Passive drift or active swimming in marine organisms?

Abstract

Predictions of organismal movements in a fluid require knowing the fluid's velocity and potential contributions of the organism's behaviour (e.g. swimming or flying). While theoretical aspects of this work are reasonably well-developed, field-based validation is challenging. A much-needed study recently published by Briscoe and colleagues in Proceedings of the Royal Society B compared movements and distribution of satellite-tracked juvenile sea turtles to virtual particles released in a data-assimilating hindcast ocean circulation model. Substantial differences observed between turtles and particles were considered evidence for an important role of active swimming by turtles. However, the experimental design implicitly assumed that transport predictions were insensitive to (i) start location, (ii) tracking duration, (iii) depth, and (iv) physical processes not depicted in the model. Here, we show that the magnitude of variation in physical parameters between turtles and virtual particles can profoundly alter transport predictions, potentially sufficient to explain the reported differences without evoking swimming behaviour. We present a more robust method to derive the environmental contributions to individual movements, but caution that resolving the ocean velocities experienced by individual organisms remains a problem for assessing the role of behaviour in organismal movements and population distributions.

Nurdle drifters around South Africa as indicators of ocean structures and dispersion

“…Opportunities to study marine larval movement in the context of population connectivity are vital for both benthic species that use the planktonic larval stages to connect sessile populations 38 and for the management and conservation of fished species that require regional management efforts 39 .…”

Observation and quantification of inertial effects on the drift of floating objects at the ocean surface

“…Examples of relevant aspects include clustering at the center of the subtropical gyres 18,19 , phenomenon supported on measurements of plastic debris concentration 7 and the analysis of undrogued drifter trajectories 18,19 , or the role of mesoscale eddies as attractors or repellers of inertial particles depending on the polarity of the eddies and the buoyancy of the particles 19,27,28 despite the Lagrangian resilience of their boundaries [48][49][50][51] , which is also backed on observations 52 . The cited phenomena, which act on quite different timescales, all require both O(1) and O(τ ) terms in (12) for their description 18,19,27,28 consistent with the slow manifold M τ in (14), rather than the critical M 0 , controlling the time-asymptotic dynamics of the τ → 0 limit of the Maxey-Riley set (5).…”

“…The assessment of motions of floating matter in the ocean is of importance for a number of key reasons. These range from improving search-and-rescue operations at sea 1,2 ; to better understanding the drift of flotsam of different nature including macroalgae such as Sargassum [3][4][5] , plastic litter 6,7 , airplane wreckage 8,9 , tsunami debris 10,11 , sea-ice pieces 12 , larvae 13,14 , and oil 15,16 ; to better interpreting "Lagrangian" observations in the ocean 17,18 . At present, largely piecemeal, ad-hoc approaches are taken to simulate the effects of ocean currents and winds on the drift of floating objects.…”

Distribution of genetic diversity reveals colonization and philopatry of the loggerhead sea turtles across geographic scales

“…Particularly, sea turtles are capable of homing on a scale of a few kilometres (Lee et al, 2018). Upon hatching, neonate enter the ocean, find the major currents to actively escape predator-rich coastal waters (Scott et al, 2014; Putman & Mansfield, 2015; Putman et al, 2016), and disappear for a period known as the “lost years” (Carr, 1987). At sexual maturity, turtles return to their natal rookery likely using a combination of geomagnetic and olfactory cues (Brothers & Lohmann, 2015; Cameron et al, 2019).…”