It’s a crab-eat-crab world for the Chesapeake Bay’s juvenile blue crabs (Callinectes sapidus). Literally. Cannibalism is the number one killer of the crustaceans that congregate in mid-salinity waters like coastal estuaries. As a result, the blue crabs are relying on the safety of some threatened shallow water habitats, according to a study published today in the journal Proceedings of the National Academy of Science (PNAS).
The lives of blue crabs are anything but boring. They spend about two months floating around the ocean as larvae, only to be swept back into the lower Chesapeake Bay in Maryland to change into juveniles. While there, they rely on sea grass to keep them safe from predatory fish. Once they grow up about one inch, many will migrate north to mid-salinity water, where the seagrass and fish are more scarce. However, another enemy is waiting in the wings—larger blue crabs.
“Blue crabs are notoriously cannibalistic,” said Anson “Tuck” Hines, a study co-author, marine biologist and director emeritus of the Smithsonian Environmental Research Center (SERC), said in a statement.
According to Hines, while cannibalism is common throughout aquatic ecosystems, long-term studies trying to quantify the phenomenon are rare. That type of data could help keep the blue crab fishery sustainable.
This new study took place over 37 years. Hines and his team sought to find out what exactly is eating young blue crabs and what makes them less vulnerable. It began in 1989 in the Rhode River. The mid-salinity tributary of Chesapeake Bay is located at SERC’s campus in Edgewater, Maryland.
The team attached juvenile crabs to small metallic rods in the water, with a three-foot-long tether so that the crabs could move around. Tethering allows the juvenile crabs to stay buried in the sediment, while avoiding predatory fish that hunt by sight. Even so, buried juvenile crabs are still vulnerable to large crabs. Instead of using vision like fish, larger crabs use chemical cues to detect prey that is buried. After 24 hours, they returned to see how the crabs had fared. Roughly 74 percent survived and over half were released back into the bay.
The team also detected signs of cannibalism on 42 percent of the crabs. The living crabs had injuries associated with cannibalism, while those that were killed had some remains left behind. There was no evidence of fish predators, suggesting that mid-salinity rivers do offer a refuge from some blue crab predators.
“We were amazed to find that over our 37-year study, cannibalism accounted for all of the predation, and we found no fish predation on tethered crabs,” Hines said.
“We’ve recorded a few of the tethering experiments with a high-resolution sonar,” added study co-author and research ecologist Matt Ogburn. “In the sonar videos, most fish didn’t show any interest in the tethered crabs, and only adult crabs attacked them.”
Compared to the medium or large juveniles, the smallest crabs were more than twice as likely to get eaten. However, they are much safer in shallow waters. A small juvenile crab was 60 to 80 percent likely to get eaten in deeper waters—1.3 to 2.5 feet deep. In the shallowest zones of half an inch to half a foot, they were roughly 30 percent likely to be cannibalized. Safety in the shallows also held true for larger juveniles.
With this data, scientists are developing a new stock-assessment model for blue crabs in the Chesapeake that is due out later this year. The long-term data can help make the new model more accurate, especially the finding that most cannibalism is focused on smaller juvenile crabs.
”Knowing that juveniles become less susceptible to cannibalism as they grow is important for getting the new model right,” said Ogburn.
However, shallow-water sanctuaries are at risk. Seawalls, layers of large stones to protect soil from erosion, and other human-made projects to harden the shoreline are shrinking the nearshore territory where young blue crabs are known to escape. Invasions by non-native species, such as blue catfish, may also impact blue crabs in these refugee areas.
“The nearshore shallows of the mid-salinity zone provide juvenile crabs with a crucial refuge habitat from cannibalism by large crabs,” Hines said.
The team concludes that with so much cannibalism and predators in all of the bay, protecting these nearshore habitats is critical for stabilizing blue crab numbers and protecting the blue crab fishery.
