Predators
The main way that fish avoid potential predators is to place themselves in a school. Schooling has a lot of advantages besides predator avoidance, but the main advantage is that it reduces the chance that any one individual fish will be eaten by a predator if they are swimming in the school with their peers. Predators have to pick out individual fish to catch, and the large amounts of movement in a school can confuse a predator. Also, predators will often become satiated (full) while feeding on fish in a school.
Schools move in a coordinated fashion, despite the fact that there is no "leader fish." Fish use their highly sensitive lateral lines (and to some extent visual cues) to respond to each other's movement.
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