Meeting Abstract
Animals have an unparalleled capacity to pump and swim through water, but these hydrodynamic properties have been largely overlooked as a factor in early animal evolution. By collectively driving water currents, and compressing corresponding diffusion gradients, even the simplest colonies of flagellated cells gain significant gas-exchange and feeding advantage. These advective effects multiply at larger length scales, particularly in combination with differentiated muscle tissue. At a cnidarian grade of organization, they introduced animals to inertial and turbulent fluid dynamic regimes with negligible metabolic investment. In concert with hydrodynamically tuned morphologies they also led to the phenomenon of swimming, and the metazoan take-over of pelagic ecology. Swimming animals actively mix and ventilate much of the modern oceans as a by-product of their feeding activities. In addition to the repackaging of dispersed surface-generated productivity as sedimenting faecal pellets, a significant fraction is transported actively to depth via diurnal vertical migration (DVM), a behavioural consequence of visual predation, muscular swimming and the size-structured tiering of pelagic food webs. Such activity both aerates the surface ocean and concentrates biological oxygen demand at depth in the form of oxygen minimum zones – independently of atmospheric oxygen concentration or net carbon burial. In this light, it is clear that the depth of DVM and the nature of OMZs must have changed systematically through time, in concert with escalatory innovations in the size and speed and of marine predators. This alone may account for the step-wise changes observed in marine redox signatures through the later Neoproterozoic and early Palaeozoic, as well as intervals experiencing mass extinction.
