How Do Hellbenders Breathe? Wrinkly Skin Explained

How does a hellbender breathe through its skin?

Breathing through the skin is called cutaneous respiration. Oxygen dissolved in the water passes through the thin outer layers of the skin and into a dense network of tiny blood vessels, called capillaries, that lie just beneath the surface. At the same time, carbon dioxide passes out of the blood into the water.

All amphibians use their skin to breathe to some degree, but the hellbender takes it to an extreme. A classic study published in 1973 measured hellbenders at water temperatures of 5, 15 and 25 °C (41, 59 and 77 °F) and found that the water supplied more than 90 percent of their oxygen at every temperature. The authors described the hellbender as the largest vertebrate without gills that relies almost entirely on breathing in water.

Why are hellbenders so wrinkly?

The wrinkles are the secret of the system. Thick, loose folds of skin run along each side of the body from the neck to the tail, and further flaps hang from the legs. These folds greatly increase the area of skin in contact with the water, so more oxygen can be absorbed, much as the many fine filaments of a fish’s gills do.

The hellbender’s flat body helps too. A flattened shape exposes a lot of skin relative to the animal’s volume, and it lets the hellbender press itself under rocks in the current, where moving water constantly washes over it. The skin is coated with slimy mucus, which keeps it moist and may protect it from abrasion and parasites.

Why do hellbenders rock from side to side?

Water right next to the skin quickly loses its oxygen, leaving a thin, stale layer around the body. In a fast stream, the current sweeps this layer away. In still or oxygen-poor water, hellbenders have been seen swaying or rocking their bodies and rippling their skin folds. Most researchers think this movement stirs the water and brings fresh, oxygen-rich water against the skin, though some have suggested other explanations.

Nesting males show how important this is. A male guarding eggs under a rock fans and rocks to keep water moving over the clutch, and he must also ventilate his own skin. Underwater video studies suggest that when oxygen levels in the nest drop, males cannot do both jobs fully at once, so poor water quality can force a choice between caring for the eggs and breathing.

Do hellbenders have lungs or gills?

Hellbenders do have lungs, large thin-walled sacs inside the body, but they play only a small part in breathing. Researchers think they mainly help the animal control its buoyancy, letting it rise or settle on the streambed.

Gills belong to the hellbender’s youth. Larvae hatch with feathery external gills, which they lose at about 18 months of age, when they are around 10–13.5 cm (4–5.3 in) long. Adults usually keep one small gill slit on each side of the neck, a leftover from the larval stage. That hidden opening gave the genus its scientific name, Cryptobranchus, meaning “hidden gill.”

Why do hellbenders need cold, fast water?

Cold water holds more dissolved oxygen than warm water, and fast, tumbling water mixes in oxygen from the air. Because a hellbender cannot breathe properly without both, it lives in cool, clear, rocky streams with riffles, and it struggles in slow, warm or muddy water.

Hellbenders are usually found in streams of about 9–22.5 °C (48–73 °F). At the same time, a warmer animal burns energy faster and needs more oxygen, so a rise in temperature squeezes the hellbender from both sides. This is why dams, which create slow, warm reservoirs, and silt, which smothers the rocks where water flows, are so damaging, and why scientists worry that warming streams will add to the threats described in Is the hellbender endangered?

Other animals that breathe through their skin

The hellbender is not the only animal to rely on its skin. The lungless salamanders (family Plethodontidae), the largest group of salamanders and very common in the same Appalachian forests, have no lungs at all and breathe through their skin and the lining of their mouth.

The Lake Titicaca water frog of Peru and Bolivia has evolved a strikingly similar solution to the hellbender’s: huge, baggy folds of skin and a habit of bobbing its body in low-oxygen water. Many frogs and turtles also take in oxygen through their skin while they spend the winter under water, as explained in Can frogs breathe underwater?

Not every aquatic salamander works like the hellbender. The axolotl and the mudpuppy keep their feathery external gills for life and use them, along with their lungs and skin, to breathe.