The position of a fish's mouth gives a general indication of where it feeds in the water column. In an aquarium, fish with up-pointing mouths like hatchetfish primarily feed on the food flakes that float or hang near the water surface. Some fish with mouths on the underside of their head, like the catfish, feed on the bottom. A catfish would be very beneficial to the Aquademics aquarium because, as a bottom-feeder, it helps eliminate unused food buildup by eating the food particles that sink to the lower levels of the tank.
The shape and size of a fish's mouth can also tell you what it eats. Since tropical fish in an aquarium have small mouths, Tetra has developed a variety of small fish food flakes, granules and tablets for daily feedings.
Fish, like most organisms, need oxygen to survive. The oxygen that fish "breathe" is dissolved in the water. The oxygen enters the water surface by diffusion or in the water from plants as a byproduct of photosynthesis. Water enters the fish's mouth, moves across the gills and passes out the gill slits or operculum. The gills are made up of a bony or cartilaginous arch supporting a large number of paired gill filaments. Numerous small projections with very thin membranes on each filament are the sites of gas exchange (oxygen to carbon dioxide). Beneath the thin membrane is a network of blood vessels. Oxygen diffuses from the water through the membrane into the blood and carbon dioxide diffuses outward.
Fish can smell odors in the water with receptors inside blind sacs, called nares, located on the head. Nares are similar to our nostrils, but fish do not use them to breathe. The sense of smell is well developed and is useful in detecting distant odors.
Neutral buoyancy, the ability to remain motionless in the water without rising or sinking, is of great importance to a fish. It allows a fish to spend energy feeding, escaping predators or mating that would otherwise be needed to maintain its position in the water. Several strategies help achieve neutral buoyancy. Most bony fish have a structure called a swim bladder, which is a balloon-like organ in the body cavity. Gases from the blood are added or removed to control buoyancy at different depths.
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