“A marine polychaete worm, Eupolymnia heterobranchia, lives in a tube in a mud flat with its front end sticking out. It stretches out its long, flexible, sticky, feeding tentacles crosswise to flow…like the main cables of an extension bridge but loaded by drag instead of gravity. According to Johnson (1993), extended straight from animal to anchor point, the tentacles could not withstand much drag. But by a combination of passive sagging and muscular activity, they extend ever farther as flow increases. Drag may increase with flow, but the tentacle-stretching tensile consequence of drag does not–tension in the tentacles remains almost constant over at least a sevenfold speed range.” (Vogel 2003:329)
Comparative Biomechanics: Life's Physical World, Second EditionPrinceton University PressJune 17, 2003
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.