Pachygrapsus crassipes at Sunset Bay - MStueve 20190605 002

One of three common shore crabs on the Southern Oregon Coast, lined shore crabs often have very fine, green stripes across their backs. This one was scuttling over the rocks at Sunset Bay last month. 

You will likely see one of them while exploring along the seaside rocks during this week’s very low tides. You might even hear them as they scurry on their hard, sharply-pointed legs over the rocks from crevice to crevice.

Every kid looks for more once one is found scuttling about: shore crabs. Small, somewhat flat and compact, shore crabs are lively and may be out in the open and out of the water. They easily catch your eye -- and ear -- and are tempting to pick up.

We have two common shore crabs on all of Oregon’s beaches: the purple shore crab (Hemigrapsus nudus) and the yellow shore crab (Hemigrapsus oregonensis). A third shore crab is rather common on the rocky beaches of southern Oregon: the lined shore crab (Pachygrapsus crassipes).

All three shore crabs are an inch to two inches across, or smaller. All have square or nearly square backs. All might be seen walking about out of water, usually looking for food.

No surprise on the names: purple shore crabs are usually purple all over, but always have purple spots on the claws. Yellow shore crabs are mottled yellowish to greenish, with pale, spot-free claws. Lined shore crabs are dark with very fine stripes across the back from side to side—often greenish, sometimes reddish. Purple shore crabs have smooth legs, while yellow shore crabs have very “hairy” legs, and lined shore crabs have ribbed legs.

All three of our common shore crabs are omnivores, munching down algae, but occasionally feeding on animal material that’s usually already dead, though not always.

All three can survive great changes in salinity, either in an estuary or in a tidepool subject to rain or evaporation, tolerating water ranging from saltier than seawater to nearly fresh. [Estuaries are places were fresh and saltwater mix, such as Coos Bay.]

All three of these shore crabs are well adapted to the rocky shore, but they do tend to sort themselves out.

While you could find them in the same place, more often rockier places have lined shore crabs and more purple shore crabs, and muddier places have more yellow shore crabs.

How is that?

Remember the hairy legs? Yellow shore crabs are hairier elsewhere, too. The openings to the gills of yellow shore crabs are lined with fine hairs that keep mud from entering and clogging the gills -- and suffocating the crab.

In fact, yellow shore crabs can make the mud their home by digging holes in estuarine marshes. Purple and lined shore crabs apparently never dig holes, although they may use holes made by others.

Additionally, yellow shore crabs are a bit more tolerant of fresher water and water with less oxygen. On the other hand, purple and lined shore crabs are a little larger, are better at avoiding predators, and more tolerant of drying out.

All three shore crabs are “shore crabs” because they can walk around out of the water for quite some time by “holding their water,” so to speak. Shore crabs can live underwater all the time, but they can still respirate when out of the water by “breathing” through the water they hold next to their gills. After wandering around on land for a period, they do have to dip back to replace or replenish the water. Lined shore crabs are best at keeping their gills adequately wetted and have been observed spending more than have their time out of the water.

All that makes purple and lined shore crabs generally more common on the rocky outer ocean coast and yellow shore crabs generally more common in the bays and estuaries. Too, on both the outer ocean coast and in estuaries, lined shore crabs are more common on the upper reaches of the intertidal than purple and yellow shore crabs.

All three of these shore crabs may sometimes compete for shelter at low tide when they share habitat. The claws that all three use to shred and eat algae and other food are also used to fend off competitors as well as predators.

Shore crabs will sometimes pinch people, too -- not particularly hard, but enough to make you drop them and likely hurt them.

Don’t overlook these lively cuties as you’re exploring the upper beach -- though it’s best to enjoy them hands-off.

For information on how you can arrange your own exploration of our fascinating natural history, contact Marty at 541-267-4027, mgiles@wavecrestdiscoveries.com, or www.facebook.com/wavecrestdiscoveries. Questions and comments about local natural history are welcome. www.wavecrestdiscoveries.com

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