I recently migrated from Michigan to Louisiana to join the faculty at Nicholls State University as assistant professor. Moving from the Great Lakes to the bayou is inevitably laced with logistical concerns; in this case they included mobilizing myself, my wife, all our stuff, and…my fishes. That’s a two-day trip across ~1200 miles, but I was pretty confident in my piscine pets’ ability to make the journey. Why so confident? Air-breathing. That’s correct, some fishes have the ability to breathe atmospheric air, as opposed to merely garnering dissolved oxygen from the water. This adaptation has allowed a diverse group of fishes to persist in habitats where more conventional “water-breathers” may not fare as well. Some even take their air-breathing on the road, literally.
Fishes have evolved multiple methods for air-breathing, but they all come down to gas exchange (air also holds more oxygen than water), allowing blood to come into contact with oxygen through vascularized tissues. These tissues may be structures associated with gills, lung-like organs that work like SCUBA tanks, or even breathing through the skin! Here are some examples of fishes that often need to take a breather:
Lungfishes are popular examples of air-breathing fishes; there are seven extant species in Africa, Australia, and South America, all of which possess one or two “lungs” and must frequently surface to gulp air (obligate air-breathers). Some species take their aerobic talents to the extreme. Four species of lungfish swim in rivers and lakes in parts of Africa, and sometimes those bodies of water dry out. Whereas most fishes would perish without water, African Lungfishes (Protopterus spp.) can burrow into the mud, envelop themselves in mucus cocoons, and drastically slow down their metabolism (estivation) until the waters return. Lab experiments on lungfish estivation indicate the fish can survive in near-suspended animation for up to five years!
Bettas (Betta splendens) are popular aquarium fish with large, colorful fins; the males were bred to fight each other (an older name is Siamese Fighting Fish). We often see Bettas in pet shops in small plastic cups or jars; although these fish will do better in a larger tank, their ability to breathe air allows them to live in seemingly small, stagnant waters. Bettas and their close relatives possess “labyrinth organs,” complex structures of bone and muscle that form chambers above their gills. These labyrinth organs contain vascularized tissue, which allows for gas exchange when the fish takes a gulp of air from the surface. Fun fact: Bettas make nests of bubbles!
A trio of air-breathing fishes is highly invasive to North America, and has been introduced to several other parts of the world. Humans have transported these fishes for food and the aquarium trade, but in some cases, the fishes creatively transport themselves as well.
Walking Catfish (Clarias spp.) are popular food fish in their native range, and invasive through parts of the southeastern United States. Not only can they breathe air (and outcompete many native species), but as their name suggests, these fish can traverse terrestrial habitats for short distances using their pectoral fins and a slithering motion. “Walking” to new habitats allows for successful invasion. Combining land locomotion with large size, lots of sharp teeth, and a voracious appetite, we find the snakeheads (Channa spp.) These predatory fishes have quickly risen to the top of the food chain in several aquatic habitats from waters in Maryland (Northern Snakehead Channa argus) to southern Florida (Bullseye Snakehead C. marulius).
Efforts to curb their populations by bowfishing and adding them to restaurant menus are underway. Rounding out this trio of invasives are armored suckermouth catfishes (Family Loricariidae), which include the aquarium “algae-eaters.” These fishes have modified stomachs that allow air-breathing, and are highly invasive in portions of the southeastern United States and even Hawaii. Voracious omnivores (several eat plant matter), at least these catfish can’t walk across land. I should note that aquarium/pet fish should never be released into the wild; they can have devastating impacts on native species.
Several sport fishes are also air-breathers; the giant South American Pirarucu (Arapaima) possesses a highly vascularized gas bladder so large that it envelops the animal’s kidneys! These massive fish are known to synchronize their trips to the surface, which unfortunately can also make them easy targets for poachers. Tarpons (Megalops) can live in freshwater, brackish, and marine environments, and are also air-breathers; they begin gulping air when merely three inches long. Also of fishery significance is the American Eel (Anguilla rostrata), which, like others in the genus can even breathe air through their skin (cutaneous respiration)!
As you can see, air-breathing fishes are a diverse group, and we’ve only reviewed a handful (fin-ful?) of these impressive species. For further reading I highly recommend Air-breathing Fishes by Jeffrey B. Graham (1997). And what about my fishes and their cross-country trip? All nine of the gars (Family Lepisosteidae) made the trek without any problems, and are currently keeping me company as I wrap up this post!