‘Pretty swimmer’ attack at low tide

David Johnson, an ecologist at the Virginia Institute of Marine Science at William & Mary, has spent more than 20 years in salt marshes at sites all along the US East and Gulf Coasts. But while doing research in a Virginia salt marsh at low tide last September, he and his colleagues noticed something they had never seen before – blue crabs attacking fiddler crabs from shallow, water-filled pits.

“It was surprising because here was an aquatic predator—one that lives, eats, breathes, and reproduces underwater—comes out of the water,” Johnson says. “It was like crocodiles ambushing wild animals in Africa.” He reports his discovery in the September issue. ecology,

Johnson and colleagues – VIMS PhD student Serina Wittingham; VIMS Laboratory and Research Associate Leah Scott; and Dr. Cora Baird of the University of Virginia—believed that these ambush-style attacks from pits at low tide are the first recorded for a blue crab or any other swimming crab, except for an earlier anecdote by Dr. Richard Heard. . University of Southern Mississippi. blue crab genus name callinectesGreek for “beautiful swimmer”, attesting to its aquatic nature.

“It was really hot – 95 degrees – and the tide was not going back for another 3 hours,” Johnson says. “But this aquatic crab had found a way to feed at low tide: dig shallow pits that are filled with water and wait for its prey to arrive. One crab was 70 meters off the shoreline. It had a body length of 800 It is as if I sank a mile underwater and hid behind a rock to attack the fish that swim.”

The researchers observed that the blue crabs emerge from the muddy camouflage of their pits, chase and snatch a fiddler crab, then move back to the pit to eat their prey, allowing the larger claws of the male fiddler crabs to form the pits. Trash the edges. Johnson says it “looked like the abandoned bones of villagers outside a dragon’s lair.”

Crustacean scientists and fiddler crabs have both long known that blue crabs feed during salt marshes. High tide; Fiddler crabs typically react by retreating into their burrows during tidal peaks to avoid being eaten by their aquatic cousins. But scientists have long thought that during low tide, the exposed marshy surface provided a fiddler’s refuge, where these semi-terrestrial crabs may feed only on detritus and algae, along with birds not to worry about. For.

“Blue crabs have been known to snatch fiddler crabs a few feet to the ground before returning to the water and eat them,” Johnson says, “but the behavior we observed was different.” The blue crabs were not chasing their prey on the ground. , They waited on the ground for their prey to come to them. It would be like you went to an Italian restaurant and were suddenly pulled under the table by a giant octopus.”

This discovery sparked a flurry of questions. How common is this behavior in blue crabs, and how successful is it? Do they dig pits or rely on existing pits? How do blue crabs deal with the risks of shore hunting, such as exposure to common fiddler-crab predators such as herons and herons? Could other aquatic creatures use similar hunting tactics?

To begin addressing those questions, Johnson returned to the same quagmire two weeks after initial observations to record blue-cake density, size and strikes. This follow-up visit, as well as subsequent video from the trail cam, confirmed the behavior and revealed more details. For one, most crabs (83%) were juveniles. They also found that most pits were generally no wider or deeper than blue crabs, suggesting that they dug the pits themselves. This is corroborated by video footage, which shows the crabs scooping out mud with their claws. But the blue crabs were not loyal to their pit and went into an empty pit – or footprint filled with water – and drove out another blue crab if necessary.

Of the 33 attacks captured on 37 hours of video, 11 (33%) were successful. It is three times more efficient than the polar bear and has about the same success rate as the domestic tabby. And the blue crabs’ muddy camouflage and motionless waiting seem to reduce their own vulnerability. “A laughing gull, a known blue-crab hunter, went to within a centimeter of a blue crab in a pit, but didn’t notice it,” says Johnson. He plans to have future tethering and video studies to test this hypothesis more rigorously.

The scientists also plan to explore another tantalizing observation from their initial fieldwork, which actually focused on another crustacean, the purple marsh crab. sesarma reticulatum, ,sesarma creates fragmented areas in salt marshes by grazing on cordgrass,” Johnson says. He thinks this more open landscape may in turn help blue crabs make it easier for them to dig pits and chase their fiddler crab prey. Preliminary measurements seem to support his hypothesis: the researchers found almost twice as many blue crabs in grazing areas as in areas with plants, and also higher fiddler crab densities, a later observation strengthened by previous research.

find that callinectes Feeds in salt marshes suggest that these environments are more important for blue crabs than previously thought. “Our observations underscore how important salt marshes are to blue crab production and blue crab fisheries,” Johnson says.

He also thinks that the feeding strategy of the blue crab may act as a link between the salt marsh and the surrounding waters. “Blue crabs that feed in salt marshes at low tide provide an attractive opportunity to study how predatory 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 salmon’s energy to the forests of the Pacific Northwest, blue crabs connect the salt marshes to the estuary.”


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