'Hide or get eaten,' urine chemicals tell mud crabs

January 8, 2018, Georgia Institute of Technology

Psssst, mud crabs, time to hide because blue crabs are coming to eat you! That's the warning the prey get from the predators' urine when it spikes with high concentrations of two chemicals, which researchers have identified in a new study.

Beyond decoding crab-eat-crab alarm triggers, pinpointing these compounds for the first time opens new doors to understanding how chemicals invisibly regulate marine wildlife. Insights from the study by researchers at the Georgia Institute of Technology could someday contribute to better management of crab and oyster fisheries, and help specify which pollutants upset them.

In coastal marshes, these urinary alarm chemicals, trigonelline and homarine, help to regulate the ecological balance of who eats how many of whom—and not just crabs.

Blue crabs, which are about hand-sized and are tough and strong, eat the mud crabs, which are about the size of a silver dollar, and thin-shelled. Blue crabs also eat a few oysters, but mud crabs eat a lot more oysters than they do. When blue crabs are going after mud crabs, the mud crabs hide and stay still, so far fewer oysters get eaten than usual.

Humans are part of the food chain, too, eating oysters as well as blue crabs that boil up a bright orange. The blue refers to the color of markings on their appendages before they're cooked. Thus, the blue crab urinary chemicals influence seafood availability for people, as well.

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Are tasty blue crabs creeping North?

March 7, 2015

Provided by Diana Kenney, Marine Biological Laboratory

WOODS HOLE, Mass.–David Johnson was standing in a salt marsh tidal creek north of Boston, Mass., when he scooped up a blue crab, Callinectes sapidus, 80 miles north of its native range. The northern migration of this commercially important species, Johnson says, could be yet another sign of climate change. Then a scientist at the Marine Biological Laboratory (MBL) Ecosystems Center, Johnson recently published his observations in the Journal of Crustacean Biology.

The historic northern limit of this species of crab (also called Atlantic blue or Chesapeake blue) is Cape Cod, Mass. They typically weren’t found in the Gulf of Maine due to its cold Canadian waters. From 2012 to 2014, however, scientists and resource managers observed blue crabs as far north as northern Maine and Nova Scotia, Canada. Johnson hypothesizes that warmer ocean temperatures in 2012 and 2013, which were 1.3°C higher than the previous decade’s average, allowed the crabs to move north.

Climate change presents a challenge

“Climate change is lowering the thermal barriers that kept species from moving toward the poles,” he says. “Climate change presents a challenge not only for ecologists, but for fisheries managers as commercially important species shift their ranges in response to warming oceans.”

Ephemeral populations of blue crabs have been documented previously in the Gulf of Maine. Johnson notes that in the 1950s blue crabs were observed in the gulf during a time of warmer waters. But once the waters returned to average temperatures, the crabs disappeared.

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