Vortical feeding currents in nauplii of the calanoid copepod Eucalanus pileatus

Abstract

The goal of this study was to quantify feeding-current generation processes in mid to late nauplii and early copepodites of the calanoid copepod Eucalanus pileatus. Using a high-speed microscale imaging system (HSMIS) to conduct both microvideography and micro-particle image velocimetry (µPIV), free-swimming nauplii of E. pileatus were shown to use a novel ‘double draw-and-cut’ continuous appendage beat pattern, which is nonreciprocal, to generate a vortical feeding current at a Reynolds number of ~0.8. The feeding current consists of a core flow towards the ventral surface and 2 laterally flanking viscous vortices reinforcing the core flow. This feeding current is spatially limited with an r-3 decay, potentially reducing predation by rheotactic predators. The feeding current displaces water at ~1.0 × 106 naupliar body volumes per day towards the mouthpart zone. This would result in a clearance rate providing sufficient food at a relatively high environmental food concentration. HSMIS videos revealed that E. pileatus nauplii combine their feeding current and swimming motion to displace algae towards their mouth for capture, and can react to an incoming alga at a 300-500 µm distance away from the nearest naupliar setae, indicating remote detection presumably via chemoreception. The r-3-decay naupliar feeding current is suggested to enhance chemoreception by more effectively elongating the algal phycosphere towards the nauplius. Compared with nauplii, E. pileatus early copepodites, being larger in size and negatively buoyant, beat appendages in a more complex, intermittent pattern to generate an r-1-decay feeding current for displacing more water, indicating a trade-off among feeding, predator avoidance, and alga perception.