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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

View larger version (33K): [in a new window]   Fig. 1. The influence of oxygen tension (mmHg) on mass-specific O2 consumption rate (O2) in large (N=13; open bars) and small (N=15; hatched bars) oscars. Values are means ± 1 s.e.m. Measurements were made over 20 mmHg intervals. Asterisks and daggers indicate significant differences (P<0.05) from the respective rates at the highest PO2.

View larger version (58K): [in a new window]   Fig. 2. A response surface describing mass-specific O2 consumption rate (O2) as a function of body mass and PO2 based on data from 35 oscars ranging in mass from 8.8 to 308 g. The model is: logO2=3.20686+0.786xlog(mass)–0.88913x0.96371PO2 (0.05–0.3 kg fish) (r2=0.948); logO2=3.28076+0.786xlog(mass)–0.92331x0.97024PO2 (0.01–0.05 kg fish) (r2=0.928), where O2 is in µmol O2 h–1, mass is in kg, and PO2 is in mmHg.

View larger version (17K): [in a new window]   Fig. 3. (A) Decrease in oxygen tensions (mmHg) over time as nitrogen is bubbled into a glass tank. Oxygen tensions were measured every 5 min in each vertical zone (see key) within the tank and there was no significant zonation of oxygen tension within the tank (P=0.771). (B) The oxygen tension at the time of surfacing of large (open bar) and small oscar (hatched bar). The asterisk indicates a significant difference (t=2.14, P=0.02, N=8); values are means ± s.e.m. The insert shows the correlation between mass of individual fish (g) and the oxygen tension at time of surfacing (P=0.04; r2=0.207).

View larger version (26K): [in a new window]   Fig. 4. (A) Gradient of oxygen tension (mmHg) in a long (2.16 m) shallow tank. Each data point is equidistant along the tank. The shaded area represents the area covered by the floating plant, Pistia stratiotes. (B) Time spent under the shelter of the floating plant, Pistia stratiotes, at the most hypoxic end of the gradient by fish from the large and small groups of oscars, held in either normoxic (130 mmHg; open bars) or hypoxic (40 mmHg; hatched bars) water for 1 h prior to being placed into the experimental set-up. There was a significant interaction between hypoxia and size (two-way ANOVA; P<0.01), with the asterisk indicating a significant effect of both size and hypoxia treatment (t-test: P<0.01, N=7 fish per group). All values are means ± s.e.m.

View larger version (31K): [in a new window]   Fig. 5. Changes in behaviour in large and small oscar during progressive hypoxia. Where significant interactions (P<0.05) between hypoxia and size masked changes in behaviour within size classes, one-way ANOVA analyses were performed post hoc to test for statistical differences. (A) Horizontal activity, (B) vertical activity and (C) aggression. Values are means ± s.e.m. Letters denote significant differences within size groups where bars sharing the same letter are not significantly different (one-way ANOVA: P<0.05) and asterisks denote a significant difference between size groups (two-way ANOVA: P<0.001). (D) Relationship between the activity scores of individual fish at different oxygen tensions (Spearman's Rank Correlation: rs=0.667, 0.698, 0.718 for PO2=40, 80 and 136 mmHg, respectively; P<0.001).