Catfish are a diverse and widespread group of ray-finned fish belonging to the order Siluriformes. This order contains over 3,000 recognized species. While the common image of a catfish is rooted in freshwater rivers and lakes, the order is not exclusively confined to these habitats. The vast majority of species inhabit inland waters, but notable exceptions thrive in marine and brackish environments.
The Freshwater Majority
Catfish are primarily a freshwater lineage, with their greatest diversity found in the rivers and tributaries of tropical South America, which hosts over 1,650 species. Freshwater species make up over 75% of the entire order and are found across every continent except Antarctica. These fish are characterized by their smooth, scaleless skin and the prominent, whisker-like barbels around their mouths. These barbels function as chemosensory organs, helping the fish locate food in murky waters.
In North America, three species dominate the freshwater landscape: the Channel Catfish, the Blue Catfish, and the Flathead Catfish. Channel Catfish are the most abundant and widely distributed, thriving in environments from small ponds to large reservoirs. The Blue Catfish is the continent’s largest species, preferring the deep channel runs and swift currents of major river systems like the Mississippi and Missouri. Flathead Catfish are also large predators, typically found lying in wait within submerged logs or rocky overhangs in large rivers.
Marine Catfish
A minority of catfish species have evolved to live in saltwater. The two main families adapted to the marine realm are the Ariidae, known as Sea Catfish, and the Plotosidae, or Eel-tailed Catfish. Ariidae is the most diverse marine group, consisting of about 143 species found throughout tropical and subtropical continental shelves globally. These sea catfish often inhabit shallow coastal areas, estuaries, and brackish bays.
Plotosidae are restricted to the Indian and Western Pacific Oceans and are distinctive for their elongated, eel-like tails. While some species in this family also occupy freshwater, many are found in coastal reefs and tidal flats. Ariid catfish also have a unique reproductive strategy where the male mouthbroods the eggs for up to two months until they hatch.
Managing Different Water Environments
The ability of catfish to inhabit both fresh and marine waters is dependent on a physiological process called osmoregulation. This mechanism allows the fish to maintain a stable internal balance of water and salt despite the contrasting external environments. Freshwater catfish live in a hypotonic environment, meaning their bodies have a higher salt concentration than the surrounding water, causing water to constantly diffuse inward. To counteract this, they rarely drink water, excrete a large volume of very dilute urine, and actively absorb salt ions from the water through specialized cells in their gills.
Marine catfish, conversely, are in a hypertonic environment, where the surrounding water is saltier than their internal fluids, causing them to constantly lose water. To maintain hydration, they must continually drink seawater and produce a very small amount of concentrated urine. The massive influx of salt from the ingested seawater is then actively pumped out of the body by chloride cells located in their gills.