Thixotropy, and the Mechanics of Burrowing in the Lugworm (Arenicola marina L.)

Abstract

Chapman and Newell¹ have recently published the following argument : (1) Lugworms can burrow into a particular beach whether covered by sea-water or not. The total downward thrust which a worm can exert is limited by its weight. If one divides the immersed weight of a worm by the area of its front end, one finds "that the maximum pressure which can theoretically be exerted cannot exceed 16 g./sq. cm.". (2) A man can walk across the same beach without sinking in, though his feet exert a pressure of about 150 gm./sq. cm. (3) "The explanation of this apparent anomaly must be sought in the physical properties of the wet sand." The authors demonstrate that the sand is thixotropic by "stepping five times with the weight of the body on the foot" ; this causes the sand to liquefy, after which it fairly rapidly sets again. They suggest that "when a worm begins to burrow in the sand, its resistance is first reduced by alternating extrusion and withdrawal of the proboscis and then the anterior end of the worm is thrust into the softened sand".