Crafty crabs

Researchers in California got a surprise when the invasive European green crabs they were attempting to eradicate made a startling comeback. One year after about 90% of the crabs from a California estuary had been removed, the number of crabs had increased 30-fold, according to a study led by the University of California, Davis.

How did this happen? The study found the population explosion was partly caused by the fact that adult decapod crustacea — such as shrimp, lobster and crab — typically cannibalize younger individuals. When most of the adults were removed from the scenario, the juveniles grew unchecked and overcompensated for the loss of adults.

"We slapped our foreheads at the time, but with thought and understanding, it's told us a lot about what we shouldn't be doing and provided a way forward for us," said lead author Edwin Grosholz, a professor and ecologist at the University of California, Davis. "The world should get less focused on total eradication and work toward functional eradication."

The Christmas Island red crab usually lives in sheltered forested areas. But once a year, usually around October and November, they migrate to the seashore to breed. During this time, the crabs create a sea of red in the island's pedestrian areas. The migration is linked to the phases of the moon and allows the crabs to take a dip in the ocean to replenish their body moisture and salts.

Baby Christmas Island red crabs hatch in the ocean, but most larvae never make it out of the water before being eaten by predators like fish, manta rays and whale sharks. Sometimes no baby crabs survive, but once or twice a decade a huge number persevere. Measuring just 5 millimeters (1/4 inch) across, many die on the journey inland. Some locals help keep them safe by brushing them off the roads.

Rather than navigating by what they see, crabs can navigate by what they hear. But a study using 12 European shore crabs found that they were able to learn how to navigate a maze. Over four weeks, researchers tested the crabs' ability to make their way through a maze using a food incentive. In week one, the median number of incorrect turns was 3.5 and this decreased to just one by week four!

Decorator crabs dress to impress, but while the end result can be dazzling the real reason is a little more sinister. These kinds of crabs attach different sorts of materials to their bodies to provide camouflage or scare off predators. Some use seaweed and shells to adorn their shells, while others use sponges and anemones. The results can be beautiful and sometimes hilarious.