Seashore, land at the edge of a sea, alternately submerged and exposed by the tides. This land-water environment produces a rich variety of life forms. Tiny plankton (animals and plants floating in the tides) are the basis of shore life, both as larvae for shore animals and as food near the bottom of the food chain. The mollusks, sponges, and other animals that feed on plankton serve as food for shorebirds and other larger animals. Rocky seashores provide surface for seaweeds, mussels, sea urchins, and other organisms to attach themselves. Tide pools left by retreating tides shelter fish and other aquatic animals. Sandy shores are less hospitable, but some clams and crabs can make their home in the loose sand. Where protected bays create muddy shores and sea water mixes with fresh river water, plants thrive and crabs and turtles are common.