Although the beach season is over for many folks, I feel exactly the opposite. I tend to avoid the crowds in the summer, and explore beaches during the colder weather months when they’re mostly empty. The parking is free, the birding is great, and beachcombing is absolutely a year-round activity. On a recent walk along Scusset Beach in Sandwich, I came across many crab shells washed up on the beach, the most numerous of which were the shells of the Atlantic rock crab (Cancer irrotatus) – one of the commonest shallow-water crabs in New England. 

Atlantic rock crabs can be identified by their broad, fan-shaped carapace, which has three low teeth between the eyes and nine mostly smooth teeth on either side of the eye socket. A similar species, the Jonah crab (Cancer borealis) has a more oval carapace, three low teeth between the eyes and nine teeth with distinctly serrated margins lateral to each eye socket. Jonah crabs can also grow larger than Atlantic rock crabs, maxing out at approximately 6 inches rather than 5, but size is certainly not the best distinguishing feature due to a great deal of overlap between the sizes of these two species. 

Despite the term “rock” in its name, it turns out that Atlantic rock crabs are not restricted to rocky habitats. In fact, this was actually a long-held misconception of mine due to the crab’s name, and I was always perplexed at the number of their shells I would find in areas with no rocky habitat nearby. In actuality, adult Atlantic rock crabs are more likely to be found in sandy or muddy substrate. Although research has shown that they do have a preference for rocky habitats, there are at least two reasons why they’re more commonly found in sandier habitats. First, soft substrates are more common than rocky habitats in this area, as well as farther south in the crab’s range. Second, where rocky areas do exist, Atlantic rock crabs tend to be outcompeted by more aggressive Jonah crabs. It is worth noting that the competition is for shelter, not for food. Jonah crabs are sub-par burrowers and rely on rocky cavities for shelter. Atlantic rock crabs, on the other hand, are fairly efficient burrowers in sand. 

The rock crab fishery was historically quite important – in the early 1900s up to 8 million crabs were brought to local markets in a good year. Today the rock crab catch on the Atlantic coast is mostly limited to incidental crabs caught in lobster pots. However, as other fisheries decline, large standing populations of these crabs and the fact that their meat is said to rival that of the popular blue crab in flavor will likely lead to an increased market for these crabs in the future.