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Midgut gland respiration in the estuarine crab, Hemigrapsus nudus (Dana) Hawke, Scott Dransfield


Weight-specific oxygen consumption of midgut gland tissue of Hemigrapsus nudus has been investigated at three levels of salinity (35%, 75% and 125% sea water), two levels of experimental temperature (5°C and 20°C) and four acute (Warburg) temperatures (5°, 10°, 15° and 20°C) in all combinations for each season (summer and winter). Metabolic-temperature curves reveal that at standard baseline conditions where the animals are held 24 hr at their respective seasonal temperature and salinity, midgut gland respiration is highest at all acute temperatures in the summer animals. Acutely measured metabolic-temperature curves for midgut gland tissue show that winter animals acclimated to their opposite seasonal conditions of temperature and salinity for 10 days demonstrate the greatest degree of acclimation. The effect of experimental temperature is statistically and biologically significant. The highest respiration rate is at 5°C. Low temperature (5°C) may provide a greater thermal stress than a high temperature (20°C) resulting in a higher rate of oxygen consumption. Experimental temperature also influences the seasonal respiratory response of midgut gland tissue to salinity. In summer animals there is no correlation of midgut gland respiration to salinity at 5°C. There is a increase in respiration rate as the osmotic gradient between the blood and medium increases at the seasonal baseline temperature of 20°C. Winter animals held at the seasonal baseline temperature of 5°C demonstrate a "V-shaped" relationship to salinity with the lowest respiratory response in 75% sea water where the gradient between the blood and medium is minimal. Animals held at 20°C increase respiration with an increase in salinity. It is suggested that the metabolic activity of midgut gland from summer animals may be related to the maintenance of a osmotic gradient between the blood and medium or alternatively to the energy demands associated with new exoskeleton formation. The proposal is put forth that midgut gland respiration in winter animals may indicate osmotic work being done to maintain the osmotic gradient between the blood and medium. The production of a urine hypotonic to the blood may also assist winter animals in regulation of blood electrolytes. The regression coefficients of weight-specific oxygen consumption as a function of body weight were not significantly different from zero at the 0.01 probability level.

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