Abstract
Many small marine planktonic organisms converge on similar propulsion mechanisms that involve impulsively generated viscous wake vortex rings, and small-scale fluid physics is key to mechanistically understanding the adaptive values of this important behavioral trait. Here, a theoretical fluid mechanics model is developed for plankton jumping, based on observations that the initial acceleration phase for a jumping plankter to attain its maximum speed is nearly impulsive, taking only a small fraction of the viscous timescale, and therefore can be regarded as nearly inviscid, analogous to a one-dimensional elastic collision. Flow circulation time-series data measured by particle image velocimetry (PIV) are input into the model and Froude propulsion efficiencies are calculated for several plankton species. Jumping by the tailed ciliate Pseudotontonia sp. has a high Froude propulsion efficiency ~0.9. Copepod jumping also has a very high efficiency, usually >0.95. Jumping by the squid Doryteuthis pealeii paralarvae has an efficiency of 0.44 ± 0.16 (SD). Jumping by the small medusa Sarsia tubulosa has an efficiency of 0.38 ± 0.26 (SD). Differences in the calculated efficiencies are attributed to the different ways by which these plankters impart momentum on the water during the initial acceleration phase as well as the accompanied different added mass coefficients.
Highlights
Marine planktonic organisms play crucial roles in marine ecosystems and biogeochemical cycling in the world ocean; most of them are of microscopic size, having no or limited swimming capabilities relative to the macroscopic water parcels within which they are embedded
For jumps of a few species considered in this study, the Froude propulsion efficiencies calculated using the newly developed elastic collision model can be compared to available previous results obtained using completely different methods
The Froude propulsion efficiencies calculated in the present study for jumps of the copepods Acartia tonsa and Calanus finmarchicus (Table 2) compare well to the previous computational fluid dynamics (CFD) simulation results that range from 0.94 to 0.98 [21]
Summary
Marine planktonic organisms play crucial roles in marine ecosystems and biogeochemical cycling in the world ocean; most of them are of microscopic size, having no or limited swimming capabilities relative to the macroscopic water parcels within which they are embedded. Froude propulsion efficiency for copepod jumping appears quite counter-intuitive with respect to the dominantly viscous water environment in which the copepod resides, it means that the part of the mechanical work done to generate the wake vortex is significantly smaller than W useful. It is unknown how the Froude propulsion efficiency varies for jumping by other small plankters that differ in body morphology and size and propulsion machinery. Here, the convergent evolution manifests itself in the behavioral traits which those small plankters possess to exert thrust on water in an astonishingly quick and impulsive fashion
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