Last September, ecologist David Johnson and his colleagues were at a Virginia salt marsh at low tide. There, they observed some unexpected behaviour by an aquatic predator.

They witnessed blue crabs waiting in shallow, water-filled pits, stalking and ambushing fiddler crabs above land, at low tide.

After capturing their prey, they would carry it back to the pit to consume it, then discard the large claws of the fiddler crab at the edge of the pit.

"It was amazing because here was an aquatic predator—one that lives, eats, breathes, and breeds under water—feeding out of the water," said Johnson, who is from William & Mary’s Virginia Institute of Marine Science.

He and his team published their findings in the September issue of Ecology journal.

At a follow-up visit to the same marsh a fortnight later, Johnson observed more instances of such behaviour. He observed that 83 percent of the blue crabs were juveniles. It was believed that the pits were dug by the crabs themselves, as they were about the crabs’ size, and there was video footage showing them scooping out mud with their claws.

In addition, they did not always stick to the same pit, and would move to an empty one if available, or even evict another blue crab from their own pit.

Based on 37 hours of video footage, the success rate of the attacks were 33 percent (11 out of 33).

"Our observations underscore how vital salt marshes are to blue crab production and the blue crab fishery," said Johnson. He plans further research with more video studies and future tethering to test his hypothesis.