Sea stars are marine invertebrates found across all the world’s oceans, from warm tropical regions to the frigid polar depths. With nearly 2,000 known species, these creatures exhibit a remarkable range of sizes, colors, and feeding behaviors. Their widespread presence and diverse dietary habits make them important for understanding marine ecosystems.
Diverse Diets of Sea Stars
Sea stars are primarily carnivores, preying on a wide variety of other marine organisms. Their diet commonly includes bivalves like clams, mussels, and oysters, as well as gastropods such as snails. Some species also consume crustaceans, including barnacles and small crabs, and even other echinoderms like sea urchins and other sea stars.
Many sea star species also function as scavengers, feeding on detritus, dead organic material, or fecal matter. Certain species exhibit specialized feeding habits, such as filter feeding on phytoplankton or absorbing dissolved organic nutrients directly from the surrounding water. A sea star’s diet depends on its species, habitat, and prey availability.
Unique Feeding Mechanisms
Sea stars possess a unique method of consuming their prey. When a sea star encounters food, particularly shelled organisms, it uses its numerous tube feet, powered by a hydraulic water vascular system, to grasp and manipulate the prey. For bivalves, the sea star attaches its tube feet to both shells and exerts steady pressure, slowly prying them open. Some species can apply significant force, up to 12 pounds of pressure, to open tightly sealed shells.
Once a small gap is created, the sea star everts its cardiac stomach out through its mouth and into the prey’s shell or around the prey’s body. This external stomach secretes powerful digestive enzymes that break down the prey’s soft tissues into a liquid mixture. The stomach then absorbs this pre-digested material and retracts back into the sea star’s body for final digestion and nutrient absorption within specialized digestive glands called pyloric caeca, located in each arm.
Ecological Role of Sea Star Diets
The feeding habits of sea stars significantly impact the structure and health of marine ecosystems. As predators, they help control prey populations, preventing species like mussels from overpopulating an area. This regulatory role is evident in their function as “keystone species” in many environments.
A keystone species is one whose presence has a disproportionately large effect on its environment relative to its abundance. For example, the ochre sea star (Pisaster ochraceus) preys on mussels; without it, mussel populations can explode, outcompeting other species and reducing biodiversity. Sea stars also contribute to nutrient cycling in marine environments as scavengers of dead organic matter. Their predatory activities, such as those of the sunflower sea star on sea urchins, are important for maintaining healthy kelp forests, which provide habitat for countless other marine creatures.