The swim-and-sink behaviour of copepods: a revisit to mechanical power requirement and a new hypothesis on function

Many copepods display a swim-and-sink behaviour, which is not energetically efficient but probably aids in perceiving and capturing diatom chains. Here, computational fluid dynamics was employed to calculate the mechanical power required by a negatively buoyant, self-propelled copepod in swim-and-sink versus hovering. The results show that upward swim-and-sink about a fixed depth always demands more power than hovering. Subsequently, high-speed microscale imaging was employed to observe the copepod Centropages sp. in swim-and-sink, specifically its encounter and handling of diatom chains for capture, along with the measured alternating swimming and sinking currents imposed by the swim-and-sink copepod. The findings suggest that during upward swimming, the copepod uses its swimming current to scan the fluid for detecting embedded diatom chains, presumably through chemoreception. Once a diatom chain is perceived, the copepod sinks and uses its sinking current to manipulate the orientation of the diatom chain before swimming upward to capture it. Overall, these results propose a hypothesis that swim-and-sink is an innate behaviour that assists copepods in perceiving and manoeuvring diatom chains for capture. In contrast with near-spherical algae, diatom chains predominately exhibit a horizontal orientation in the ocean, necessitating vertically oriented copepods to possess a handling behaviour that manoeuvres diatom chains for capture.