Page 22 - Purple Shore Crab
At low tide the hundreds of purple shore crabs that earlier scurried about looking for food, disappear under rocks, or into rock crevices, or among seaweeds or creep into tidal pools. Their thin flat bodies allow them to crawl easily into narrow passageways or dig backwards under large rocks. They fold their two heavy claws in front of themselves to seal the entrance to their under-rock shelters. When the tide is out the wet seaweeds and rock shelters protect the shore crabs from the drying sun and from the hungry gulls. As long as shore crabs remain hidden in dark wet places they are fairly safe from the picking and probing of those long sharp bills as the hungry gulls search the shore for food.
On the rocks just below the mussel beds live the purple or orange sea stars. When covered with seawater sea stars move and hold onto rocks with their many tube feet. At the tip of each tube foot is a water bulb that forms a suction cup, something like a bathroom plunger. When sea stars move the tub feet work as plungers and levers to pull and shove their bodies forward.
This star has a big appetite and eats huge numbers of mussels. When attacking a mussel it uses its tube feet to pull and pull and pull, until slowly the mussel tires and the two shells start to open. Then, quick as a wink, the sea star slips its bag-shaped stomach from itself and into the mussel. It may take two or three days, but the hungry sea start digests the mussel right inside its own shell.
When the tide goes out to sea, that purple or orange sea star below the mussel beds no longer glides gracefully over the rocks or through tidal pools. A sea star out of water stops moving. A sea star needs water to support its large heavy body, and water to make its arms and tube feet work. Before the tide goes out a sea star takes in seawater and firmly attaches its tube feet to rocks or to whatever it happens to be eating. As long as the tide is out the sea star remains firmly attached and its hard spiny skin keeps the wetness in. Should a sea star become stranded by the falling tide, its long colorful arms hang down as if the animal is dead. A limp sea star is easy to flip over, and is easy food for gulls and other shorebirds.