Dr. David Johnson has spent more than two decades in salt marshes on the East and Gulf coasts. He and his colleagues saw blue crabs ambushing fiddler crabs from shallow, water-filled pits while doing research in a Virginia salt marsh.
An aquatic predator that lives, eats, breathes, and breeds under water was feeding out of the water. It was similar to crocodiles ambushing wildebeests. They were reported in the September issue of Ecology.
The first ambush-style attacks on the blue crab are thought to have been recorded by Johnson and colleagues. The blue crab's name Callinectes is Greek for beautiful swimmer.
The tide wasn't going to come back for another 3 hours because it was so hot. The aquatic crab was able to feed at low tide by digging shallow pits that filled with water and waiting for prey to come to you. The crab was close to the shoreline. There are 800 body lengths. If I dove a mile underwater and hid behind a rock to ambush fish, it would be like that.
It looked like villagers' bones were thrown away.
Both crustacean scientists and fiddler crabs have known for a long time that blue crabs feed in the salt marsh during high tide, and that fiddler crabs respond by retreating to their burrows to avoid being eaten by their aquatic cousins. During low tide, the exposed marsh surface provides a fiddlers' refuge, where these semi-terrestrial crabs can feed on things other than birds.
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, but the behavior we saw was different. Blue crabs were not chasing their prey on the ground, they were waiting for their prey to come to them. If you went to an Italian restaurant and were dragged under the table, it would be like that.
There were a lot of questions about the discovery. How common is this behavior among crabs? They dig the pits or rely on depressions. What do blue crabs do to deal with the risks of hunting on the beach? Is there a similar hunting strategy used by other animals?
Johnson returned to the same marsh two weeks later to record blue-crab densities, sizes, and attacks. The behavior was confirmed by this follow-up visit and video from trail cameras. The majority of the crabs were under the age of 18. He found that most of the pits were not as deep as the blue crabs suggested. Video footage shows the crabs scoop mud out of their mouths. The blue crabs were not loyal to their pit and would move into an empty one, or a water filled footprint, if necessary.
Only 11 of 33 attacks were successful. That is three times more efficient than a polar bear and the same as a domestic tabby. The blue crabs seem to be less vulnerable due to their muddy camouflage. A laughing gull, a blue-crab predator, walked within centimeters of a blue crab in a pit, but didn't seem to notice it. He wants to test this hypothesis more rigorously.
The purple marsh crab Sesarma reticulatum is one of the crustaceans that the scientists intend to explore. The areas in the salt marsh are denuded. He thinks a more open landscape will help the blue crabs by making it easier for them to find their prey. The researchers found almost twice as many blue crabs in grazed areas than in areas with plants and higher densities of fiddler crabs.
The environments in which Callinectes feeds suggest they are more important to blue crabs. Blue crab production and the blue crab fishery are dependent on salt marshes.
The blue crab may act as a link between the salt marsh and adjoining waters. Blue crabs feeding in salt marshes at low tide offer a fascinating opportunity to study how predator behavior can affect the movement of energy.
More information: David S. Johnson, Beautiful swimmers attack at low tide, Ecology (2022). DOI: 10.1002/ecy.3787 Journal information: Ecology
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