Maryland’s crabs are voracious cannibals, decades of studies show

Crabs cannibalize each other with surprising predation in parts of the Chesapeake Bay

By Jackie Flynn Mogensen edited by Claire Cameron

Chesapeake Bay’s crabs are tearing apart. A decades-long study of the blue crabs that live along the Maryland coast suggests that cannibalism is so widespread that the crabs are their own main predator.

Cannibalism is common in the animal kingdom—it’s been seen in a wide variety of creatures, from caterpillars and praying mantises to giant salamanders and octopuses—but how, where, and when it occurs is less understood.

In this study, between 1989 and 2025, researchers released 2,687 young crabs, tying them to posts at different times of the year and at different depths of Maryland’s Rhode River, a tidal estuary of the Chesapeake Bay. After about 24 hours, the researchers would look for signs of predation — basically if the crabs were dead or injured. Incredibly, they found that a whopping 97 percent of the crab kills or injuries could be attributed to cannibalism — meanwhile, fish were nowhere to be found.

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The fact that the crabs preyed on each other was not a shock, said Anson “Tuck” Hines, director emeritus of the Smithsonian Environmental Research Center and lead author of the study. “What was surprising was that we found no fish predation here — not a single case of fish predation.”

“All the predation was due to cannibalism by other crabs,” he says.

The way Hines and his team figured out what had killed or maimed the freed crabs was by looking for clues in their remains. If the crabs were killed by fish, other research suggested that there would be no crab remains left at the end of the tether line. Instead, it would be a fish—”sort of like fishing with a live bait,” Hines explains. But if the crabs were attacked by their own kind and their shell-crushing pincers, you’d expect to find bits of “shield”—crab shell—or a damaged crab at the end of the row.

At the end of the study period, slightly more than 40 percent of all tethered young crabs that fell into the river showed signs of predation. Of those, about 56 were killed “with remains” left on the line, the authors found, and 41 percent were left “alive and injured” — both smoking guns for a cannibalistic crab culprit. In only 3 percent of cases did the crab disappear entirely – but without a fish at the end of the line, the researchers could not directly attribute these disappearances to any specific cause. (Yet, even in these cases, the predators were assumed to be adult crabs.)

Hines’ research suggests that estuaries like the Rhode River can provide an “important refuge” for young Bay blue crabs, which try to survive by burrowing into the sediment. Fish tend to be visual predators, he says, while blue crabs use “chemical and tactile cues” — they dig around in the sediment to hunt, which in some areas can make them better at uncovering a hidden young crab.

The results could help fisheries better assess the blue crab population in the Chesapeake Bay, information that is important to another known crab predator: us. In fact, an estimated 50 percent of all blue crabs harvested in the United States for human consumption come from the Chesapeake Bay.

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