Recent studies document linear elastic response of muddy marine sediments to load and deformation on temporal and spatial scales relevant to animal movement, with burrowers making openings for movement in such sediments by fracture. Cracks propagate through linear elastic solids in mode I (opening-mode crack growth) when the stress intensity factor (K I) at the crack tip exceeds the material's fracture toughness (K Ic). Fracture mechanics depend on material stiffness as well as fracture toughness, and we prepared a range of transparent gels that varied in stiffness and fracture toughness to assess the dependence of burrowing behavior on these material properties. When the polychaete Nereis virens elongated its burrow, it altered its body shape and behavior across these gels in a manner consistent with fracture mechanics theory. We modeled burrow elongation as stable, wedge-driven crack growth, and calculated that K I values at the tips of the burrows reached K Ic values of most gels without pharynx eversion and exceeded K Ic when the pharynx was everted. In materials with higher fracture toughnesses, worms everted their pharynges to become thicker and blunter wedges, as predicted from simple wedge theory. In stiff materials with low toughness, worms moved their heads from side-to-side to extend crack edges laterally, relieving elastic forces compressing them and allowing them to maintain body shape more easily. This solution extends the crack in small increments that each require relatively little force. We introduce a dimensionless "wedge" number to characterize the relative importance of work to fracture the material and extend the burrow and work to maintain body shape against the elastic restoring force of the material. The mechanism of burrowing by crack propagation is utilized across a range of material properties found in natural muds, and variation in these properties strongly influences burrowing behaviors. These results demonstrate how quantifying the mechanical properties of muds can improve our understanding of bioturbation. On spatial and temporal scales relevant to burrower activity, variations in these properties may impact particle mixing by influencing burrower behavior.

Mudblister worms Polydora websteri bore holes into oyster shells, and oysters respond by secreting extra layers of shell, creating a mudblister. When shucked, mudblisters can burst and release anoxic mud. Thus, infestation devalues oysters, particularly on the half-shell market. This study quantified oyster condition index and worm abundances over 2 full growing seasons at commercial oyster farms on the US Gulf of Mexico coast, and our results indicate that oyster growth rate, manipulated through ploidy and stocking densities, had little effect on worm infestation. Larval spionid worms were found year-round. Larval abundances were slightly higher within than away from farms, and larval size distributions were skewed toward smaller larvae within the farms, suggesting that farms may be a source of larvae. Triploid oysters had higher or comparable condition index values to diploids, but during summer months, when worm infestation was high, worm infestation was not correlated with condition index. Previously infested shells deployed at farms became more infested than uninfested shells at moderate infestation levels, but re-infestation was influenced more by farm location than by previous infestation condition. Higher infestation at a farm with more variable salinity as well as higher infestation in 2017 when salinity was lower suggest that salinity may be a potential driver of mudblister worm infestation. Results indicate that oyster farmers on this coast should use desiccation to treat oysters for mudblister worms frequently during the summer, but that manipulating stocking density or ploidy is unlikely to be effective in preventing mudblister worm infestation.

Infauna exhibit a range of behavioral responses to declining dissolved oxygen concentrations that affect their burrowing and feeding behaviors. Diel oxygen cycles, common in shallow coastal areas, may drive changes in faunal behavior that affect sediment mixing. In this laboratory study we exposed 3 species, the burrowing ophiuroid Hemipholis cordifera, the tube-dwelling polychaete Owenia fusiformis, and the burrowing clam Ameritella versicolor, to 60 h of diel cycling dissolved oxygen in the overlying water, with oxygen concentration varying between 2 and 7 mg l-1. We observed the study organisms’ behaviors and evaluated their sediment mixing activity using luminophores applied to the sediment surface. We found that sediment mixing activity of all 3 taxa, measured as percent decrease in luminophore coverage, varied proportionally with dissolved oxygen concentration during the diel cycle. Observations of animal behavior did not reveal a diel pattern, though this was likely due to the temporal and spatial scale of observations. Our results also indicated that diel cycling oxygen may change faunal effects on the sediment in ways that only emerge after more than a single cycle. Measuring sediment mixing in sustained full oxygen saturation may produce misleading estimates over time, and future research should investigate how faunal responses to short-term variability can scale to have longer-term effects.