MOSS LANDING >> When Ed Ricketts collected data for his seminal 1939 book “Between Pacific Tides,” he described the lined shore crab as a vegetarian and part-time scavenger that feeds primarily on seaweed.

The misconception endured for more than 75 years — until last April when researchers at the Monterey Bay Aquarium Research Institute accidentally left the small crabs in a laboratory tank with some juvenile mussels.

“They ate them, which really surprised us,” said postdoctoral fellow and lead researcher Joshua Lord. “A lot of scientific research and guidebooks, including Ricketts, described them as primarily scraping seaweed off rocks to feed.”

To further test the carnivorous, predatory nature of the lined shore crab, Lord and his team set out 400 juvenile mussels in the intertidal zone this summer.

“As soon as the tide came in, the crabs converged on the mussels. They ate all but three of the mussels within 24 hours and we caught it all on a waterproof camera,” said Lord.

The lined shore crab is the Dr. Jekyll and Mr. Hyde of the intertidal zone. When the tide is out, they scramble about the rocky shore collecting seaweed and occasionally scavenging. But when the tide comes in, the voracious 1- to 2-inch crabs devour young mussels, a variety of sea snails and even juvenile abalone.

“As it turns out, they’re really aggressive, opportunistic predators on some important species — and they’re really common,” Lord said. “They can be absurdly high density. We’ve counted 200 crab in one square meter.”

Considering a single lined shore crab is capable of eating between up to 40 juvenile mussels per day, Lord’s discovery may have important implications for how scientists view the intertidal ecology as a whole.

“They can really alter the number of mussels and other species,” Lord said. “Especially in sheltered places like Moss Landing Harbor.”

Lord said their diet is seasonal, however. The lined shore crabs aren’t capable of eating older, larger mussels. As a result, the mussels are only vulnerable to predation in the summer when they’re young.

The lined shore crab, which is a native species, has few predators because of its speed and agility. However, the abundant crab still faces a murky future.

Much of Lord’s research focuses on how the interaction of intertidal species is changing due to climate change. The lined shore crab is highly vulnerable to the effects of ocean acidification, the ongoing decrease in the pH of the Earth’s oceans, which is caused by the uptake of carbon dioxide from the atmosphere.

“These guys do decline with ocean acidification conditions, which opens the door to some of the other invasive species like green crabs, which aren’t affected,” Lord said.

For information about the Monterey Bay Aquarium Research Institute, visit mbari.org.