Feeding Posture of Modern Stalked Crinoids

Abstract

MOST modern crinoids (Echinodermata) are comatulids, which lack the stalk characteristic of Palaeozoic crinoids. The specialisation and adaptation to different ecological niches made possible by loss of the stalk have resulted in several different ways of spreading the arms and pinnules in an array for suspension feeding. Until the 1960s, most observations were based on specimens in aquaria. These suggested that the comatulids sit on the substratum with the mouth orientated upwards, the arms stretched out in the form of a broad funnel or collecting bowl. The crinoid was thought to wait passively for a rain of its food material from above¹. This feeding posture is used by some deep water crinoids^2,3. Direct observations of the feeding behaviour of a shallow water comatulid in the Red Sea during the 1960s demonstrated that the arms are withdrawn during the day, but at night are arrayed in a vertical plane to form a filtration fan. This fan is orientated perpendicular to a prevailing weak current, filtering some 40,000 1 of water at a current speed of 2 cm s⁻¹ during one period of activity⁴. These observations and a review of deep-sea photographs indicated that modern crinoids favour an environment with moderate currents and are to some degree current-seeking (rheophilic)⁵. Application of these ideas to the palaeocology of Palaeozoic stalked crinoids suggested that most were rheophilic, using the stalk to raise the calyx above the substratum and allowing the arms to be outspread in a planar, circular filtration fan². The morphology of some ancient forms, however, was considered indicative of a rheophobic mode of life.