Meeting Abstract
Sessile marine organisms rely on ambient flow for nutrient supply and waste removal. Many cnidarians have flexible tentacles that sway and bend under the influence of waves and currents. Using high speed videography and PIV measurements, we recorded the flow and tentacle motion of three cnidarian species, in-situ and in a standing wave lab flume. Tentacles exhibited an unintuitive motion: they oscillated with the same frequency as the waves, but preceded the waves by around a ¼ of wave period, generating an out-of-phase motion. Our observations (>120) led to two research questions: 1. is there a benefit in out-of-phase motion, in terms of mass transfer? 2. what mechanism permits such motion? We used numerical simulations to estimate absorbance of dissolved oxygen from the environment to the tentacles in a wide range of phase differences. Our simulations show that the observed out-of-phase motion improves mass transfer by up to 2-fold compared to moving in-phase. We tested a simple mechanical model where tentacles were represented as mass-spring systems and used it to non-intrusively measure the spring coefficient of Dipsastrea favus tentacles (κ=1.13±1.24 [dyn cm / rad]). The model suggests the motion of tentacles is due to the mechanical properties of the tissue. We postulate that out-of-phase motion is a general phenomenon shared by cnidarian tentacles (and possibly other flexible marine organisms). Corals can modulate tentacle expansion, possibly indicating that they can adjust the spring coefficient and enhance mass transfer. Our findings demonstrate how these animals, often treated as immobile, can actively affect their interaction with the flow and harness wave motions to improve mass transfer.
