Sea stars are marine invertebrates belonging to the phylum Echinodermata, a group characterized by spiny skin and radial symmetry. They inhabit diverse environments from tide pools to the deep sea. The ecological function of a sea star is often misunderstood, particularly regarding its contribution to nutrient cycling. This article clarifies the sea star’s actual function, differentiating its predatory and scavenging behavior from the process of true decomposition.
Understanding Ecological Classifications
To understand the sea star’s role, it is helpful to first define the three main ecological classifications for organisms within a food web. Producers, such as phytoplankton and algae, create their own food using sunlight or chemical energy. Consumers are organisms that obtain energy by feeding on other living or recently dead organisms. Sea stars fall into this broad category as they rely on external food sources.
Decomposers are specialized organisms, primarily bacteria and fungi, that break down dead organic matter into basic inorganic substances like carbon dioxide and mineral nutrients. This chemical breakdown is performed at a microbial level and returns nutrients to the ecosystem for reuse by producers. While a sea star may consume dead material, it does not perform the microscopic chemical conversion that defines a true decomposer.
The Sea Star’s Primary Role as a Consumer
The sea star is functionally a consumer, acting as both a predator and an opportunistic scavenger within its marine environment. Its diet is largely composed of living prey, including bivalves like mussels and clams, barnacles, and snails. This predatory role directly controls the populations of other species in its immediate habitat.
The sea star’s feeding strategy is highly unusual and involves a process called stomach eversion. Using hundreds of tiny tube feet, the sea star attaches to the shells of its prey and applies steady force to pull the shells slightly apart. Once a gap is created, the sea star extends its stomach out of its body and into the prey’s shell.
The everted stomach secretes powerful digestive enzymes that liquefy the soft tissues of the prey while it is still inside its own shell. The sea star then absorbs this nutrient-rich liquid before retracting its stomach back into its body. This external digestion allows the sea star to consume prey that would otherwise be too large to ingest whole.
The consumption of dead organisms, known as scavenging, helps clear organic debris from the seabed. However, this is a physical act of consumption, not the microbial nutrient recycling characteristic of true decomposition. Therefore, the sea star is correctly classified as a consumer, specifically a predator and scavenger.
Keystone Impact on Biodiversity
Beyond its role as a consumer, the sea star’s broader function is defined by its status as a keystone species. A keystone species is an organism whose presence has a disproportionately large effect on the structure and stability of its environment. The removal of such a species can lead to the collapse of the entire ecosystem structure.
The ochre sea star (Pisaster ochraceus) provides the classic example of this impact in the rocky intertidal zones along the Pacific coast of North America. This species primarily preys on the California mussel (Mytilus californianus), which is a dominant competitor for space on the rocks. Without the sea star’s predation, the mussels rapidly take over, forming dense monocultures that exclude almost all other invertebrate species.
By consuming mussels and creating open patches on the rock surface, the sea star creates space for other, less competitive species to settle and thrive. This predatory action maintains a high level of species diversity among algae, barnacles, and other invertebrates. When scientists experimentally removed the sea stars, the species diversity dropped dramatically, confirming their role in structuring the habitat.
This demonstrates that the sea star’s most significant contribution is regulating the ecological balance of the entire community. Its feeding habits prevent a single species from dominating and ensure that the habitat remains diverse for numerous other organisms. The sea star is therefore a controller of marine biodiversity, a role distinct from that of a simple decomposer.