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First published online March 17, 2006
Journal of Experimental Biology 209, 1197-1205 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02090

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Tribute to R. G. Boutilier: The effect of size on the physiological and behavioural responses of oscar, Astronotus ocellatus, to hypoxia

Katherine A. Sloman^1,*, Chris M. Wood², Graham R. Scott³, Sylvia Wood², Makiko Kajimura², Ora E. Johannsson⁴, Vera M. F. Almeida-Val⁵ and Adalberto L. Val⁵

¹ School of Biological Sciences, University of Plymouth, Devon, PL4 8AA, UK
² Department of Biology, McMaster University, Hamilton, Ontario, Canada L8S 4K1
³ Department of Zoology, University of British Columbia, Vancouver, Canada V6T 1Z4
⁴ Department of Fisheries and Oceans, Canada Centre for Inland Waters, Burlington, Ontario, Canada L7R 3A6
⁵ Laboratory of Ecophysiology and Molecular Evolution, INPA, Manaus, Brazil

^* Author for correspondence (e-mail: katherine.sloman{at}plymouth.ac.uk)

Accepted 12 January 2006

The physiological and behavioural responses of two size groups of oscar (Astronotus ocellatus) to hypoxia were studied. The physiological responses were tested by measuring _O2 during decreasing environmental oxygen tensions. Larger oscars were better able to maintain oxygen consumption during a decrease in P_O2, regulating routine _O2 to a significantly lower P_O2 threshold (50 mmHg) than smaller oscars (70 mmHg). Previous studies have also demonstrated a longer survival time of large oscars exposed to extreme hypoxia, coupled with a greater anaerobic enzymatic capability. Large oscars began aquatic surface respiration (ASR) at the oxygen tension at which the first significant decrease in _O2 was seen (50 mmHg). Interestingly, smaller oscars postponed ASR to around 22 mmHg, well beyond the P_O2 at which they switched from oxyregulation to oxyconformation. Additionally, when given the choice between an hypoxic environment containing aquatic macrophyte shelter and an open normoxic environment, small fish showed a greater preference for the hypoxic environment. Thus shelter from predators appears particularly important for juveniles, who may accept a greater physiological compromise in exchange for safety. In response to hypoxia without available shelter, larger fish reduced their level of activity (with the exception of aggressive encounters) to aid metabolic suppression whereas smaller oscars increased their activity, with the potential benefit of finding oxygen-rich areas.

Key words: oxygen, Amazon, predation, social, aquatic surface respiration (ASR), oscar, Astronotus ocellatus

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