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First published online November 28, 2008
Journal of Experimental Biology 211, 3790-3799 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.018721
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Haemoglobin as a buoyancy regulator and oxygen supply in the backswimmer (Notonectidae, Anisops)

Philip G. D. Matthews* and Roger S. Seymour

Ecology and Evolutionary Biology, School of Earth and Environmental Sciences, Darling Building, University of Adelaide, Adelaide, SA 5005, Australia


Figure 1
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  		Fig. 1. Diagram of the backswimmer Anisops deanei from above (ventral side) and laterally. The grey shaded areas on the abdomen indicate the hair-covered grooves that contain the air-store. Scale bar is 2 mm.

 

Figure 2
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  		Fig. 2. The ventral abdominal surface of first instar Anisops deanei, submerged (A) and at the surface (B). The hydrophobic hairs (h) completely cover the air-store when submerged and expose it at the surface. The spiracles (s) connected to the tracheoles that invade the haemoglobin cells (Hb) are visible.

 

Figure 3
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  		Fig. 3. Apparatus used for measuring the buoyancy of a submerged backswimmer.

 

Figure 4
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  		Fig. 4. The volume of air extracted from a backswimmer immediately after diving voluntarily. The variation in collected air volume is affected by length of dive and amount of struggling during capture, but lies between 19.4% above to 4.9% below neutral buoyancy.

 

Figure 5
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  		Fig. 5. The relationship between the weight of the backswimmer and the initial volume of air carried at the beginning of a forced dive. Filled circles indicate data from control backswimmers, and open circles indicate data from CO-treated backswimmers. Neutral buoyancy occurs along the solid line, where the air-store volume balances the insects' negative buoyancy.

 

Figure 6
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  		Fig. 6. Typical traces showing percentage decrease in air-store volume in untreated (solid line) and 15% CO-exposed (dotted line) backswimmers. Spikes in volume at the end of each trace were produced by the energetic movements of the backswimmers as they attempted to surface.

 

Figure 7
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  		Fig. 7. Typical changes in the PO2 of a submerged backswimmer's air-store, before (solid line) and after (dotted line) exposure to 15% CO.

 

Figure 8
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  		Fig. 8. Three phases of the air-store's PO2 during a dive (solid black line), defined by the rate of PO2 change (PO2, dashed line). The initial steep PO2 decline in phase 1 (P1) enters a plateau phase (P2) once PO2 has decreased by half its initial rate (point a). The arbitrary end of P2 (point b) is marked by the maximum rate of decrease in PO2. The low air-store PO2 forces the backswimmer to surface during P3. The effective oxygen contributions of the air-store and haemoglobin were calculated by assuming that in the absence of haemoglobin the air-store would supply oxygen until point c, while the oxygen contributed by the haemoglobin extends the dive time to point b. The initial PO2 was taken from the first stable 20 s of the trace (first dotted box), while the plateau region was defined as the middle two-thirds of the PO2 trace between P1 and P3 (second dotted box).

 

Figure 9
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  		Fig. 9. Relationship between aquatic PO2 on voluntary dive time at 20°C. Line shows least-squares regression. Error bars indicate 95% CI of mean dive times. N=8 backswimmers per treatment, six dives recorded from each animal (N=32 in total).

 

Figure 10
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  		Fig. 10. Effect of water temperature on voluntary dive time. Line shows least-squares regression (–12.45°C+467.8). Error bars indicate 95% CI, N=8 backswimmers per treatment, six dives recorded from each animal (N=40 in total).

 

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© The Company of Biologists Ltd 2008