Sediment properties and burrowing of Abarenicola pacifica and A. claparedi vagabunda in False Bay

Recent research on burrowing mechanics suggests that animals may be moving through crack propagation and that the material properties of a sediment may be important for the ability of animals to move it. Two species of Abarenicola who inhabit distinct areas of False Bay were examined. It was hypothesized that a sediment’s stiffness would be related to its grain-size composition and to animal distributions, and that worms would burrow more successfully in their own sediment. In sites with either A. pacifica or A. claparedi, grain size distribution was analyzed and a modified penetration test was used to quantify the stiffness of sediments. Additionally, each species was timed burrowing in the field in either its own sediment or the sediment of the other species. A. pacifica lived in stiffer, poorly-sorted sediment with a high proportion of silts and clays, while A. claparedi were found in less stiff, well-sorted sediments consisting mostly of fine sands. Despite living in distinct sediment types, worms did not burrow more quickly in their own sediment. However, burrowing angle and speed changes throughout burrowing suggest A. claparedi struggled to burrow successfully in the stiffer sediment of A. pacifica. Our results suggest that the material properties of sediment do vary with the grain size composition and that these material properties may play an important role in determining animal success in an area. However, the affect of the material properties of sediment on burrowing of Abarenicola in the field is not as simple as originally hypothesized