A green crab’s super power: eating through its gills

A photo of a solitary European green crab on a sandy substrate.
The European green crab (Carcinus maenas) has spread far beyond its native continent. It's now found in waters off North and South America, Asia, and Australia. It's a voracious eater and poses a nuisance to shellfish farmers. (Filip Nuyttens, World Register of Marine Species)

The phrase “to inhale your food” evokes images of hot dog eating contests or late night fast food binges. But for the European green crab, the phrase is a bit more literal—these crabs can actually absorb food through their gills.

Gills are the equivalent of lungs in sea creatures like fish and crustaceans, enabling them to capture oxygen from water rather than air. A new study found that in addition to absorbing oxygen, the gills of a green crab are specially equipped to absorb specific nutrients. Although this type of feeding is common among other invertebrates like mussels and oysters, it’s the first-time nutrient uptake through the gills has been recorded in arthropods, a group that includes shrimps, lobsters and crabs.

A green crab has nine pairs of gills that sit just below its hard body shell, and previous studies have shown that four of these are specifically adapted to also take in salts. The slightly thicker gill tissues are lined with special pumps that shuttle the salt from the surrounding water into the crab’s hemolymph—the invertebrate version of blood. Perhaps, researchers thought, in addition to shuttling salt, these gills would be capable of shuttling larger molecules, like amino acids, which are the building blocks of proteins.

Preview Diagram showing gills on a crab absorbing nutrients
Green crabs filter nutrients out of the water using their gills which sit just under the hard exoskeleton. (Altered from J. Patrick Fischer via Wikimedia Commons and CSIRO Marine Research)

To test this idea, scientists at the Bamfield Marine Sciences Centre removed gills from several green crabs and hooked them up to a special contraption that simulates water filtration as though it were still in a living, breathing crab. In the setup, a pump acts like a heart circulating man-made hemolymph through the gill submerged in a beaker of seawater. As the hemolymph passes through the gills it absorbs whatever enters through the gill’s filtration system. When scientists added radioactive amino acids to the seawater the scientists found that the hemolymph leaving the gills suddenly became radioactive, evidence that the gills were picking up the nutrients. And just as they predicted, the extra fleshy gills were highly efficient—capable of absorbing three times the nutrients when compared to the thinner gills.  

Called the shore crab in its native waters around Europe, the European green crab is an invasive species that began wreaking havoc in New England coastal ecosystems following its introduction in the 1800s. It has no predators or competitors and also a killer appetite—crab, fish, young lobster, and shellfish are no match for its nimble, yet crushing claws. Digging up and eating as many as 40 small clams a day isn’t out of the norm for one individual crab. Their tolerance to changes in temperature, oxygen levels, and salt allows the species to take up residence in many types of environments.

But the discovery of nutrient uptake through the gills adds a new layer to the crab’s super survival skills. While the crabs may simply use this as another way to eat, it could be one of the reasons they are so good at surviving tough times. In a food shortage, they can subsist off the amino acids acquired through their gills. It may not equate to a full stomach, but in dire circumstances the little bits of nutrition may be the difference between life and death.

January 2018