Sag-Mediated Modulated Tension in Terebellid Tentacles Exposed to Flow

The long, compliant feeding tentacles of the terebellid polychaete Eupolymnia heterobranchia not only stretch out over a sandflat substratum but also extend into flow. Tentacles suspended perpendicular to flow responded to increasing velocity by increasing their sag. An analysis of tension in these tentacles, mathematically analogous to that applicable to suspension bridges, shows that sagging permits the tentacles to avoid increases in tension that would otherwise occur as flow increases. Force modulation was achieved by active muscular control rather than by passive material properties. Although these tentacles would certainly break in the experimental flows if they did not sag, the low tension achieved suggests that some other reason, such as limitations on the adherence of cilia and mucus, accounts for the level of tension observed. Because drag is maximum on tentacles oriented perpendicular to flow, reorientation of tentacles, either by sagging or by dangling parallel to flow, additionally reduces tension by reducing drag. Theoretical estimates of drag on tentacles oriented parallel to flow show that they are never in danger of being broken. Drag is sufficient, however, to assist in passive extension of tentacles. While reorientation is a common mode of drag reduction among marine organisms, sagging represents a novel mechanism of mediating structural forces resulting from flow.