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Blue Crab’s strange feeding habits caught on unprecedented video: “It was really hot—95 degrees”

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It was amazing to witness an aquatic predator—one that breeds, lives, and feeds solely underwater—eating out of the water at a scorching-hot temperature of 95 degrees.

The Virginia Institute of Marine Science’s Dr. David Johnson has spent more than 20 years studying salt marshes at locations all along the East and Gulf coastlines of the United States.

But last September, while conducting research in a salt marsh in Virginia at low tide, he and his colleagues witnessed something unique: blue crabs ambushing fiddler crabs from small, water-filled pits.

It was incredible to see an aquatic predator—one that breeds, lives, and eats underwater—feeding from the water, according to Johnson.

“It was like crocodiles ambushing wildebeests in Africa.”

The findings are detailed in his article for Ecology’s September issue.

With the exception of an earlier anecdotal account from Dr. Richard Heard of the University of Southern Mississippi, Johnson and colleagues—VIMS PhD student Serina Wittyngham, VIMS Laboratory and Research Associate Leah Scott, and Dr. Cora Baird of the University of Virginia—believe these ambush-style attacks from pits at low tide are the first ever documented for the blue crab or any other swimming crab. The blue crab’s aquatic nature is indicated by its genus name, Callinectes, which is Greek for “beautiful swimmer.”

“It was really hot—95 degrees—and the tide wasn’t going to return for another 3 hours,” explains Johnson. “But this aquatic crab had figured out a way to feed at low tide: dig shallow pits that fill with water and wait for prey to come to you. One crab was 70 meters away from the shoreline. That’s 800 body lengths. It’d be like if I dove a mile underwater and hid behind a rock to ambush fish that swam by.” 

The experts watched as the blue crabs emerged from the muddy hiding of their pits, pursued and snatched a fiddler crab, and then scurried back to the pit to eat their food, leaving the rims of the pits covered in the enormous claws of the male fiddler crabs. According to Johnson, it appeared to be “like the discarded bones of villagers outside a dragon’s lair.”

Fiddler crabs often respond to tidal peaks by returning to their burrows to avoid being eaten by their aquatic cousins; this behavior has been observed by both crustacean biologists and fiddler crabs for a long time. Blue crabs dine within the salt marsh during high tide.

However, for a long time, scientists believed that during low tide, the exposed marsh surface served as a fiddlers’ haven, where these semi-terrestrial crabs could eat on waste and algae with only birds as a concern.

According to Johnson, “blue crabs have been known to dash a few feet onto land to snatch fiddler crabs before returning to the water to dismember and eat them.” 

However, the behavior they observed was “different”. 

“Blue crabs were not chasing their prey on land; they were waiting on land for their prey to come to them. It’d be like if you went to an Italian restaurant and were suddenly dragged under the table by a giant octopus.” 

It sparked many questions. How often do blue crabs do this, and how well does it work? Do they dig the pits themselves or rely on pre-existing depressions? How can blue crabs manage the dangers of onshore hunting, such as coming into contact with common fiddler-crab predators like herons and egrets? Could this hunting technique be used by other aquatic creatures?

Two weeks following the initial observations, Johnson went back to the same marsh to record blue-crab populations, sizes, and attacks in an effort to start answering those questions. This later visit and subsequent trail cam video corroborated the behavior and provided more information. For starters, the majority of the crabs (83%) were young. The fact that the majority of the trenches were often not much broader or deeper than the blue crabs further supported the theory that they had excavated the pits themselves.

This was supported by video footage that showed the crabs using their claws to remove the muck. However, the blue crabs were not devoted to their hole, and if necessary, they would evict another blue crab and move into an empty one or a footprint covered in water.

On 37 hours of video, 33 attacks were recorded, and 11 (or 33%) of them were successful. That’s roughly the same success rate as a domestic tabby and three times as effective as a polar bear.

In addition, the blue crabs’ murky camouflage and immobility appear to lessen their vulnerability. Johnson notes that “a laughing gull, a known blue-crab predator, walked within centimeters of a blue crab in a pit, but didn’t appear to notice it.”  He intends to do additional tethering and video investigations to rigorously evaluate this theory.

Another intriguing finding from their original fieldwork, which was actually centered on a different crustacean, the purple marsh crab Sesarma reticulatum, will also be investigated, according to the researchers. By grazing on cordgrass, sesarma “creates denuded areas in the salt marsh,” claims Johnson.

He believes that the blue crabs may benefit from a more open environment since it will be simpler for them to dig pits and hunt for their fiddler crab prey.

Initial measurements support his theory. The study discovered almost twice as many blue crabs in grazed areas as in plant-filled areas. They also found more fiddler crabs, which was supported by earlier research.

The fact that Callinectes feeds in salt marshes shows that blue crabs are more dependent on these settings than previously believed. According to Johnson, “our observations underscore how vital salt marshes are to blue crab production and the blue crab fishery.”

Additionally, he believes that the blue crab’s eating habits may serve as a connection between the salt marsh and nearby waterways. At low tide, Johnson notes, “blue crabs feeding in salt marshes at low tide offer a fascinating opportunity to study how predator behavior can affect the movement of energy from one ecosystem to another.”

“Just like crocodiles link the river to the savanna, and grizzlies carry the energy of salmon into the forests of the Pacific Northwest, blue crabs connect the salt marsh to the estuary.”

Source: 10.1002/ecy.3787

Image Credit: Stil from the video

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