The intertidal zone of Acadia National Park contains numerous invertebrates (animals lacking a backbone). Some of these animals, including barnacles, can survive the daily physical stresses (e.g., extreme heat, freezing, or drying) of the high intertidal zone and are common there. However, many invertebrates, including sea urchins and sea stars, are only found in the lower intertidal zone, low tide pools, and below the intertidal zone because they cannot tolerate the drying stress of the intertidal habitat.

The invertebrates within the rocky intertidal zone include filter feeders, herbivores, and predators.

• Filter feeders such as barnacles and mussels are dependent upon the incoming tide to bring them food, because they feed on microscopic unicellular algae in the phytoplankton. A number of gastropod mollusks (periwinkles and limpets) and some small crustaceans (amphipods and isopods) feed on intertidal seaweeds; many of the seaweeds that are abundant in the intertidal zone have defenses to avoid grazing (e.g., calcification or production and storage of compounds such as tannins).
• The major herbivore of the region just below the low tide mark is the sea urchin, which commonly feeds on brown seaweeds called kelps.
• Predators in the intertidal zone include whelks, sea stars, and crabs. These animals control the abundance and vertical distribution of their prey. For example, mussels are not abundant below the lower intertidal zone because sea stars feed on them.

The photographs in the field guide will help you identify some of the common invertebrates found in the intertidal zone of the park. The field guide was developed by Sarah Hall and Joseph Stachelek, under the guidance of Professor Susan Brawley, for their senior capstone project at the University of Maine.