Green Crab Monitoring
Native shore crabs of Arness Park and Slough
Purple shore crabs have polka dots on their claw arms and a smoother body shape. (Pictured above & below right.)
Useful links
Visit Washington Sea Grant’s website for more extensive information about native crab species, European green crabs, and how you can help forestall their invasion.
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Visitors to Arness Park and the estuary across South Kingston Road (AKA “the Slough”) will encounter numerous small crabs in the shallows. These aren’t babies of the larger Dungeness and red rock crabs found deeper in the cove. And they aren't the infamous invasive European green crab that has been in the news and that volunteers at more than 50 sites around Puget Sound (including "the Slough") have been watching out for as part of an 'early warning' system Washington Sea Grant's Crab Team has been managing to prevent their invasion of and settlement in Puget Sound.
Rather the small crabs in the estuary and cove are adults of two species with many common names, known to biologists as Hemigrapsus oregonensis (AKA the hairy, green or Oregon shore crab) and Hemigrapsus nudus (AKA the purple or naked shore crab). These species both reach a maximum of 2 inches across at the widest part of their shell or carapace and come in an overlapping variety of colors, making them difficult to tell apart. However, some distinctive features and their different ecologies – ways of living – can help distinguish them. Hairy shore crabs are usually mottled greenish brown, sometimes with lighter patterning. The easiest way to distinguish them from purple shore crabs (which can be green, grey or reddish purple) is by the crew-cut hairs on their walking legs. Purple shore crabs have “nude” legs and distinctive purplish polka dots on their claw arms. Hairy shore crabs better tolerate fresh water, so are more numerous in the estuary where salinity fluctuates widely with rain and tide. Where there are no rocks or logs to hide under, they dig burrows in the mud for shelter when the tide goes out. Purple shore crabs are more commonly found in the saltier water of the cove and shelter under rocks rather than dig burrows. Both species feed primarily on algae but will also eat dead animals. Both are food for raccoons, shorebirds, seabirds, anemones and sculpins. You can avoid getting pinched and still check out these crabs by looking for “molts”, the entire exoskeletons they cast off as they grow (including legs!). Sometimes mistaken for dead crabs, molts are lighter weight and don’t stink. When a crab is ready to “shed its skin”, it grabs onto something with its claws and backs out of its old exoskeleton, leaving it to wash ashore with the tide. Molts turn reddish, then white in the sun. It is harder to see hairs on hairy shore crab molts, but if you find purple shore crab molts with claws still attached, the purple-pink polka dots stand out.
Stillwaters’ volunteers monitor crabs in the estuary monthly from April to September as part of a state-wide effort to control invasive European green crab, which is a serious concern for shellfisheries. Fortunately, in seven years of monitoring, these invaders have not yet been found in Kingston!
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Please keep an eye out for European green crabs both in the water and as molts (cast-off shells) on the beach. They can be most easily distinguished from our native crabs by the number of ‘marginal teeth’ behind each eye. Green crabs have five on each side, compared to only three on the shore crabs and ten on the ever popular Dungeness Crab. Crab Team asks you NOT to kill any suspected green crabs you may find but rather contact their Green Crab Hotline: 360-358-3561 and take a photo before releasing it to email to [email protected].
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How you can help!
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