You might not think of the lobster you're munching as a vicious predator but green crabs would beg to differ. Researchers recently discovered that, much to their surprise, lobsters (aka Homarus americanus), which troll the ocean floor along the east coast of North America from Labrador to North Carolina, move inshore at night with the high tide, hunting for prey.

"We shone our lights around and there were lobsters everywhere, all cruising around the intertidal zone," says Patricia Jones, a recent Cornell University graduate now headed to a graduate program in ecology evolution and behavior at the University of Texas at Austin. Jones is co-author of a paper reporting the findings in Journal of Experimental Marine Biology and Ecology.

The intertidal zone is usually underwater, and especially so at high tide, but can be exposed during neap (lowest) tide. It had been thought that adult lobsters were not major predators there, but rather stayed mostly in the subtidal zone, which is nearly always underwater.

What was known about Atlantic or American lobsters came mostly from catching them in deeper waters, or observing the young ones hiding under rocks along the shoreline. But Jones discovered an abundant population of the adult critters while diving at night around Appledore Island off the Maine coast. The adults were nowhere to be seen at low tide or during the day. This suggests, she says, they follow the rising and dipping tides when night falls in search of food.

Jones and co-author Myra Shulman, a marine biologist and her former professor at Cornell, say the lobsters were probably chasing green crabs (Carcinus maenas). Larger lobsters tend to eat more crab, which aids in their growth and reproduction.

Some of the lobster's distant relatives demonstrate similar behavior. Four years ago, Carlos Robles, a biologist at California State University, Los Angeles, reported in Oecologia that he found crayfish (also called spiny lobsters) roaming the Pacific shoreline at night during high tide in search of mussels to munch. That cast doubt on the "refuge hypothesis," a long-held view of wave-beaten rocky shores as refuges from predation for sedentary species, such as mussels, that make their home there. Jones says it also raised the question of whether Atlantic lobsters do the same.

"People just weren't looking at night at high tide, when the community is covered in water and things move up from the subtidal," Shulman says. "So the lobsters weren't thought to have any role there at all."