Dr. David Johnson, an ecologist at the Virginia Institute of Marine Science at William and Mary, has spent more than 20 years in salt marshes at locations throughout the eastern and Gulf Coast of the United States. But while doing research in a Virginia salt marsh at low tide last September, he and his colleagues saw something they’d never seen before—blue crabs waiting for fiddler crabs from shallow, water-filled pits.
“It was amazing because here was an aquatic predator — one that lives, eats, breathes and reproduces underwater — feeding off the water,” Johnson says. “It was like crocodiles stalking wildebeest in Africa.” He reports their discovery in the September issue of Ecology.
Johnson and colleagues—VIMS PhD student Serena Whittingham; VIMS Laboratory and Research Associate Leah Scott; and Dr. Cora Baird of the University of Virginia — believe these low-tide pit ambush-style attacks are the first recorded for blue crabs or any other swimming crab, except for an earlier anecdotal account by Dr. Richard Hurd University of Southern Mississippi. The genus name of the blue crab CallinectesGreek for “beautiful swimmer”, attests to his aquatic nature.
“It was really hot — 95 degrees — and the tide wasn’t going to come back for another 3 hours,” Johnson says. “But this water crab had devised a way to feed at low tide: dig shallow pits that fill with water and wait for the prey to come to you. One crab was 70 meters from the shoreline. That’s 800 body lengths. it’s like diving a mile underwater and hiding behind a rock to stalk a swimming fish.”
The researchers watched the blue crabs emerge from the muddy camouflage of their burrows, stalk and grab a fiddler crab, then run back to the burrow to devour their prey, leaving the male fiddler crabs’ large claws to litter the edges of the burrows. Johnson says it looked “like the discarded bones of peasants outside a dragon’s lair.”
Both crustacean and fiddler crab scientists have long known that blue crabs feed in the salt marsh during High high tide; fiddler crabs usually respond by retreating to their burrows during peak tides to avoid being eaten by their aquatic cousins. But scientists have long thought that at low tide, the exposed marsh surface provides a refuge for fiddler crabs, where these semi-terrestrial crabs can feed on detritus and algae without worrying about birds.
“Blue crabs have been known to dash several feet onto land to grab fiddler crabs before returning to the water to dismember and eat them,” says Johnson, “But the behavior we saw was different. Blue crabs did not pursue their prey on land; they waited on land for their prey to come to them. It would be like going to an Italian restaurant and suddenly being dragged under the table by a giant octopus.”
The discovery raised a wave of questions. How common is this behavior among blue crabs and how successful? Do they dig the pits or rely on existing depressions? How do blue crabs cope with the risks of shore hunting, such as exposure to common fiddler crab predators such as egrets and egrets? Could other aquatic creatures use a similar hunting strategy?
To begin answering these questions, Johnson returned to the same marsh two weeks after the initial observations to record blue crab densities, sizes and attacks. That follow-up visit, plus subsequent surveillance video, confirmed the behavior and revealed more details. For one thing, most of the crabs (83%) were young. He also found that most of the burrows were usually not much wider or deeper than the blue crabs, suggesting that they dug the burrows themselves. This was confirmed by videos showing the crabs scooping up mud with their claws. But blue crabs were not loyal to their burrow and would move to an empty – or water-filled footprint – and drive out another blue crab if necessary.
Of the 33 attacks captured on 37 hours of video, 11 (33%) were successful. This is three times more effective than a polar bear and about the same success rate as a domestic tabby. And the blue crabs’ muddy camouflage and immobile waiting seem to reduce their own vulnerability. “A laughing gull, a known blue crab predator, was walking within inches of a blue crab in a pit but didn’t seem to notice it,” says Johnson. He plans future tethering and video research to test this hypothesis more rigorously.
The scientists also plan to investigate another tantalizing observation from their initial fieldwork, which is actually focused on another crustacean, the purple swamp crayfish Sesarma reticulatum. “Sergeant creates bare areas in the salt marsh by grazing on cordgrass,” Johnson says. He thinks this more open landscape may in turn help the blue crabs by making it easier for them to dig burrows and pursue their prey. The initial measurements support his hypothesis: The researchers found nearly twice as many blue crabs in grassland areas as in vegetated areas, as well as a higher density of fiddler crabs, the latter observation reinforced by previous research.
The discovery that Callinectes foods in salt marshes suggests that these environments are more important to blue crabs than previously thought. “Our observations highlight how important salt marshes are to blue crab production and to the blue crab fishery,” says Johnson.
He also thinks that the blue crab’s feeding strategy may act as a link between the salt marsh and neighboring waters. “Blue crabs feeding in salt marshes at low tide offer an amazing opportunity to study how predator behavior can affect the movement of energy from one ecosystem to another,” says Johnson. “Just as crocodiles connect the river to the savanna, and grizzlies carry the energy of salmon to the forests of the Pacific Northwest, blue crabs connect the salt marsh to the estuary.”