Starfish, correctly known as sea stars, are marine invertebrates belonging to the class Asteroidea. They are part of the phylum Echinodermata, which is characterized by radial symmetry and spiny skin. Starfish inhabit all the world’s oceans, from shallow tidal zones to the deepest abyssal plains. Their presence fundamentally influences the structure of marine communities.
Starfish as Primary Invertebrate Predators
Most sea star species function as active carnivores, feeding primarily on slow-moving or sessile prey like bivalves, barnacles, and snails. They are equipped with hundreds of tube feet that operate using a hydraulic water vascular system, which allows them to move slowly and adhere firmly to surfaces. This system also enables them to exert a powerful, sustained pull on the shells of prey like clams and mussels, often applying up to 12 pounds of pressure to create a small gap.
The sea star’s unique feeding strategy, called stomach eversion, allows it to consume prey that are larger than its mouth. Once a small opening is created in the shell, or when feeding on soft-bodied prey, the sea star pushes its cardiac stomach out through its mouth. This exposed stomach then secretes powerful digestive enzymes directly onto the prey’s soft tissues, liquefying the meal outside of the body. The resulting nutrient-rich fluid is absorbed by the stomach before the organ is retracted back into the body cavity.
Defining the Keystone Role
By preying on competitively dominant species, certain sea stars function as keystone species, meaning their impact on the ecosystem is disproportionately large relative to their overall abundance. The classic example of this ecological role involves the ochre sea star, Pisaster ochraceus, found along the rocky intertidal zone of the Pacific Northwest. This predator selectively targets the California mussel, Mytilus californianus, which is the superior competitor for space on the rocks.
Mussels are foundation species that, if left unchecked, would monopolize the substrate, creating vast monoculture beds. Pisaster’s predation clears patches of rock, creating open space for dozens of other, less competitive species to settle and thrive. This action dramatically increases the overall biodiversity of the intertidal community, allowing species like limpets, barnacles, algae, and anemones to flourish. Experimental removal of Pisaster led to the rapid expansion of the mussel population and a significant decrease in the number of species present. This demonstrates that the sea star’s predatory actions maintain the ecological diversity of the entire community.
Secondary Ecological Contributions
Beyond their role as a keystone predator, sea stars contribute to the marine environment in several other ways. They act as scavengers and detritivores, consuming dead organic matter, microorganisms, and waste that settles on the seafloor. This feeding habit helps recycle nutrients and prevents the accumulation of decaying material, contributing to the overall cleanliness of the benthic environment. Sea stars are also a food source for other animals, particularly when they are young and their protective spiny skin is less developed.
Larger predators, including sea otters, sharks, rays, and large crabs, consume adult or juvenile sea stars. The health of sea star populations also serves as an indicator for the condition of the broader marine habitat. The mass mortality event known as Sea Star Wasting Syndrome (SSWS), which began in 2013 and affected over 20 species, revealed profound ecological consequences. For instance, the decline of the sunflower sea star (Pycnopodia helianthoides) led to a surge in its prey, sea urchins, which then overgrazed kelp forests. This demonstrated how sea star populations regulate diverse food webs, connecting invertebrates to large algae necessary for healthy ocean ecosystems.