Strategies of burrowing in soft muddy sediments by diverse polychaetes

Muddy sediments are elastic solids through which morphologically diverse animals extend burrows by fracture. Muddy sediments inhabited by burrowing infauna vary considerably in mechanical properties, however, and at high enough porosities, muds can be fluidized. In this study, we examined burrowing behaviors and mechanisms of burrow extension for three morphologically diverse polychaete species inhabiting soft muddy sediments. Worms burrowed in gelatin, a transparent analog for muddy sediments, and in natural sediments in a novel viewing box enabling visualization of behaviors and sediment responses. Individuals of Scalibregma inflatum and Sternaspis scutata can extend burrows by fracture, but both also extended burrows by plastic deformation and by combinations of fracture and plastic deformation. Mechanical responses of sediments corresponded to different burrowing behaviors in Scalibregma; direct peristalsis was used to extend burrows by fracture or a combination of plastic deformation and fracture, whereas a retrograde expansive peristaltic wave extended burrows by plastic deformation. Burrowing speeds differed between behaviors and sediment mechanical responses, with slower burrowing associated with plastic deformation. Sternaspis exhibited less variability in behavior and burrowing speed but did extend burrows by different mechanisms consistent with observations of Scalibregma. Individuals of Ophelina acuminata did not extend burrows by fracture; rather individuals plastically deformed sediments similarly to individuals of the related Armandia brevis. Our results extend the range of natural sediments in which burrowing by fracture has been observed, but the dependence of burrow extension mechanism on species, burrowing behavior, and burrowing speed highlights the need for better understanding of mechanical responses of sediments to burrowers.