Active feeding behavior of and current modification by the sea lily Metacrinus rotundus (Echinodermata: Crinoidea)

All modern crinoids, categorized as articulate crinoids, possess muscular articulations in their arms that allow them to maintain their feeding posture in changing current conditions. We tested the responses of the isocrinid Metacrinus rotundus to the introduction of suspended particles in the water column and to the chemical properties of the suspended particles (organic versus inorganic), and measured changes in flow patterns during feeding. The crinoids demonstrated specific behavior in response to organic particles in the current, and chemical extracts consisting of organic matter induced a change in the posture. The currents around the crinoids were disturbed by the crinoid arms only when the particles were passing through the arms, and the current speed and direction were maintained within a certain range. Crinoids are considered to actively form an effective filtration fan in the current, detect if organic matter is present in the current, and control and maintain the current condition to maximize their filter feeding. Living stalked crinoids are restricted to the deep sea, and the abovementioned crinoid strategies are key characteristics for their life in an oligotrophic environment. The development of muscular articulation provides not only a free locomotion but also a sophisticated filter feeding mechanism by acquiring an idealized filtration fan.