Starfish, correctly known as sea stars, are marine invertebrates that belong to the class Asteroidea within the phylum Echinodermata. These animals are found in all the world’s oceans, inhabiting environments from the intertidal zone to the abyssal depths. They play a significant ecological role as both active predators and opportunistic scavengers on the seabed. Their diet is diverse, and they are known for unique methods of consuming prey larger than their mouths.
Common Food Sources
The primary diet of many sea stars consists of slow-moving or sessile organisms, which they are well-equipped to handle despite their limited speed. Bivalves, such as clams, oysters, and mussels, are a staple food source for many larger predatory species. These mollusks are often found attached to rocks or buried in soft sediment, making them accessible targets.
Sea stars slowly climb onto their chosen prey, using the thousands of tiny, hydraulically-controlled tube feet on their underside to gain purchase. They also consume other invertebrates, including snails, barnacles, and various marine worms. Because they are unable to chew or bite, their preferred meals are those they can physically overwhelm and digest externally.
The Unique Mechanism of Starfish Feeding
The method by which a sea star eats focuses on external digestion. Once a starfish has secured a bivalve, it uses the suction power of its tube feet, which are operated by a water vascular system, to exert a continuous pull on the two shells. Some species are capable of applying up to 12 pounds of pressure to pry the shells apart just a fraction of a millimeter.
Through this tiny gap, the sea star performs stomach eversion, pushing its large cardiac stomach out through its mouth, which is located on its underside. The everted stomach wraps around the soft tissue of the prey, allowing the sea star to digest the meal outside of its body cavity. Powerful digestive enzymes are secreted directly onto the food, liquefying the tissue that can then be absorbed by the stomach.
Once the meal is digested externally, the starfish retracts the cardiac stomach back inside its body. The partially digested nutrients then travel into the pyloric caeca, which are digestive glands extending into each arm, where the final absorption takes place. This adaptation allows the sea star to consume prey items that would otherwise be far too large to fit through its small central mouth.
Dietary Variations Across Starfish Species
While the predatory consumption of shelled mollusks is the most recognized feeding behavior, the Asteroidea class exhibits a wide range of specialized diets. Some species, categorized as detritivores, feed primarily on dead organic matter, including decomposing organisms and fecal material that settles on the seabed.
Other sea stars are specialized suspension feeders, using their tube feet and mucus to capture plankton and fine organic particles floating in the water column. Species sometimes associate with sponges to utilize the water currents the sponges create. This contrasts sharply with the Crown-of-Thorns starfish, a predator that feeds almost exclusively on the soft polyps of living stony corals.