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First published online May 24, 2004
Journal of Experimental Biology 207, 2361-2370 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.01023

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Upper thermal tolerance and oxygen limitation in terrestrial arthropods

C. Jaco Klok, Brent J. Sinclair and Steven L. Chown^*

Spatial, Physiological and Conservation Ecology Group, Department of Zoology, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa

View larger version (26K): [in a new window]   Fig. 1 Standardized data output of thermolimit respirometry outlined as (A,B) changes in metabolic rate (CO2; ml CO2 h-1) and (C) activity (V) for a tracheal breathing beetle (A,C) and a pleopodal breathing terrestrial isopod (B,C) at increasing temperatures starting at 30°C. Analogous to the law of tolerance (Schwerdfeger, 1977; Shelford, 1931) as interpreted by Pörtner (2001, 2002a), the CO2 profiles characterise the rise in metabolic rate in both species across their range of aerobic capacity (above 30°C), culminating in a maximum metabolic rate CO2max corresponding to a temperature here termed the TMetMax (equivalent to Pörtner's TpII - upper pejus temperature). Beyond the TMetMax (=TpII) increasing temperatures cause the onset of a progressive decrease in metabolic rate. This short temperature range is called the deleterious range. This metabolic rate decline culminates in the respirometry CTmax, signalled by a brief spike in CO2 emission. With temperature increases beyond the CTmax, metabolic breakdown continues and eventually leads to death and the subsequent release of residual CO2 from the body. Motor activity also increases with increasing temperature and these responses are equivalent in both test species. The cessation of coordinated motor function characterises onset of the activity CTmax (sensu Lutterschmidt and Hutchison, 1997). The respirometry CTmax often corresponds closely with the activity CTmax (see vertical line). Furthermore, respiratory recordings in the tracheal breather show spiracular activity and this ceases in concert with cessation in coordinated motor function shown in the activity recordings (J. R. B. Lighton and R. J. Turner, personal communication; see also Lighton and Turner, 2004). The primarily diffusive pleopodal breathing isopod does not show this.

View larger version (30K): [in a new window]   Fig. 2. Representative data for the thermal limit respirometry experiments on Armadillidium vulgare. (A) 21% oxygen, (B) 10%, (C) 2.5%, (D) 40%. (Ai-Di) Metabolic rate CO2, (Aii-Dii) activity in arbitrary units (V), where both negative and positive values indicate activity.

View larger version (30K): [in a new window]   Fig. 3. Representative data for the thermal limit respirometry experiments on Gonocephalum simplex. (A) 21% oxygen, (B) 10%, (C) 2.5%, (D) 40%. (Ai-Di) Metabolic rate CO2, (Aii-Dii) activity in arbitrary units (V), where both negative and positive values indicate activity.