Functional diversity of infaunal burrowers: Towards a mechanistic understanding of animal-sediment interactions

底栖生物群落由种类繁多的生物组成，具有重要的生态和生物地球化学作用。它们将有机碳转化为生物量，再转移到更高的营养水平，再生营养物质，并决定污染物和埋藏在沉积物中的有机碳的命运。在许多沿海环境中，人为压力，包括富营养化和由此造成的缺氧、拖网捕捞和渔业干扰，以及污染物，对物种多样性有负面的、往往是巨大的影响。然而，如果不了解物种的功能作用，评估物种多样性变化的生态和生物地球化学影响几乎是不可能的。在沉积环境中，确定功能对于与沉积物关系密切的生物尤其重要，例如在沉积物中生活和移动时摄取沉积物的动物群。对单个物种的洞穴行为和形态进行了研究，但自从在不同的分类群中比较广泛的洞穴行为以来，几十年过去了。此外，这种比较在很大程度上忽略了沉积物的机械响应，这类似于研究游泳而不考虑流体力学。从那时起，动物-沉积物相互作用的物理学取得了几项重大进展。泥质沉积物是弹性固体，洞穴通过裂隙延伸穿过。相反，砂子是粒状材料，其力学机制由作用于单个颗粒的重力决定，而不是由主导泥浆力学的粘性聚合物基质的粘附性和内聚力决定。使用明胶作为泥浆的清晰模拟物，使钻探可视化以及力和运动学分析成为可能。这项研究将结合在明胶和沙子模拟中挖掘的结构和解剖学研究和运动学分析，以及真实沉积物的机械测试和数值模拟。解剖结构、形态和行为与沙土和泥土的挖洞功能之间存在联系。多毛类环节动物是底栖动物群落的一个丰富而多样的组成部分，将成为重点分类单元。洞穴底栖动物的功能分组一直基于形态和营养作用，但沉积物力学的进展表明，类似的形态在沙子和泥浆中可能具有不同的功能(例如，可膨胀结构扩展泥浆中的裂缝，但在沙子中是锚)。此外，看似不同的形态可能具有相似的功能(例如，中华鳖的咽部和环状环状动物的肌肉前部都施加背腹应力，以扩大骨折后的洞穴)。将功能与洞穴动物的形态和行为联系起来，对于理解底栖动物的功能作用和由此产生的底栖群落的功能多样性是重要的。这项研究揭示的洞穴机制的多样性将使人们能够概括不同环境下的洞穴机制。洞穴运动的重要特征将被确定为不同的穴居者所共有的特征。对于来自不同环境的密切相关的分类群，将对比不同的沙和泥的不同物理约束如何规定挖洞机制，以及如何影响挖洞动物的形态和行为。研究和教育将通过从不同领域招收本科生，并让他们参与与这项跨学科研究相关的团队研究项目来整合。目标是使这些未来的科学家能够发展成功的跨学科沟通和合作所需的技能。这里提出的可视化洞穴动物的方法是经济的，而且很容易融入课堂。此外，将相关物理，特别是连续介质力学纳入生物学研究是重要的，但在适当的课程中往往被忽视。课程将通过ASLO和SICB教育网站开发和共享。由于果冻中的蠕虫已经引起了公众的兴趣，这些结果的广泛传播应该会加强对蠕虫在沉积物中挖洞的机制以及相关话题的科学理解。将与SIO的公共信息办公室斯克里普斯通信公司合作，制作关于这项研究的视频播客。该播客将在斯克里普斯海洋研究所的《探索》电子杂志(Explorations.ucsd.edu)上播出，该杂志拥有1.4万订户，并将通过SciVee与更多的观众进行比较。

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.