Great stretches of rocky shoreline are characteristic of the Pacific Coast. The coastal substrate varies from soft sandstone or clay shale to very hard granite. The type of rock is particularly important to the rock-boring forms and to the organisms that will attach to the surface of the substrate. A sheer cliff is often found adjacent to the rocky shore, and rocky pools, undercut ledges and tidal crevices present a large number of microhabitats for marine organisms. These organisms have adapted to wave shock, currents, and the physical changes caused by tidal exposure.

The wind-driven waves and currents constantly pound, abrade and pull at the inhabitants of the rocky shore. Organisms must be tough, flexible, or firmly attached to the substrate. The amount and type of wave shock and current action depends upon the degree of coastal exposure. A protected outer coast is shielded from direct wave exposure by a point of land, submerged reefs or sandbars, extensive kelp beds, or even a peculiar refraction of waves. The open coast is usually characterized by wave-swept rocky headlands jutting out into fairly deep water. The protected outer coast has a more abundant biota because of the reduction in wave shock and currents.

Many changes in the physical characteristics of the rocky shore are caused by the ebb and flow of the tides. For example, the daily and seasonal temperature variations of the rocky shore water are more extreme than the temperature changes in the open sea. Accordingly, the rocky shore organisms are subjected to rather severe temperature changes that may limit their distribution. The changes in water temperature will affect the amount of dissolved oxygen available and the salinity of the water. The amount of dissolved oxygen will decrease and the salinity will increase as the water temperature increases.

The splash and high-tide zones are more exposed to the effects of evaporation than the lower tidal zones. Accordingly, the upper areas are often more saline. During the rainy season the inflow of fresh water may dilute the salinity of the pools. Organisms living in the upper zones must be able to tolerate, avoid, or physiologically regulate against the salinity changes. Exposure to air tends to desiccate (dehydrate) the exposed tidal organisms and the severity of dehydration is directly correlated with the tidal zone. Many mobile organisms will seek shelter, while the non-mobile forms have specialized structural adaptations such as shells, plates, or coverings of sand or debris to reduce water loss from the tissues. The extent of tidal exposure creates a gradient of each limiting factor. Extremes are reached in splash areas and minimal changes occur in the lowest ranges of the tide. The most adaptable organisms live in the upper tidal areas, those that are least tolerant live in the lower areas. The most convenient way to study these organisms is according to their distribution in the tidal gradients or zones.* *Taken from Natural History of Southern California Ancinec,Radford & Schwenkmeyer.