In monodirectional currents, arms on the downcurrent side twist so that all ambulacra face downcurrent. These fans sometimes widen into funnels or bowls, the latter grading into the radial and parabolic fan postures. The fan is usually slightly concave upcurrent, so this posture grades into the next. Some clinging species, particularly the comasterid Capillaster multiradiatus and the colobometrid Cenometra bella , often interrupt the radial fan with several irregularly arrayed or curled arms. In parabolic fans, characteristic of stalked isocrinids, the oral surface orients downcurrent and arms recurve into the current.
Some arms assist in anchoring.
Under unidirectional flow, arms on the upcurrent side may form a more regular multilayered fan with some twisting of downcurrent arms. This is an active feeding posture assumed under strong flow conditions and should be distinguished from the meridional posture described below under slack water. Under slack water conditions, comatulids orient arms in a variety of ways. Diurnally cryptic species curl up in crevices, under ledges or among coral branches. Exposed taxa assume a meridional posture with arms arched over the oral surface.