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First published online September 19, 2008
Journal of Experimental Biology 211, 3139-3146 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.021907

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Investigating onychophoran gas exchange and water balance as a means to inform current controversies in arthropod physiology

Susana Clusella-Trullas^* and Steven L. Chown

Centre for Invasion Biology, Department of Botany and Zoology, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa

^* Author for correspondence (e-mail: sct333{at}sun.ac.za)

Accepted 3 August 2008

Several controversies currently dominate the fields of arthropod metabolic rate, gas exchange and water balance, including the extent to which modulation of gas exchange reduces water loss, the origins of discontinuous gas exchange, the relationship between metabolic rate and life-history strategies, and the causes of Palaeozoic gigantism. In all of these areas, repeated calls have been made for the investigation of groups that might most inform the debates, especially of taxa in key phylogenetic positions. Here we respond to this call by investigating metabolic rate, respiratory water loss and critical oxygen partial pressure (P_c) in the onychophoran Peripatopsis capensis, a member of a group basal to the arthropods, and by synthesizing the available data on the Onychophora. The rate of carbon dioxide release (_CO2) at 20°C in P. capensis is 0.043 ml CO₂ h^–1, in keeping with other onychophoran species; suggesting that low metabolic rates in some arthropod groups are derived. Continuous gas exchange suggests that more complex gas exchange patterns are also derived. Total water loss in P. capensis is 57 mg H₂O h^–1 at 20°C, similar to modern estimates for another onychophoran species. High relative respiratory water loss rates (34%; estimated using a regression technique) suggest that the basal condition in arthropods may be a high respiratory water loss rate. Relatively high P_c values (5–10% O₂) suggest that substantial safety margins in insects are also a derived condition. Curling behaviour in P. capensis appears to be a strategy to lower energetic costs when resting, and the concomitant depression of water loss is a proximate consequence of this behaviour.

Key words: metabolism, hypoxia, respiratory water loss, cuticular water loss, discontinuous gas exchange, invertebrate, velvet worm, respirometry

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