Many fish species are structurally specialised to feed on one or a few of these food types, so that the food can be taken in and digested as efficiently as possible. These fish are called ‘specialists’. There are also ‘generalists’: fish species that aren’t good at eating specific kinds of food but that can find food and eat it under nearly any conditions. When a new habitat is created (a lake formed by water held back by a new dam, for example), these fish are the first colonists to be able to take advantage of what the new location has to offer. These fish are also known as ‘opportunists’.
Fish that eat hard food (shellfish, seeds, nuts) need sturdy, stout, long jaws for strength. Stout, short jaws, on the other hand, are best for scraping food (e.g., algae) from rocks. A fixed cheekbone is helpful for applying force; this requires sturdy, stout jaw muscles. Large jaw muscles are, however, not handy for big pieces of food.
Teeth are needed for grasping and breaking up food. A large, round tooth surface is suitable for handling a lot of force; these teeth are also close to the point of rotation. Teeth at an angle of 90 degrees to the jaw can withstand a great deal of force, while a narrower angle (facing inside) is handier for tearing.
There are fish species with no mouth teeth (cyprinidae: carp-like fish - the largest family of freshwater fish, with more than 2000 species). These fish have toothed jaws in the throat, to which the same rules apply as the jaws and teeth of the mouth. Cichlids have teeth in both the mouth and the throat.
The orientation of the mouth opening reveals the direction in which food is sought. The mouth of most predators, for example, opens to the front. A mouth facing downward is used for bottom feeding or for eating vertically up against a plant or rock formation (algae eaters). An upward-facing mouth opening is handy for fish that feed on the surface, or for ‘ambush predators’ that attack from a hiding place.