Worming its way through mud

Written by Thomas Hesselberg

Tuesday, 04 December 2007

A ragworm with its pharynx (or jaws) extended as viewed under the microscope.

For many of us, worms are long pink creatures found in the ground with no visible head and mainly good for attaching to hooks when we go fishing. However, the segmented worms in the phylum Annelida comprise other animals than just the earth worms. The ragworms for instance in the class Polychaeta have a distinct head region with eyes, palps and antenna as well as appendages on each of the body segments, which is used as leg during crawling or as paddles during swimming. But like the earth worm burrows in the soil, the ragworm burrows in the muddy sediment in the shallow coastal waters where it lives.

Now American scientists have investigated how the large ragworm Nereis virens burrows through the sediment. Instead of using actual mud, which would make observing the burrowing process difficult, the researchers used gelatine as a substitute. A further advantage of using gelatine is that the material, if viewed with a polarising filter, changes light intensity when stressed. Thus the material itself gives valuable information on the forces produced by the worm during burrowing.

The scientists discovered that the worms seem to burrow through the mud by crack propagation. By extending their pharynx they creates small cracks in the gelatine, which due to the material properties, causes large cracks to appear behind the initial small crack. The scientists then created finite element models on the computer and found that the burrowing mechanism observed in gelatine, would also work in muddy sediments. Thus it seems that ragworm burrowing requires considerably less energy that hitherto believed.

Sources

Dorgan, K. M., Arwade, S. R. and Jumars, P. A. (2007). Burrowing in marine muds by crack propagation: kinematics and forces. Journal of Experimental Biology 210, 4198-4212.

Dorgan, K. M., Jumars, P. A., Johnson, B. D., Boudreau, B. P. and Landis,
E. (2005). Burrow elongation by crack propagation. Nature 433, 475.

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