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After 75 years, what was previously known about the lined shore crab… has gone out with the tide.
Since 1939, after Ed Ricketts published “Between Pacific Tides,” it was believed that the crab was a vegetarian and part-time scavenger, who primarily fed on seaweed.
However, last April, researchers at the Monterey Bay Aquarium accidentally stumbled upon the truth. The researchers left some lined shore crabs in a lab tank with some juvenile mussels, and when they came back… the mussels were eaten. Curious? We know.
Joshua Lord is a postdoctoral fellow and researcher who commented, “They ate them, which really surprised us. A lot of scientific research and guidebooks, including Ricketts, described them as primarily scraping seaweed off rocks to feed.”
Lord and his team decided to further test the theory by placing 400 juvenile mussels in the intertidal zone.
The Monterey Bay researcher noted, “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.”
What does this mean? Basically the lined shore crab is the “Dr. Jekyll & Mr. Hyde of the intertidal zone.” When the tide was out, the crabs scooted around the rocky shore to collect seaweed. However, when the tide came in, the crabs would prey upon the young mussels along with sea snails and abalone.
After catching the crabs on camera, Lord commented, “As it turns out, they’re really aggressive, opportunistic predators on some important species — and they’re really common. They can be absurdly high density. We’ve counted 200 crab in one square meter.” Since the lined shore crab is unable to eat the larger mussels, it makes their carnivorous diet seasonal. The crabs are only able to prey upon mussels in the summer when they are juvenile. So the part-time scavenger is also a part-time carnivore.
The majority of Lord’s research focuses on how climate change affects intertidal species. This particular species of crab has few predators due to its speed and agility, but it can’t seem to stay out of the way of ocean acidification. It is highly vulnerable to its effects and will likely see a decline in population.