Photo Credits: Mason Cummings, NPS

In the narrow band between land and sea, plants and animals must endure being pounded by icy waves at some times while being parched by hot, dry air at others. Despite these extreme conditions, wave-swept rocky shores are full of life because they also have plenty of light, nutrients, and oxygen.

Bounded by high and low tides, the intertidal zone is rich in algae and invertebrates, but the particular mix of species varies with proximity to shore. The high intertidal zone, which is inundated only during high tides, has species including rockweed, acorn barnacles, turban snails, and lined shore crabs. The middle intertidal zone, which is exposed to the air at least once a day, is home to creatures such as sea lettuce, aggregating anemones, chitons, gooseneck barnacles, mussels, and ochre stars. The low intertidal zone, which is exposed only during very low tides, is inhabited by coralline algae, giant green anemones, purple sea urchins, and bat stars.

While resilient to the normal extremes of their environment, intertidal species are also sensitive to fluctuations in salinity and temperature, and so serve as indicators of environmental change. In the San Francisco Bay Area Network, park staff do both long-term monitoring and rapid assessments of the intertidal zone. This monitoring can reveal environmental shifts driven by factors such as development, chemical spills, and climate change, as well as facilitate timely responses to catastrophes such as oil spills. Find out more about what has been learned about these amazing habitats through the links on the upper right of this page.