Marine intertidal zones-those areas between the highest high-tide and lowest low-tide marks-appear to be difficult places for organisms to live. The rocky shore is subject to crashing waves, rapidly changing temperatures, and predation from both land and sea. Yet, astonishingly, the richness, productivity, and diversity of the intertidal rocky community-especially in the world's temperate zones-are matched by very few other places. There is intense competition for space. Life abounds.

One reason for the great diversity and success of organisms in the rocky intertidal zone is the large quantity of food available. The junction between land and ocean is a natural collection sink for living and once-living material. Minerals dissolved in water running off the land serve as nutrients for the inhabitants of the intertidal zone as well as for plankton in the area. The crashing surf and strong tidal currents keep nutrients stirred and ensure a high concentration of dissolved gases to support a rich population of autotrophs (organisms, like plankton, that are capable of synthesizing their own food from inorganic substances). Many of the larval forms and adult organisms of the intertidal community depend on plankton as their primary food source.

Another reason for the success of organisms here is the large number of habitats and niches to be occupied. The habitats of intertidal animals and plants vary from hot, high, salty splash pools to cool, dark crevices. These spaces provide hiding places, quiet places to rest, attachment sites, jumping-off spots, cracks from which to peer to obtain a surprise meal, footing from which to launch a sneak attack, secluded mating nooks, and darkness to shield a retreat. The niches of the creatures in this community are varied and numerous: Encrusting algae produce carbohydrates, snails scrape algae from rocks, hermit crabs scavenge for bits of food, and octopuses wait in ambush for likely meals.

The most obvious and important physical factor in intertidal communities is the rise and fall of the tides. Organisms living between the high-tide and low-tide marks experience very different conditions from those residing below the low-tide line. Within the intertidal zone itself, organisms are exposed to varying amounts of emergence and submergence. Because some organisms can tolerate many hours of exposure while others can tolerate only a very few hours per week or month, the animals and plants sort themselves into three or more horizontal bands, or subzones, within the intertidal zone. Each distinct zone is an aggregation of animals and plants best adapted to the conditions within that particular narrow habitat. The zones are often strikingly different in appearance, even to a person unfamiliar with shoreline characteristics.

For intertidal areas exposed to the open sea,wave shock is a formidable physical factor. Fist-sized rocks have been thrown 100 meters (330 feet) into the air by the force of breaking waves. Large intertidal plants must be immensely strong, elastic, and slippery to avoid being shredded by wave energy. Mobile animals move to protecting overhangs and crevices, where they hide during intense wave activity. Attached, or sessile, animals hang on tightly, often gaining assistance from rounded or low-profile shells, which deflect the violent forces of rushing water around their bodies. Some sessile animals have a flexible foot that wedges into small cracks to provide a good hold; others (like mussels) form shock absorbing cables that attach to something solid.

Desiccation (drying) by exposure to air and sunlight is another source of intertidal stress. Again, mobile organisms have an advantage because they can move toward water left in tidal pools or muddy depressions by the retreating ocean. Attached animals and plants must await the water's return, huddled in low spots, moist pockets,or cracks in the rocks, or within tightly closed shells. Water trapped within a shell can keep gills moist for the needed exchange of gases. A protective mucous coating can retard evaporative water loss from exposed soft body parts or blades of seaweed.