Adults of many invertebrate species are attached permanently to the rock or other organisms (epibiota). These include members of the phyla Porifera (sponges), Cnidaria (hydroids and sea anemones), Annelida (tube-building polychates), Arthropoda (barnacles), Mollusca (mussels and clams), Bryozoa (moss animals), and Chordata (tuni-cates). Suspension feeding - either by pumping water through a sieve structure or trapping particles carried on induced or external currents - is a common feature of sessile animals and serves to transfer inputs of energy and nutrients produced in the water column into the intertidal zone via the ingestion of plankton. Additionally, by feeding on locally derived detritus, suspension feeders capture some of the nutrients that are produced by neighboring inhabitants.
Sessile intertidal animals are often physically or chemically defended against predation and display plastic phenotypes in response to changing environmental conditions because they are fixed in place and cannot move to avoid predators. For example, the presence of the predatory gastropod Acanthina angelica induces change in the shell shape of its barnacle prey Chthamalus anisopoma, and the barnacle forms a curved shell making it more difficult for the predator to attack.
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