RESPIRATION AND DEPTH CONTROL AS POSSIBLE REASONS FOR SWIMMINGOF NORTHERN ANCHOVY, ENGRAULIS MORDAX, YOLK-SAC LARVAE

Larval northern anchovy in the yolk-sac (nonfeeding) stage exhibit regular bursts of continuous swimmingduring the first 3 days after hatching. Ithas been suggested that this behavior may have a respiratory function. A different possibility is depth control, countering the tendency of the larvae to sink when motionless. This paper includes a theoretical and experimental investigation ofthe possible functions of these swimming bouts. The theoretical approach was to define a model and calculate the oxygen available to the larva when resting and while moving, and experiments were performed as a check of the theoretical results. The experiments were conducted on yolk-sac larvae in sealed tanks with varying dissolved oxygen concentrations to determine the effects of reducing the available oxygen on the frequency and duration of the swimming bursts. Results of the experiments confirmed the theoretical model. They indicate that the swimming bouts both help the larva stay at a constant depth and have a respiratory function when the oxygen concentration in seawater is less than 60% ofsaturation. Newly hatched northern anchovy, Engraulis mordax, larvae exhibit a pattern of regular short bouts of continuous swimming interspersed with periods of resting. These larvae are still in the yolk-sac stage and are not feeding so that the locomotory behavior must have some other pur­ pose, as these motions are energy consuming and also endanger the animal by attracting predators (Lillelund and Lasker 1971). Hunter (1972) suggested that these swimming bouts might have a respiratory function. Respiration has to be by cutaneous diffusion through the 2-3 /-tm thick skin (Lillelund and Lasker 1971) of the larvae as the gills develop only at a later stage. The purpose of this paper is to test this hypothesis and another possibility, depth control, to counter sinking due to the negative buoyancy, using theoretical and experimental methods. First, I develop a theoretical model for oxygen transport to motionless and swimming yolk-sac larvae and estimate the possible oxygen uptake. Next, I describe the experiments to test the predic­ tion of the theory for both proposed mechanisms and compare their results.