Crinoids are ancient marine invertebrates belonging to the phylum Echinodermata, which also includes sea stars and sea urchins. Commonly known as sea lilies or feather stars, these organisms possess a distinctive radial symmetry and a flower-like appearance. Crinoids have a long evolutionary history, with fossil records dating back to the mid-Cambrian period. They are characterized by a central body cup and flexible, often multi-branched arms, allowing them to thrive in various marine environments from shallow reefs to the deep sea.
Structure Relevant to Feeding
The physical structure of a crinoid is adapted for capturing food from the water column. The central body, known as the calyx, is a cup-like structure housing the mouth and digestive organs on its upward-facing oral surface. Extending from the calyx are the jointed arms, or brachials, which can number from five to over two hundred in some species.
These arms are lined with numerous smaller side branches called pinnules, giving the crinoid its feathery appearance. A shallow channel, the ambulacral groove, runs along the upper surface of each arm and pinnule toward the mouth, directing captured particles inward.
The Primary Diet
Crinoids are microphagous suspension feeders, capturing tiny, suspended particles from the surrounding seawater. Their diet consists primarily of various forms of plankton, including microscopic algae (phytoplankton) and small zooplankton, such as copepods and invertebrate larvae.
A significant portion of their intake is fine organic detritus, which is non-living particulate matter suspended in the water. Crinoids are passive feeders, using their arms to sieve particles from the natural flow of the current rather than actively hunting prey. They are highly selective based on the size of the particle they can capture and transport, though generally non-selective regarding the type of organic material.
The Filter Feeding Mechanism
The feeding process begins when the crinoid positions its arms into a broad, fan-like array, oriented perpendicularly to the water current. This arrangement maximizes the surface area exposed to the flow, creating a net to intercept particles. Specialized tube feet, or podia, extend from the pinnules, forming a sticky, fine-meshed barrier.
The podia are covered in a mucus layer that acts as an adhesive trap for minute food items. Once a particle adheres, the tube feet flick the combined food and mucus into the nearest ambulacral groove.
Within the groove, numerous microscopic, hair-like structures called cilia begin to beat rhythmically. The cilia propel the mucus-coated food, now formed into a cohesive bolus, along the groove toward the central mouth. This ciliary action moves the captured material efficiently, where it enters the digestive tract.