Energetic cost of position-holding behavior in the planktonic mysid Mysidium columbiae

Abstract

In situ videotapes indicate that the planktonic mysid Mysidium columbiae exhibits positive rheotactic behavior and can use prop roots and other objects as visual cues to maintain position in currents of changing speed within the mangrove prop root environment. The ability of mysids to hold position in currents of different speed was measured in the laboratory in a flow-through chamber. The response of mysids to visual cues was tested with an optokinetic drum. When the drum rotates, the mysids swim at the same speed and in the same direction as the moving stripes. Swimming speed of the mysids was measured using video-computer motion analysis techniques. By varying the speed of currents in the flow-through chamber or by changing the rotational speed of the optokinetic drum, mysids can be forced to swim at various speeds. By measuring the decrease in oxygen concentration within a sealed chamber, while simultaneously monitoring swimming speed with a video camera, the metabolic cost if increased swimming speeds was calculated. The respiration rates of M. columbiae more than double when swimming at sustained high speed (ca25mm s-1). These mysids are preyed on by a wide range of planktivorous fish under laboratory conditions, and their survival may depend upon their ability to maintain their position within the safety of the prop root habitat during daylight hours in spite of currents and turbulence that would tend to disperse them into adjacent open water habitat.