By Jonny Lupsha, Wondrium Staff Writer
The world’s largest, recorded freshwater fish has been caught. A Cambodian fisherman landed a 13-foot, 660-pound stingray in the Mekong River. Rays are specialized for swimming on sea and river beds.
Freshwater fish, unsurprisingly, are any fish that spend their whole lives in freshwater. Since bodies of freshwater are far smaller than oceans, it’s less common for exceptionally large fish to be found there as compared to the depths of the Pacific or Atlantic.
However, a Cambodian fisherman, south of Stung Treng, just shattered a world record by catching a 13-foot, 660-pound stingray, making it the largest, recorded freshwater fish ever caught. The previous record was a 646-pound Mekong giant catfish, which was snagged in Taiwan in 2005.
In his video series Zoology: Understanding the Animal World, Dr. Donald E. Moore III, director of the Oregon Zoo, reveals the incredible world of the several species of ray around the world.
Had to Google “Osmoregulatory”
Rays have flattened bodies especially made for skirting along the floors of oceans and rivers. They may look like big, gray pancakes, but their “wings” are actually pectoral fins that have greatly enlarged over time thanks to evolution. When those fins undulate, they propel rays through the water.
“Amazonian river rays are the only rays known to permanently dwell in freshwater, and in this trait, they differ from all other living rays and their shark relatives,” Dr. Moore said. “These freshwater rays have lost any ability to migrate between freshwater and their ancestral marine environments due to a highly refined osmoregulatory system that allows them to survive in freshwater.”
Osmoregulation is an organism’s way of maintaining an internal balance of water and dissolved materials. According to Dr. Moore, other sharks and rays have evolved physiologically to allow them to osmoregulate in saltwater. Since ocean waters are saltier than most fish blood, ocean-dwelling rays and sharks evolved to regulate their internal salt concentrations to match those of saltwater. They do this by maintaining sufficient amounts of enzymes and organic solutes in their bodies in the presence of salty bodily fluids.
Freshwater rays have the opposite problem: Instead of worrying about losing water to an external saltwater environment, they have to make sure they don’t gain water from the freshwater environment, since their salt levels are already higher than the rivers in which they live.
“Freshwater and ocean rays all have two rows of five gill slits on the bottom, or ventral, side of their bodies,” Dr. Moore said. “They also have two, modified sixth gill slits on the upper, or dorsal, side of their bodies and these are called spiracles. Their spiracles are located behind the eyes on the top of the body.”
Most rays’ mouths are often on the watercourse bottom, at, or even below, mud level. Because of this, water enters the spiracles on the dorsal surface, which prevents the gills from getting clogged with silt. This process aids rays’ respiration. According to Dr. Moore, this evolutionary adaptation helps rays breath more easily when they’re hiding in the sand and mud.
“The rays have long, whip-like tails that are armed with one or more spines,” he said. “These spines have venom glands at their base. The spines can seriously wound or even kill a predator or a human, and the wounds can take a long time to heal.”