Collaborative Research: Dynamic similarity or size proportionality? Sensory ecological adaptations of Euchaeta to viscosity

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Small planktonic organisms like copepods live at the interface of laminar and turbulent regimes, which is a fluid environment that is not well understood. It is not turbulent, like the rumbly wake behind a ski boat. It is not predictable laminar flow, like the steady flow creeping by a smooth stone. In this transitional environment, even small changes in the viscosity of the water can impact an organism’s behavior and sensory perception in unexpected ways. Waters in the polar regions have twice the viscosity as that in the subtropics. In addition to viscosity there are thermal effects on physiology and differences in organism size. Nevertheless, pilot studies indicate that polar species are dynamically similar to the subtropical ones. This suggests their fluid-object interactions with their surrounding environment is very similar from the poles to the subtropics. The goal of this study is to measure nerve impulse conduction velocities, respiration rates, swimming and escape speeds, and muscle mass to determine whether and what metabolic compensation is occurring to maintain this dynamic similarity. The broader impacts include training early career scientists at different stages of their education to work across STEM disciplines. Eight trainees ranging from undergraduate to post-doctoral levels are working within the fields of fluid dynamics, marine biology and neurophysiology to address questions surrounding the evolution of key organisms in the ocean. By creating an educational ladder in the lab, students are learning to mentor other students as they learn the scientific method. Outreach is focused on incorporating results from this project into exhibits at the Museum of Design Atlanta, Georgia to share how planktonic organisms can become part of innovative design using solutions from nature to improve the way problems are solved. Flow regimes at intermediate Reynolds number are characterized by the transition between viscous and inertia-dominated realms. Zooplankton like copepods operate within this interface. These small organisms detect prey, predators and mates by sensing small changes in the fluid that surrounds them. However, fluid viscosity alters the fluid signals that are created and perceived by the organisms and how this affects the performance of individual copepods is poorly understood. The goal of this project is to investigate the role viscosity plays as an evolutionary force leading to adaptations in body size, volume of flow field, sensor length and neural function, swimming speeds and muscle mass. The model system for this study is a group of three species in the genus Euchaeta. The target species have evolved to live in a gradient of fluid regimes spanning temperatures from 0 to 23ºC and viscosities from 1.84 to 1 Centistokes. The species vary in length by three-fold and swim at speeds from less than 1 to over 103 millimeters per second. Nerve impulse conduction velocities, respiration rates, swimming and escape speeds, and muscle mass are being measured experimentally under a range of viscosities to elucidate underlying mechanisms of metabolic compensation involved in the maintenance of dynamic similarity from the subtropics to the poles. The focus on the congeners offers a natural experiment to examine the effects of viscosity in an organism that lives at intermediate Reynolds number where viscous forces are important.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.