Echinoderms are exclusively marine invertebrates, known as Echinodermata, which translates to “spiny skin.” This diverse group includes approximately 7,000 living species, comprising the five main classes: Asteroidea (starfish), Ophiuroidea (brittle stars), Echinoidea (sea urchins and sand dollars), Holothuroidea (sea cucumbers), and Crinoidea (sea lilies and feather stars). All echinoderms share a unique five-part radial body plan and a distinct water vascular system, which uses hydraulic pressure to operate their tube feet. These anatomical characteristics support a wide variety of feeding strategies, ranging from active predation to passive filter feeding.
Active Predation and External Digestion
Starfish (Asteroidea) engage in active predation, often targeting slow-moving or sessile prey like bivalves, snails, and barnacles. They position themselves over the prey, typically a clam or oyster, and use their numerous tube feet to secure a firm grip on the shell. The tube feet, powered by the water vascular system, exert steady force, slowly pulling the two halves of the shell apart.
Once a minute gap is created between the shells, the starfish employs external digestion. It everts its lower cardiac stomach through the mouth opening located on its underside, inserting the soft, sac-like stomach directly into the shell opening to wrap around the prey’s soft tissues. The everted stomach secretes powerful digestive enzymes onto the prey, breaking down the mollusk’s body outside the central body cavity. This process liquefies the prey, which the stomach tissue absorbs before the stomach is retracted; final digestion occurs within the pyloric ceca, a pair of digestive glands extending into each arm.
Specialized Grazing Mechanisms
Sea urchins and sand dollars (Echinoidea) primarily rely on a grazing diet, consuming algae, detritus, and organic matter scraped from hard surfaces. Their feeding is centered around a complex, jaw-like apparatus known as Aristotle’s Lantern, a structure unique to the class. This intricate organ consists of five large, calcareous plates that function as teeth, arranged in a circular, pyramid-like structure.
The lantern complex is operated by muscles that allow the sea urchin to extend the five teeth and move them laterally. This movement facilitates a powerful scraping and chewing action, enabling the animal to efficiently graze algae off rocks and coral substrates. The five teeth are continually sharpened as they are worn down through use, with new tooth material growing from the top of the lantern structure. This continuous growth ensures the grazing tool remains sharp and functional throughout the animal’s life.
Deposit and Suspension Filtering
The remaining echinoderm classes utilize passive feeding methods, capturing particulate matter from the sediment or the water column. Sea cucumbers (Holothuroidea) are the primary deposit feeders, consuming seabed sediment to extract organic detritus, bacteria, and microalgae. They use a ring of specialized tentacles surrounding their mouth to scoop up the mud or sand.
Other sea cucumber species, known as dendritic-tentacled holothurians, are benthic suspension feeders that extend highly branched, sticky tentacles into the water column. These feathery structures trap suspended plankton and organic particles. The sea cucumber then inserts each tentacle into its mouth sequentially to “lick” the food off.
Crinoids (feather stars and sea lilies) are almost exclusively passive suspension feeders, relying on water currents to deliver food particles directly to them. They position their arms and feathery appendages, called pinnules, to form a filtration fan that intercepts the flow of water. The numerous tube feet lining the arms and pinnules are coated in mucus, which captures plankton and other particulate matter. Cilia then transport the food-laden mucus down ciliated grooves along the arms to the mouth. Brittle stars (Ophiuroidea) often combine scavenging and deposit feeding, using their flexible arms and tube feet to gather small organic particles from the seafloor and move them toward the mouth.