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Journal of Experimental Biology 96,53-67 (1982)
Published by Company of Biologists 1982

The Transition to Air Breathing in Fishes: : I. Environmental Effects on the Facultative Air Breathing of Ancistrus Chagresi and Hypostomus Plecostomus Loricariidae

JEFFREY B. GRAHAM ¹ and TROY A. BAIRD ¹

¹ Physiological Research Laboratory, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California 92093

In response to progressive aquatic hypoxia, the armoured loricariid catfishes Ancistrus chagresi and Hypostomus plecostomus become facultative air-breathers and utilize their stomachs as accessory air-breathing organs. Hypostomus initiates air breathing at a higher aquatic O₂ tension (P_{w, Ow, O2}) than does Ancistrus (60 v. 33 mmHg). Once begun, the air-breathing frequencies of both species increase with decreasing P_{w, Ow, O2}; the frequency of Ancistrus, however, is greater than and increases more with hypoxia than does that of Hypostomus, which appears to be a more efficient air breather.

Hypoxia acclimation reduces the air-breathing rate of both species. A larger rate reduction occurs in Ancistrus, which, however, continues to require more frequent breaths than Hypostomus. Hypoxia acclimation does not affect the air-breathing threshold of either species, suggesting that external O₂ receptors initiate facultative air breathing.

In progressive aquatic hypercapnia Ancistrus has a lower air-breathing CO₂ threshold (8.7 mmHg) than Hypostomus (12.8 mmHg). However, in some tests, individual fish of both species did not initiate air breathing even at P_{w, COw, CO2} as high as 21 mmHg. Also, air breathing evoked by hypercapnia was short-lived; both species quickly compensated for this gas and resumed exclusively aquatic respiration within a few hours of exposure. Thus, CO₂ is not an important regulator of air breathing in these species. Between 25 and 35 °C, the P_{w, Ow, O2} air breathing threshold of Ancistrus is temperature-independent, but air-breathing frequency increases with temperature.

Ancistrus and Hypostomus do not breathe air in normoxic (air-saturated) water; their air-breathing responses are evoked by environmental hypoxia. This is fundamentally different from other fish species that breathe air in normoxia in order to meet heightened metabolic demands. Also, the facultative air-breathing adaptations of Ancistrus and Hypostomus differ in scope and magnitude from those utilized by species that breathe air in nor-moxia and adapt to hypoxia by increasing air-breathing rate.

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