I first heard about the Asian shore crab (Hemigrapsus sanguineus) several years ago. Dr. Nancy J. OíConnor, a biologist at the University of Massachusetts Dartmouth, was monitoring the spread of its population in southern New England. I started looking for the animal along the cobble, rocky beaches on the Cape. The crab hides under these pebbles and stones, similar to other species such as its competitor, the green crab (Carcinus maenas).
The Asian shore crab is easy to spot with its three spines along its carapace or shell. The stripes on its legs alternate from yellow to brown. The crab is no larger than two inches across, and feeds on algae and small mollusks along the shoreline. Like other crustaceans the animal grows by molting, a process in which the carapace splits in the back and the growing crab emerges from the tail end.
Similar to many invasive marine organisms, the Asian shore crab was introduced into this area in the ballast water of ships, presumably from Japan. Such species also have an enormous reproductive rate in which a female crab can release thousands of eggs up to four times a year. The fertilized eggs are easily spread to other areas because the larvae can remain part of the floating plankton for several weeks.
It didnít take long for me to find this species on the Cape. At first I noticed their population was limited to small, specific areas like one side of a jetty, or under a small pile of rocks in a mudflat. However, recently I have seen dozens of these animals congregating in a larger area such as the cobble beach on the Sandwich side of the canal jetty. At one point these habitats were primarily occupied by green crabs, but these animals seem to be out-competed by the more productive Asian shore crab. The irony is, of course, that the green crab was also introduced to these waters.
Itís not clear yet whether or not other animals have developed an appetite for the Asian shore crab. Still, itís interesting to observe these intertidal changes over time. Could there be a market for this animal? After all, the introduced green crab and periwinkle are eaten in other parts of the world, particularly Europe. Or are we witnessing the spread of an invasive species that will eventually drive others to extinction? Maybe several shorebirds will acquire a taste for this crab, resulting in an increase in birds. Time will tell as to whether or not the Asian shore crab becomes another well-adapted species to the Capeís environment, or a potential ecological disaster.
Copyright Gil Newton 2009 Thanks to Chris Dumas for logo image.