The giant freshwater stingray may be the world’s largest freshwater fish. It’s also cloaked in mystery. No one is sure how many giant stingrays are left, which habitats they prefer, or even if they ever venture into the ocean, where their more commonly known relatives live.

These ancient fish, little changed over many millions of years, can reach 16.5 feet long, including the tail. There are reports of giant stingrays weighing up to 1,300 pounds, though such accounts are not verified because weighing the enormous and awkward animals is very difficult.

Brown to gray in color, the giant stingrays are wide and flat in form, and they sport long, whip-like tails. They are known to prowl river systems in Thailand, Cambodia, Malaysia, and Indonesia, often burying themselves in sandy or silty river bottoms. They breathing through holes, or spiracles, on the top of their bodies.

Stingrays locate prey, usually clams and crabs, with sensors located around their mouth that can detect an animal’s electrical field.

Inquisitive creatures

Although stingrays do not readily attack humans, they are one of the few megafishes that can pose a real danger to those who handle them. Each ray has a sharp barb on the base of its tail that can easily penetrate human skin and bone, much like a hunting arrow. This stinger can be as long as 15 inches and typically introduces toxins to the victim’s wound. Experts, however, stress that the rays are non-aggressive and inquisitive.

A major reason why scientists know so little about giant freshwater rays is because they hide at the bottom of the river. They’re not considered a good food fish in Southeast Asia, so they’re rarely targeted by fishers, though they occasionally get caught in nets. If hooked, the car-size stingrays can put up a ferocious fight, and there are reports of boats dragged around for hours on the river by hooked rays, and even pulled underwater.

Discoveries

The giant freshwater stingray has a complicated history. It belongs to the Dasyatidae family, one of four families of stingrays. First identified in Indonesia in 1852 by Dutch ichthyologist Pieter Bleeker, it was essentially forgotten for more than a century before being described as a new species in 1990. In 2008, scientists concluded that this new species was in fact the same as the one Bleeker had identified, and it came to be named Urogymnus polylepis.

A slightly smaller stingray found in rivers in northern Australia was previously considered a regional subpopulation of the giant freshwater stingray. In 2008 it was classified as its own species, Urogymnus dalyensis.

Some scientists believe populations in Thailand and Cambodia, and those in Sumatra and Borneo, may also be genetically different enough to constitute different species.

Threats to survival

The giant freshwater stingray is classified as endangered, and there are indications that stingray numbers have dropped dramatically in recent years as the fish’s river habitats have degraded. While an unstudied but relatively healthy population exists in the upper part of Cambodian section of the Mekong River, it appears the rays no longer inhabit some parts of their historical range.

On the Mae Klong River in Thailand, cyanide-containing waste discharged from an ethanol plant in 2016 killed at least 70 giant stingrays, with few large rays having been seen there since.

The rays have also been targeted for the burgeoning aquarium trade, though it is now illegal to fish for stingrays in Thailand.