Gas exchange and control of breathing in the electric eel, Electrophorus electricus
The electric eel Electrophorus electricus is an obligate air breather. Its mouth is structurally adapted for air breathing by an extensively diverticulated and richly vascularized oral mucosa. Air is regularly taken into the mouth and later expelled at the opercular openings. The present investigation concerns the respiratory properties of blood, the dynamics of gas exchange and the control of breathing in the electric eel.
Fishes were anesthetized and catheters implanted for sampling of gas in the mouth and blood from the jugular vein draining the mouth respiratory organ, and from a systemic artery. A blood velocity transducer was implanted on the ventral aorta. Following recovery, gas from the mouth, blood gases, blood pH as well as other respiratory and circulatory parameters, were monitored during normal breathing cycles and in response to low and high oxygen tensions in both the aquatic and aerial environment surrounding the fish. In addition, the fish were exposed to a CO2 enriched environment.
Table 2 summarizes the respiratory properties of blood. The high oxygen capacity and oxygen affinity may be an adaptive measure against the mixed conditions of arterial blood. The oxygen capacity was largely unaffected by CO2.
Electrophorus showed arterial CO2 tensions higher than for typical aquatic breathers and other air breathing fishes studied. PCO2 is increased due to the shunting of blood from the mouth organ to the venous side of the systemic circulation. For the same reason arterial oxygen tensions are normally much below the P100 value. The blood bicarbonate concentration is higher than in typical aquatic breathers.
The gas exchange ratio was very low for the mouth respiratory organ and tended to decrease still further in the intervals between air breaths. The gills and/or skin are hence important for CO2 elimination.
The interval between air breaths rarely exceeded two minutes in intact free-swimming fish surrounded by aerated water and normal ambient air. The fish was irresponsive to changes in oxygen and CO2 tensions in the water, but breathing of hypoxic and hypercarbic atmospheres caused marked and very prompt increase in the rate of air breathing. Inhalation of a hyperoxic atmosphere caused a depression of air breathing.
Heart rate and cardiac output values were higher than earlier reported values for fish. Calculations showed that marked changes occured in the fractional distribution of the cardiac output related to the phase of the breathing cycle and the oxygen tension in the mouth organ.
When long intervals prevailed between air breaths the heart rate and cardiac output declined late in the breath interval. Inflation of the mouth organ with oxygen or nitrogen both prompted cardioacceleration and increased blood flow. The changes were of reflex nature and caused by pressure or tension changes inside the mouth.