2202 INTRODUCTION Burrowing has been considered to be much more energetically expensive than other forms of locomotion, but it has been argued that the high cost of burrowing is justified because sediments provide habitat and a refuge from predators (Hunter and Elder, 1989). Trevor calculated high energetic cost of burrowing per distance traveled for several polychaetes and suggested that soft-bodied burrowers are less efficient than those with a rigid exoskeleton (e.g. Emerita) (Trevor, 1978). Crawling with a hydrostatic skeleton has been found by several authors to be costly (Denny, 1980; Casey, 1991; Berrigan and Lighton, 1993). Generalizing locomotory strategies across animals with hydrostatic skeletons is questionable, however, as the mechanics of burrowing in saturated sediments differ substantially from those of terrestrial crawling (Dorgan, 2010). Whereas costs of transport for running, flying and swimming are measured from oxygen consumption rates, previous estimates for burrowing by polychaetes were based on external work converted to metabolic work, assuming an efficiency constant. This approach makes comparisons between burrowing and other modes of locomotion difficult. Moreover, these estimates of external work to burrow (Trevor, 1978; Hunter and Elder, 1989) implicitly assumed that sediment plastically deformed around the worm to create a burrow. Recent work, however, has shown that marine muds on the short time and length scales relevant for burrowing worms are elastic solids (Johnson et al., 2002; Boudreau et al., 2005), and that worms extend burrows through muds by fracture (Dorgan et al., 2005; Dorgan et al., 2007; Dorgan et al., 2008; Che and Dorgan, 2010). For example, the polychaete Nereis virens Sars everts its pharynx to apply stress to the walls of its burrow, perpendicular to its direction of travel. This stress is amplified at the crack tip just in front of the worm, and when enough stress is applied to exceed the fracture toughness of the sediment, the burrow extends by fracture (Dorgan et al., 2005). This mechanism allows the worm to apply forces over relatively small distances to either extend the crack out in front of the pharynx or to simply extend the crack tip by moving forward in the burrow like a wedge (Dorgan et al., 2008). Intuitively, applying small forces that are amplified through mechanical advantage to extend a crack through the substratum should require less work than plastic deformation of the mud, suggesting that external work to burrow was probably overestimated. At the very least, previous estimates of the external work are clearly based on an inappropriate mechanical model and therefore should be re-evaluated. A major limitation on measuring the energetic cost of burrowing has been that oxygen consumption measurements in muds are confounded by high abundances of bacteria and fauna that consume oxygen at rates that are difficult to quantify or control. Trevor measured oxygen consumption rates of burrowing polychaetes but pointed out that his measurements were probably inaccurate and instead used external work to calculate the energetic cost of burrowing (Trevor, 1978). That organisms in addition to the burrower were consuming oxygen during his experiments is suggested by the fact that his estimate of oxygen consumption of 6.9 mg O 2 (g wet mass) –1 h –1 The Journal of Experimental Biology 214, 2202-2214 © 2011. Published by The Company of Biologists Ltd doi:10.1242/jeb.054700 RESEARCH ARTICLE Energetics of burrowing by the cirratulid polychaete Cirriformia moorei Kelly M. Dorgan 1, * ,† , Stephane Lefebvre 2 , Jonathon H. Stillman 1,2 and M. A. R. Koehl 1 1 Department of Integrative Biology, University of California, Berkeley, CA 94720-3140, USA and 2 Romberg Tiburon Center for Environmental Studies, San Francisco State University, Tiburon, CA 94920, USA *Present address: Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093-0202, USA † Author for correspondence (kdorgan@ucsd.edu) Accepted 18 March 2011 SUMMARY Burrowing through marine sediments has been considered to be much more energetically expensive than other forms of locomotion, but previous studies were based solely on external work calculations and lacked an understanding of the mechanical responses of sediments to forces applied by burrowers. Muddy sediments are elastic solids through which worms extend crack- shaped burrows by fracture. Here we present data on energetics of burrowing by Cirriformia moorei. We calculated the external energy per distance traveled from the sum of the work to extend the burrow by fracture and the elastic work done to displace sediment as a worm moves into the newly formed burrow to be 9.7 J kg –1 m –1 in gelatin and 64 J kg –1 m –1 in sediment, much higher than for running or walking. However, because burrowing worms travel at slow speeds, the increase in metabolic rate due to burrowing is predicted to be small. We tested this prediction by measuring aerobic metabolism (oxygen consumption rates) and anaerobic metabolism (concentrations of the anaerobic metabolite tauropine and the energy-storage molecule phosphocreatine) of C. moorei. None of these components was significantly different between burrowing and resting worms, and the low increases in oxygen consumption rates or tauropine concentrations predicted from external work calculations were within the variability observed across individuals. This result suggests that the energy to burrow, which could come from aerobic or anaerobic sources, is not a substantial component of the total metabolic energy of a worm. Burrowing incurs a low cost per unit of time. Supplementary material available online at http://jeb.biologists.org/cgi/content/full/214/13/2202/DC1 Key words: burrowing, energetics of locomotion, cost of transport, polychaete locomotion, aerobic metabolism, anaerobic metabolism, tauropine. THE฀JOURNAL฀OF฀EXPERIMENTAL฀BIOLOGY