National Geographic : 1961 Nov
water is trapped at low tide have the best chance to survive." He went on to explain that plants and an imals living in an environment of alternate submersion and exposure must have the pow er to resist sun and wind drying. Thus, during low water, anemones contract into ugly slime-covered lumps, such as we had seen at Yachats. Bivalves draw in their soft tissues and snap shut. Worms retreat in to casements, and some mollusks hide behind trap doors. Crabs and many other inverte brates take refuge amid seaweed, or vanish into sand or moist crevices. Sea stars and sea urchins have armorlike coverings which help prevent the rapid escape of internal moisture. Exposed limpets, snails, and chitons simply cling tightly to rocks, thus protecting themselves from desiccation. But there are limits. Should these living things be exposed much longer than from one tide to another, deterioration and even death could sweep across this world of be twixt and between. Sea Plants Bend to Surf's Whiplash Several days later my wife and I explored Cape Arago, two miles south of Sunset Bay. On this excursion we noted that, in addition to defense against exposure, dwellers of the intertidal zone must possess incredible re sistance to physical shock. On a ledge less than a dozen yards from where we stood grew a petite forest of up right sea plants, Postelsiapalmaeformis, each a moplike clump of drooping leathery fronds supported by a smooth yellowish-brown trunk about 16 inches high (page 724). Clutch ing "roots," known to the marine botanist as holdfasts, anchored them firmly to the rock. I climbed down to collect one of the closer plants, but pull and jerk as I might, I could not dislodge it. I quickly scrambled back to a higher perch where Eda stood, for an enormous swell was forming not far out. When it struck and ex ploded, every member of the tiny forest yield ed freely to the rush of water, bending and tossing like a palm in a tropical hurricane. When the breaker's force was spent and its water had foamed back into the sea, the little palms were again erect, unbroken and un harmed. Elasticity, in this instance, is the secret of survival against the whiplash of the surf's fury. Other organisms resist by virtue of their hard, tough character. Take those mussels tightly wedged in beds all around us. This 726 Surf-sculptured rocks suggest monoliths species, Mytilus californianus,anchors itself with strong tufts of hairlike filaments called byssus threads. It requires more than ordi nary strength to wrench a mussel free by hand. These common bivalves are sedentary, although they can shift position by loosening the threads on one side and extending new ones on the other. Mussels exposed at low tide keep their valves tightly closed. But when submerged, they gape, enabling the animal inside to strain nutrient plankton from the sea water. Both Pacific and Atlantic mussels are regarded as tasty delicacies, but as with mushrooms, it is wise to eat only those of whose history and purity one is sure. At certain times of the year outbreaks of poisonous plankton may occur.