Fish comprise a diverse range of water-dwelling creatures that have skulls and, typically, backbones. They breathe through specialized gills, which are openings located on their skin. Their bodies are streamlined and designed for swimming, and they have fins that allow them to travel rapidly through water. Fish are classified as either freshwater fish or saltwater fish based on their habitat.
Freshwater fish have gills that function to diffuse water (not allowing impurities inside) while ensuring bodily fluids remain inside the fish. Freshwater fish have large, well-developed kidneys that are able to process vast quantities of water. Saltwater fish lose large quantities of internal body fluids through their gills because of osmosis. Since saline water is less dilute than the internal fluids of the fish, the saline water rushes in to replace the internal fluids in an effort to form an equilibrium. They replace lost water by consuming large quantities of saltwater.
Freshwater fish are adapted to live in a diverse range of habitats. Some species can survive in mild temperatures (24 degrees Celsuis), while others thrive in temperatures between 5 to 15 degrees Celsius. Freshwater fish are found in shallow wetlands, lakes and rivers, where the salinity of water is less than 0.05 percent.
Saltwater fish are found in various habitats, ranging from the cold Antarctic and Arctic oceans to warmer tropical seas. Habitats that best suit saltwater fish include coral reefs, salt ponds, mangroves, seagrass beds and the deep sea, and a range of fish have developed to thrive in each of these conditions.
Freshwater fish include catfish, charr, cisco, mooneye, gar, shiner, trout (apache, blueback, brook, brown and cutthroat), sunfish, pike, salmon (pink, coho, chum, Chinook and altantic) and whitefish.
Saltwater fish includes albacore, certain types of bass, bluefish, common dolphin, butterfish, eels, flounder, cod, marlin, mackerel, herring, shark, snapper, tuna and yellowtail.
Freshwater fish range in size, from the tiny Philippine gobbies (that measure less than an inch in length) to the white sturgeon (which weighs approximately 400 pounds)--one of the world's largest freshwater fish.
The smallest saltwater fish is the Marshall Islands goby fish (which measures 0.47 of an inch), and the largest known saltwater fish is the whale shark (which averages 12.5 meters long and weighs more than 21.5 tonnes).
Sturgeon and catfish have whisker-like feelers that allow them to taste and touch food before ingesting it. Swordfish, marlin and sailfish stun their prey with their unique bills before feeding on it. The paddlefish stirs up bottom-dwelling organisms with its paddle-shaped snout to feed on them. The goosefish (or angler) has an enticing appendage located the upper portion of its snout. It entices prey by wiggling it like a worm, luring the food to itself.
Saltwater fish have evolved structural variances that allow them to find food. Predators have saclike stomachs with thick walls that grind food. Some fish have pharyngeal teeth (in their throats), others have palatine and vomerine teeth (on the roof of their mouths and the tongue) and others have teeth around the edges of their mouths (maxillary and premaxillary).