UPSTREAM AND DOWNSTREAM CAPTURE DURING SUSPENSION FEEDING BY OLIGOMETRA SERRIPINNA (ECHINODERMATA: CRINOIDEA) UNDER SURGE CONDITIONS.

The crinoid Oligometra serripinna is a suspension feeder that usually experiences unidirectional tidal currents from which it extracts food particles by downstream capture (i.e., while the food grooves face downcurrent). However, near slack tide, wave surge may cause brief current reversals, each lasting about 2 s at roughly 10 s intervals. To test if a crinoid can engage in upstream capture (i.e., while the food grooves face upcurrent) during brief current reversals, we approximated these surge conditions in a laboratory flume. In the laboratory, as in the field, the crinoid oriented its food grooves downstream with respect to the predominant current, and the body posture did not change during the brief intervals of reversed flow. Brine shrimp cysts were added to the flume, and video recordings were made of the crinoid capturing these particles. Under surge conditions, the crinoid (1) captured 16.2% of the approaching particles while its food grooves faced downstream and (2) captured 8.0% of the approaching particles while its food grooves faced upstream. Thus O. serripinna used its filter both for upstream capture and for downstream capture, although the former was only about half as efficient as the latter.