Date of Award

12-2007

Level of Access Assigned by Author

Open-Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Oceanography

Advisor

Peter A. Jumars

Second Committee Member

Bernard P. Boudreau

Third Committee Member

Eric N. Landis

Abstract

Marine muds are elastic solids through which animals move by propagating a crack-shaped burrow. Dilations previously considered anchors serve to exert dorsoventral compressive stresses on the burrow walls that, through elastic behavior of the medium, focus strongly at the tip of the burrow. This focused stress breaks adhesive or cohesive bonds, propagating a crack for the animal to follow. The force exerted by the polychaete, Nereis virens, to propagate a crack has been measured in gelatin, an analogue of muddy sediment, through photoelastic stress analysis. Finite element analysis was used to convert measured forces to those exerted in natural sediments based on differences in stiffnesses between gelatin and mud. From linear elastic fracture mechanics theory, it is predicted that a crack propagates when the stress intensity factor, a measure of stress amplification at the crack tip, exceeds a critical value, the fracture toughness. Stress intensity factors, calculated from measured forces using finite element modeling, fall within the range of critical values measured in gelatin and exceed those in natural sediments. Stress intensity factors were also calculated from the shapes of worms burrowing in transparent gels with varied mechanical properties, and fell close to or exceeded respective critical values. These results, using two independent measurements, strongly support that the mechanism underlying burrowing is crack propagation. Behavioral differences were observed by worms burrowing in gels with different mechanical properties, and can be explained by the differences in mechanics. This mechanism of burrowing by fracture is consistent with descriptions of burrowing across phyla and helps explain long-puzzling anatomies and behaviors of burrowing animals. Understanding of this mechanism raises questions about the reputed high energetic cost of burrowing, feeding guild classifications—specifically surface deposit feeders, and identifies some potential artifacts in benthic studies of chemistry and bioturbation. Both behaviors of burrowers and responses of sediments to forces exerted by burrowers depend on the mechanical properties (stiffness and fracture toughness), and understanding of that relationship will lead to advances in automaton modeling of bioturbation. Any serious mechanical analysis of swimming involves relevant physical properties of the medium. Going forward, the same will now be true of burrowing.

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