METABOLISM AND RAM GILL VENTILATION IN JUVENILE PADDLEFISH, POLYODON-SPATHULA (CHONDROSTEI, POLYODONTIDAE)

Metabolic rate, branchial morphology, and modes of gill ventilation were studied in young (2-10 g) North American paddlefish, Polyodon spathula, with anatomical, behavioral, and physiological methods. Polyodon lacks the oral and opercular values that are typical for fishes that rely on a buccal pump system to ventilate the gills, and the jaw opening system of Polyodon is poorly suited for regular pumping movements. Unrestrained, undisturbed juvenile paddlefishes swim constantly at a mean speed of 1.1-1.5 body lengths . s-1 (bls). The maximum speed sustainable for > 10 min is 1.6-1.8 bls. When forced to swim at slow speeds in flow tanks or water tunnels, ventilation of the gills by buccal pumping occurs at a frequency of 50-80 . min-1. As swimming speed increases, buccal ventilation becomes intermittent and continuous ram ventilation occurs above 0.6-0.8 bls, which means that Polyodon is essentially an obligate ram ventilator under normal conditions. Oxygen consumption (MO2), carbon dioxide production (MCO2), and the gas exchange ratio (R) were determined as a function of inspired PO2 during undisturbed swimming in still water at 25-degrees-C. Oxygen consumption, buccal pressure, and swimming performance were also measured at set swimming speeds in a flow tank and small water tunnel. Oxygen consumption at the preferred swimming speed of 1.25 bls was 6-7-mu-mol O2 . g-1 . h-1. Carbon dioxide production was 3-4-mu-mol CO2 . g-1 . h-1, yielding an R of 0.5-1.0. Paddlefishes are O2 regulators in mild hypoxia (150 down to 90 mmHg) but die quickly at PO2 <90 mmHg. During steady swimming in normoxia, paddlefishes normally maintain 70%-80% of the maximum sustainable speed. This results in a normal minimum metabolic rate that is about twice that of the minimum (resting) rate of other acipensiform fishes. From a phylogenetic standpoint, other acipenseriforms also use ram ventilation, leading to the hypothesis that the evolutionary origin of a reliance on ram ventilation in Polyodon probably predates the origin of the filter feeding habit. Constant swimming may be metabolically expensive, but it would appear to allow some energy to be conserved by ram ventilation. This may be particularly advantageous for species such as P. spathula that combine filter feeding and ram ventilation.