Video frame of an everted Hemipodus simplex proboscis. (A) Dorsal view, with visible crack walls. (B) Lateral view of a different proboscis eversion and corresponding thresholded image ©. In C, the small upper patches of light posterior of the proboscis result from body stress (B). The light patches at the anterior end of the proboscis result from tensile stresses at the crack tip (T), and middle patches result from compressive stress © from the force of pharynx eversion and are the only pixels included in force calculations. (D) Scheme of the worm as a wedge of profile f(z–b) indicating the half-thickness (h) and distances from the crack tip to the point of contact (a) and from the crack tip to the anterior end of the worm (b) used in Eqns 1 and 2. Scale bars, 0.005 m.
Burrow extension with a proboscis: mechanics of burrowing by the glycerid Hemipodus simplex
Abstract
Burrowing marine infauna are morphologically diverse and ecologically important as ecosystem engineers. The polychaetes Nereis virens and Cirriformia moorei extend their burrows by crack propagation. Nereis virens does so by everting its pharynx and C. moorei, lacking an eversible pharynx or proboscis, uses its hydrostatic skeleton to expand its anterior. Both behaviors apply stress to the burrow wall that is amplified at the tip of the crack, which extends by fracture. That two species with such distinct morphologies and life histories both burrow by fracturing sediment suggests that this mechanism may be widespread among burrowers. We tested this hypothesis with the glycerid polychaete Hemipodus simplex, which has an eversible proboscis that is much longer and everts more rapidly than the pharynx of N. virens. When the proboscis is fully everted, the tip flares out wider than the rest of the proboscis, creating a shape and applying a stress distribution similar to that of N. virens and resulting in relatively large forces near the tip of the crack. These forces are larger than necessary to extend the crack by fracture and are surprisingly uncorrelated with the resulting stress amplification at the crack tip, which is also larger than necessary to extend the burrow by fracture. These large forces may plastically deform the mud, allowing the worm to build a semi-permanent burrow. Our results illustrate that similar mechanisms of burrowing are used by morphologically different burrowers.