Benthic communities comprise diverse and abundant organisms with important ecological and biogeochemical roles. They convert organic carbon into biomass that is transferred to higher trophic levels, regenerate nutrients, and determine the fate of pollutants and organic carbon buried in sediments. In many coastal environments, anthropogenic stresses, including eutrophication and resulting hypoxia, trawling and disturbance from fisheries, and pollutants have negative and often dramatic affects on species diversity. Assessing the ecological and biogeochemical impacts of changes in species diversity is nearly impossible, however, without understanding the functional roles of the species. In sedimentary environments, determining functionality is especially important for organisms closely associated sediments, such as infaunal deposit feeders that ingest sediments while living in and moving through them.
Burrowing behaviors and morphologies have been examined for individual species, but decades have passed since even broad burrowing behaviors were compared across diverse taxa. Moreover, such comparisons largely ignored the mechanical response of sediments, an omission similar to studying swimming without considering fluid mechanics. Since that time, there have been several major advances in the physics of animal-sediment interactions. Muddy sediments are elastic solids through which burrows are extended by fracture. In contrast, sands are granular materials whose mechanics are governed by gravitational forces acting on individual grains, rather than by adhesion and cohesion of the mucopolymeric matrix dominating mud mechanics. Use of gelatin as a clear analog for muds has enabled visualization of burrowing and analyses of forces and kinematics. This research will combine structural and anatomical studies and kinematic analyses of burrowing in gelatin and sand analogs with mechanical testing and numerical modeling of real sediments. Linkages would be made among anatomies, morphologies, and behaviors to burrowing function in sands versus muds. Polychaetous annelids, a diverse and abundant component of benthic communities, will be the focal taxon.
Functional groupings of burrowing infauna have been based on morphologies and trophic roles but advances in sediment mechanics suggest that similar morphologies may have different functions in sands versus muds (e.g., expansible structures extend cracks in muds but are anchors in sands). In addition, seemingly different morphologies may have analogous functions (e.g., the pharynx of Nereis virens and the muscular anterior of the cirratulid Cirriformia moorei both exert dorso-ventral stress to extend burrows by fracture). Linking functions to morphologies and behaviors of burrowers is important in understanding functional roles of infauna and resulting functional diversity of benthic communities. The diversity of burrowing mechanisms revealed in this study will enable generalizations about burrowing mechanics in different environments. Important characteristics of burrowing locomotion will be identified as those shared by diverse burrowers. How the different physical constraints of sand and mud specify burrowing mechanics and affect morphologies and behaviors of burrowers will be contrasted for closely related taxa from different environments.
Broader Impacts. Research and education will be integrated by recruiting undergraduates from different fields and involving them in team research projects associated with this interdisciplinary study. The goal is to enable these future scientists to develop skills necessary for successful communication and collaboration across disciplines. The methods of visualizing burrowers proposed here are economical and easy to incorporate in classes. Moreover, incorporating relevant physics, especially continuum mechanics, in biology studies is important, yet often neglected, in suitable courses. Curricula will be developed and shared through ASLO and SICB educational websites. Because worms in Jell-O have already captured the interest of the general public, broad dissemination of these results should enhance scientific understanding of the mechanics of worm burrowing in sediments, and related topics. In collaboration with Scripps Communications, the public information office of SIO, a video podcast about this research will be produced. The podcast will be featured in Scripps Institution of Oceanography's "Explorations" electronic magazine (explorations.ucsd.edu), which has 14,000 subscribers and would reach comparisons of additional viewers through SciVee.
