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

In s ~t u videotapes indicate that the planktonic m y s ~d Mysidlum columb~ae exhibits positive rheotactic behavior and can use prop roots and other objects as visual cues to m a ~n t a i n position in currents of changing speed within the mangrove prop loot environment The ability of mysids to hold poslt~on In currents of different speed was measured in the laborato~y in a flow-through chamber The response of mysids to vlsual cues was tested with an optokinetlc drum When the drum rotates, the mysids srvlm at the same speed and in the same d~rection as the moving stripes Swimming speed of the m y s ~d s was measured using video-computer motion analysis techniques By varylng the speed of currents in the flow-through chamber or by c h a n g ~n g the rotational speed of the optokinetic drum, n ~y s ~d s can be 'forced' to swim at vanous speeds By measuring the decrease in oxvqen concentration within a sealed chamber, while simultaneously monitonng swlnlming speed with a video camerd, the metabolic cost of increased swimming speeds was calculated The respiration rates of M columbiae more than double when swimming at sustained high speed (ca 25 mnl S ') These n ~y \ ~d s are preyed on by a wide range ot planktivorous fish under laboratory conditions, and their survlval may depend upon their abillty to maintaln their position within the safety of the prop root habitat d u n n g darlight houis In spite of currents and turbulence that rvould tend to d ~s p e i s e them into adjacent open water habitat KEY WORDS-Mysids .