Sea stars (often called starfish) are marine invertebrates that inhabit oceans across the globe. They belong to the phylum Echinodermata, a group that also includes sea urchins and sea cucumbers. Sea stars are instantly recognizable by their body plan, which typically features five arms radiating from a central disk, a configuration known as pentaradial symmetry. Their unique body structure and complex internal hydraulic system support their movements and unusual feeding habits, which often involve a predatory lifestyle.
Classification of Starfish Feeding Habits
The majority of the nearly 2,000 species of sea stars are active predators, meaning they are carnivores. Their slow, deliberate movements belie a highly effective hunting strategy focused on slow-moving or sessile prey. This predatory nature is the defining characteristic of their diet across most marine habitats.
However, feeding habits are not uniform across all species. Some species are omnivores, consuming both animal matter and plant material. Others act primarily as scavengers, feeding on dead or decaying organic matter, or as specialized detritivores that ingest sediment to extract nutrients. These varied roles allow sea stars to occupy a wide range of ecological niches from the shallow intertidal zone to the deep sea floor.
Specific Diets and Prey Selection
Predatory sea stars primarily consume shelled invertebrates. Bivalves, such as mussels, clams, and oysters, are a common food source targeted using hydraulic power. They also consume other hard-shelled organisms like barnacles and gastropods, such as snails.
Certain species, such as the ochre sea star (Pisaster ochraceus), are well-known for consuming mussels, which helps regulate those populations. In tropical regions, some sea stars, including the crown-of-thorns sea star, are specialized predators of coral polyps, directly impacting reef health. Other generalist predators may consume sea urchins, sponges, or even small, slow fish. Less predatory species may rely on micro-algae, organic detritus, and marine snow that settles on the substrate for their nutrition.
The Unique Mechanics of Starfish Consumption
Sea stars consume their prey using a unique process. When a sea star encounters a bivalve, it first uses the hundreds of tiny, suction-cupped tube feet on its underside to secure a grip on the two shells. It then exerts a steady pull using its water vascular system, which acts like an internal hydraulic machine. This sustained force, which can reach up to 12 pounds, is enough to fatigue the bivalve’s adductor muscles, causing the shell to open a tiny amount, sometimes only three or four millimeters.
Once the slightest gap is created, the sea star performs its signature feeding move: stomach eversion. The sea star pushes its cardiac stomach out through its mouth, located on its underside. This stomach is inserted directly into the small opening of the prey’s shell, enveloping the soft body of the mussel or clam. The sea star then secretes digestive enzymes onto the prey, beginning the process of external digestion. The enzymes liquefy the prey’s tissues, turning the meal into a nutrient-rich “soup” that the sea star’s stomach then absorbs. After the meal is complete, the stomach is retracted back into the body, leaving behind an empty shell.
Starfish as Keystone Species
The carnivorous diet of certain sea stars grants them an ecological role disproportionate to their numbers, qualifying them as keystone species. A keystone species is one whose presence is fundamental to maintaining the structure and diversity of an ecosystem. The ochre sea star, Pisaster ochraceus, is the classic example in the rocky intertidal zones of the Pacific coast.
By actively preying on mussels, particularly the California mussel, the sea star prevents this dominant competitor from taking over all available space. Without this predatory pressure, the mussels would form dense, continuous beds, excluding other organisms like barnacles, limpets, and algae. The sea star’s feeding creates open spaces on the rock, allowing numerous other species to settle and thrive, enhancing the overall biodiversity of the community. The removal of this predator leads to a collapse in species richness, demonstrating the importance of their carnivorous habit.