Ecologists Found Blue Crabs Ambushing Fiddler Crabs from Pits at Low Tide

Dr. David Johnson, an ecologist at William & Mary's Virginia Institute of Marine Science, and his colleagues witnessed something unprecedented in Sep. 2021 when blue crabs ambushed fiddler crabs from shallow, water-filled pits in the Virginia salt marsh. The ecologist was astounded, comparing it to crocodiles ambushing wildebeests in Africa. The research was published in Ecology.

(Photo: Katrin Schulz)
Blue Crab

Blue Crab in Salt Marsh

A study published in Springer states that adult blue crabs move and live in various biogenic habitats, including seagrass meadows, salt marshes, and oyster reefs, when they are foraging and looking for shelter.

The genus name Callinectes, which means "beautiful swimmer" in Greek, attests to the blue crab's aquatic nature.

Clams, oysters, mussels, smaller crustaceans, freshly dead fish, plant and animal detritus, and smaller and soft-shelled blue crabs are all included in a blue crab's diet, according to NOAA Fisheries. Crabs, on the other hand, are eaten by large fish, fish-eating birds like great blue herons, and sea turtles.

Blue Crab finds its Way to Feed on Low Tide

According to Johnson and colleagues, the ambush-style attacks from pits at low tide are the first ever recorded for the blue crab or any other swimming crab.

At 95 degrees, Johnson described the weather as "really hot." He predicted that the tide would not return for another three hours. On the other hand, the aquatic crab had discovered a way to feed at low tide and dug shallow pits that filled with water and waited for prey to come. He said that one crab was 800 body lengths from the shoreline, which is 70 meters away.

Blue crabs emerged from the muddy camouflage of their pits, stalked and snatched a fiddler crab, then dashed back to the pit to devour their prey, leaving the large claws of the male fiddler crabs to litter the pit's edges. According to Johnson, it looked like discarded villagers' bones outside a dragon's lair.

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Ecologists Find the Answer to Questions on the Blue Crab's Hunting Strategy

Blue crabs have been observed sprinting onto land to capture fiddler crabs before returning to the water to dismember and eat them. However, the behavior that the researchers observed was distinct. Blue crabs were not pursuing prey on land. They were waiting for their prey to come to them on land. Johnson described it as going to an Italian restaurant and being dragged under the table by a giant octopus.

The discovery raised a number of questions regarding the blue crab's behavior. The researchers want to know if the crabs dig the pits or rely on existing depressions.

Johnson returned to the same marsh two weeks after the initial observations to record blue-crab densities, sizes, and attacks to begin answering those questions. This follow-up visit and subsequent trail cam video confirmed the behavior and revealed more details.

He discovered that 83% of the crabs were juveniles. He also found that most pits were not much wider or deeper than the blue crabs, implying that they dug the pits themselves. This was supported by video footage of crab claws scooping mud. On the other hand, the blue crabs were not loyal to their pit and would move into an empty one or a water-filled footprint if necessary, evicting another blue crab.

Eleven out of the 33 attacks captured on 37 hours of video were successful. That's three times as effective as a polar bear and roughly the same as a domestic tabby. And the blue crabs' muddy camouflage and motionless waiting appear to make them less vulnerable.

Johnson believes that the blue crab's feeding strategy could serve as a link between the salt marsh and adjacent waters. It provides an intriguing opportunity to investigate how predator behavior affects the movement of energy from one ecosystem to another. Blue crabs connect the salt marsh to the estuary in the same way that crocodiles connect the river to the savanna, and grizzlies carry the energy of salmon into the forests of the Pacific Northwest.