Feather stars (comatulids) represent the largest group within the class Crinoidea, an ancient lineage of marine echinoderms. Unlike their stalked relatives, the sea lilies, feather stars are motile, using prehensile appendages called cirri to perch or swim freely. They are obligate suspension feeders, capturing their entire diet from particles suspended in the surrounding seawater. This strategy allows them to thrive in tropical and subtropical marine environments, often found on coral reefs where currents deliver a continuous food supply.
The Diet: What Feather Stars Filter
The diet consists almost entirely of suspended particulate matter (SPM), a broad category of microscopic organic material floating in the water. This material is filtered indiscriminately from the current passing over their outstretched arms. The primary components captured fall into three main categories: phytoplankton, zooplankton, and organic detritus.
Phytoplankton are microscopic, plant-like organisms that form a significant food source. Zooplankton consists of tiny animals, including marine protozoa and various larvae, captured as they drift by. Organic detritus is composed of non-living decaying matter and fragments of dead organisms containing usable nutrients.
Particle size is a determining factor, as the filtering structures are adapted for very small morsels. Feather stars extract a substantial portion of their energy from zooplankton, supporting their energy needs through the sheer volume of water they filter.
Specialized Capture Mechanisms
Food capture begins when the feather star extends its numerous, highly flexible arms into the water current, often forming a fan-like shape to maximize exposure. These arms are lined with specialized, feather-like side branches called pinnules, which create the filtering net. A feather star can have anywhere from five to over 200 arms, greatly increasing its surface area for feeding.
The pinnules are lined with rows of tiny, finger-like structures called tube feet (podia), which are the primary contact points for food particles. These tube feet are specialized for chemoreception and mechanical capture, not locomotion. They are often arranged in groups of three: one long foot searches the water, and two shorter ones assist in particle manipulation.
When a particle contacts a tube foot, the foot quickly sweeps the particle toward a groove running along the arm. The pinnules and arms secrete a sticky mucus that adheres to the captured particle, trapping it. This mucus coats the food, bundling it into a manageable sphere called a food bolus, which prevents it from being washed away.
Food Transport and Ingestion
Once the food bolus is secured by the tube feet, the specialized transport system takes over. The particle is transferred into the ambulacral grooves, which are ciliated channels running the length of each pinnule and arm, converging toward the central body disc. These grooves function as a biological conveyor belt.
The ambulacral groove surface is lined with microscopic, hair-like structures called cilia. These cilia beat in a coordinated, unidirectional wave, generating a current that moves the mucus-bound food bolus along the groove. In some species, this transport occurs at a speed of approximately four centimeters per minute toward the mouth.
As the food moves, the feather star can exercise selection, discarding unsuitable particles using contact chemoreceptors within the groove epithelium. All ambulacral grooves from the arms eventually meet at the central mouth, located on the upper surface of the body disc. The mouth ingests the collected food bolus.
The digestive tract is relatively simple and typically U-shaped, with the anus located near the edge of the central disc, close to the mouth. After ingestion, the food travels through a short esophagus and into a coiled intestine for nutrient absorption. The U-shaped gut efficiently processes the continuous stream of microscopic food delivered by the filtering apparatus.