PADDLEFISH BUCCAL FLOW VELOCITY DURING RAM SUSPENSION FEEDING AND RAM VENTILATION

A micro-thermistor probe was inserted into the buccal cavity of freely swimming paddlefish to measure flow velocity during ram ventilation, ram suspension feeding and prey processing. Swimming speed was measured from videotapes recorded simultaneously with the buccal flow velocity measurements. Both swimming velocity and buccal flow velocity were significantly higher during suspension feeding than during ram ventilation. As the paddlefish shifted from ventilation to feeding, buccal flow velocity increased to approximately 60 % of the swimming velocity. During prey processing, buccal flow velocity was significantly higher than the swimming velocity, indicating that prey processing involves the generation of suction. The Reynolds number (Re) for flow at the level of the paddlefish gill rakers during feeding is about 30, an order of magnitude lower than the Re calculated previously for pump suspension-feeding blackfish. These data, combined with data available from the literature, indicate that the gill rakers of ram suspension-feeding teleost fishes may operate at a substantially lower Re than the rakers of pump suspension feeders.

Periodic respiration of gill-breathing fishes

Rhythmic and arrhythmic patterns of breathing are common among gill-breathing fishes. Irregular, short bouts of apnea occur in most fishes during feeding, while long apneic periods are routine for many open-water fishes such as scombrids, which ram ventilate during cruise swimming. During ram ventilation, the work of gill ventilation is transferred from the respiratory to the swimming musculature, with energy savings due to reductions in drag and inertial losses. Noncontinuous swimmers, such as some benthic and midwater marine and freshwater species, seldom cease rhythmic respiratory movements or resort to ram ventilation. When quiescent, they may adopt patterns of secondary cycling, alternating between respiratory pauses and short periods of rhythmic branchial pumping. Types and locations of chemo- and mechano-receptors that trigger changes in respiratory patterns of fish are being identified, as are the reflex pathways linking them to brainstem respiratory centers. A new mechanoreceptor is described that overlies the adductor mandibulae jaw muscles and may be of use in the modulation of cyclic respiratory movements. Respiratory switching control between rhythmic and ram gill ventilation is discussed.

The Transition between Branchial Pumping and Ram Ventilation in Fishes: Energetic Consequences and Dependence on Water Oxygen Tension

1. Ram ventilation has been demonstrated in rainbow trout, Salmo gairdneri. The swimming speed eliciting the transition in mode of ventilation increased with declining ambient water oxygen tension (PwOO2). 2. The mode of ventilation of fish swimming at a constant speed could be altered by controlled variations in PwOO2. 3. Oxygen consumption (V·OO2) decreased by 10.2% when rainbow trout, swimming at a constant speed, shifted from active to ram ventilation. This difference can be ascribed to a lowering of the energetic cost of active ventilation as well as improved drag characteristics. 4. Water velocity required for the transition from active to ram ventilation in the resting sharksucker, Echeneis naucrates, increased with decreasing PwOO2. 5. The results show that water oxygen tension is an important stimulus for setting the ventilatory mode in rainbow trout and sharksucker.

O2 and CO2 transport in relation to ventilation in the Atlantic mackerel, Scomber scombrus

Adult mackerel actively ventilate their gills whilst swimming at low speeds (20–60 cm∙s−1; ca. 0.6–2.0 body lengths∙s−1) in a swim tunnel at 15 °C. There was a regular alternation of a buccal pump, with dominant positive pressure phase, and an opercular pump, with dominant negative pressure phase. The two pumps cooperated to produce a maintained differential pressure across the gills (buccal to opercular cavities) so that water would flow in this direction for most, if not all, of the respiratory cycle. At swimming velocities between 60 and 80 cm∙s−1 (2.0–2.6 body lengths∙s−1), mackerel were found to suspend their cyclic buccal–opercular pump and rely instead on ventilation of the gills by forward movement through the water column (ram ventilation). The transition from one type of ventilation to the other caused no major change in the oxygenation of arterial blood, though CO2 was eliminated readily at the higher swimming speeds despite its being produced in larger quantities. Variations in arterial acid–base relationships were largely due to the mackerel's response to capture and subsequent experimental procedures, and not to the change from cyclic to ram ventilation. It is concluded that arterial blood oxygenation and CO2 output are regulated within limits that suggest the gills are adequately ventilated at all swimming speeds.

Analysis of Ram Ventilation of Fish Gills with Application to Skipjack Tuna (Katsuwonus pelamis)

Some species of fish use "ram ventilation" to pass water over the gills while swimming. Hydrodynamic analysis provides estimates of the pressure losses as the water passes through the gill spaces, and of the resistance force that must be overcome by the swimming muscles. For a 44-cm skipjack tuna (Katsuwonus pelamis), swimming at its basal speed of 66 cm/sec, the gill resistance is estimated to be 1090 dynes or 7% of the total swimming resistance. If oxygen consumption doubles, gill resistance increases to 27% of the total. The energetic cost of respiration lies between 1 and 3% of the total metabolism at basal swimming speed.