The ochre sea star (Pisaster ochraceus) is a marine invertebrate found along the Pacific coast of North America. These sea stars display five stout arms radiating from a central disk, ranging from 10 to 25 centimeters in diameter. While often recognized for their vibrant purple coloration, they can also appear in shades of orange, yellow, reddish, or brown. Their rough surface is covered with small spines, part of an internal skeleton made of interlocking plates. Ochre sea stars inhabit intertidal zones, including rocky shores and tide pools, clinging to surfaces with tube feet.
Their Primary Diet
Ochre sea stars are carnivores, preying on sessile or slow-moving invertebrates. Their diet heavily consists of mussels, especially the California mussel (Mytilus californianus). Beyond mussels, they also consume barnacles, limpets, snails, and sometimes chitons or small crustaceans. Diet composition varies based on location and prey availability; for instance, where mussels are less abundant, they may consume more barnacles and whelks. As larvae, Pisaster ochraceus are filter feeders, consuming plankton before transitioning to their adult diet.
How They Feed
Ochre sea stars use a distinct feeding mechanism. They use hundreds of tube feet, equipped with suckers, to grasp prey like bivalve shells. The sea star then exerts traction, pulling the two valves of a mussel or clam slightly apart. Once a small gap, often as narrow as 0.1 millimeters, is created, the sea star everts its cardiac stomach through its mouth and inserts it into the shell opening. Digestive enzymes are released externally onto the prey’s soft tissues, liquefying them. This external digestion can take several hours to a few days, depending on prey and environmental conditions, after which dissolved nutrients are absorbed.
Role in the Ecosystem
The ochre sea star’s feeding habits significantly impact its environment, making it a keystone species in the intertidal zone. By consuming dominant competitors like mussels, the sea star prevents mussel beds from monopolizing rocky surfaces. This predation opens up space, allowing a wider variety of species, such as algae, barnacles, and other invertebrates, to establish and thrive. Their presence directly contributes to increased biodiversity within the intertidal community. Without their regulatory influence, mussel populations could overgrow, potentially reducing overall diversity in these coastal habitats.