Starfish are frequently misunderstood regarding their dietary habits. Despite their common name, these marine invertebrates are not fish, and they are certainly not omnivores. While their diverse marine habitats might suggest a varied diet, most starfish species are specialized predators. Their primary role in marine ecosystems involves consuming other animals, making them predominantly carnivorous.
Starfish Diet: What They Really Eat
The majority of starfish species are carnivores, preying on marine invertebrates. Their diet commonly includes bivalves such as clams, oysters, and mussels, which they are adept at opening. Starfish also feed on snails, barnacles, and other slow-moving organisms. Some species can even consume crustaceans, worms, and small fish. This predatory behavior helps maintain the balance of marine communities.
Starfish are often considered keystone species in many marine environments. For instance, the purple sea star (Pisaster ochraceus) plays a significant role in controlling mussel populations in intertidal zones of the Pacific Northwest. Without these predators, mussel populations can expand unchecked, decreasing the ecosystem’s overall biodiversity.
Unique Feeding Mechanisms
Starfish possess distinct methods for consuming their prey. A primary feeding strategy involves everting, or pushing out, their stomach through their mouth, which is located on their underside. This eversible cardiac stomach then envelops the prey, even those with hard shells that are too large to ingest whole. Digestive enzymes are secreted onto the prey, allowing for external digestion where the tissues are liquefied.
Once the prey is partially digested into a liquid form, the starfish retracts its stomach back into its body. The liquefied nutrients are then further processed in the pyloric stomach and absorbed by digestive glands extending into each arm. Their numerous tube feet, which operate through a hydraulic water vascular system, are instrumental in this process. These tube feet are used to grip prey, pry open shells, and maneuver food towards the central mouth.
Dietary Diversity Among Starfish Species
While many starfish are predators, there is significant dietary diversity across the approximately 1,900 known species. Some starfish are detritivores, feeding primarily on decaying organic matter and detritus found on the seafloor. These species consume leftover food particles and decomposing organisms, contributing to their habitats’ health. An example is the sand-sifting starfish, which burrows through sand to find small invertebrates and organic debris.
Other starfish species are suspension feeders, filtering small particles from the water column. These specialized feeders use their tube feet or mucus nets to capture microscopic food items. The crown-of-thorns starfish (Acanthaster planci) represents another specialized diet, exclusively preying on coral polyps, making it a corallivore. This range of feeding strategies underscores that while not omnivores, starfish exhibit a wide spectrum of specialized diets beyond general carnivory.