Johnny B. Goode
The effect of body size on metabolic rate in the goldfish (Carassius auratus).
The effect of body size on energy metabolism determines many aspects of an animal's life history. For example, small animals generally have higher mass-specific metabolic rates than large animals. As a result, small animals must eat relatively more per day than larger animals and have to spend more time searching for food. Such scaling effects have been demonstrated for a wide variety of animals (Calder, 1984). The causes of this allometric effect are a matter of intense debate. Information on additonal taxa are likely to shed light on the phenomenon. Data for fish are relatively incomplete. Goldfish (Carassius auratus) are suitable for such laboratory studies because they are available in a variety of sizes and are tolerant of very low levels of dissolved oxygen. The purpose of this experiment is to determine the effect of body size on oxygen consumption in goldfish.
Materials and Methods
Goldfish were obtained from a local pet store. Individuals were weighed, body length measured, and placed in sealed jars of 0.5-L volume for one hour. The temperature of the water was held constant (25oC) by placing the jars in an upright incubator. At the 30-minute mark respiratory rate was determined by counting the number of bucal-opercular movements during a 30-s interval. The oxygen concentration was measured before and afterward using the Winkler titration. Oxygen consumption (MO2) was calculated accordingly. Class results were pooled and recorded.
Our fish was somewhat active during the experiment, as were those of other students. Fish were of typical size, and maintained normal respiratory rates during the measurements (Table 1).Table 1. Characteristics of goldfish used in the study.
Average N Body mass (g) 6.1 7 Body length (cm) 9.5 7 Oxygen consumption (mg/g h) 2.5 7 Respiratory rate (/min) 101 6
Larger fish consumed more oxygen than smaller fish; therefore, total oxygen consumption increased with increasing body mass. However, mass-specific MO2 was greater in smaller fish than larger fish. The pooled MO2 data were plotted as a log-log function of body mass (Mb). Log MO2 decreased linearly with increasing log Mb (Figure 1). A least squares regression performed on the data yielded the following equation: MO2 (ml/g.min) = 1.4Mb-0.25 (r2 = 0.85).
Figure 1. The effect of body mass on oxygen consumption in the goldfish.
Our exponent was -0.25, which is commonly observed in numerous taxa (Calder, 1984). As size increases, mass increases with length3, while surface area increases only with length2. Hence, an exponent of 2/3 or .67 is predicted. If oxygen consumption per unit surface area is constant, then the exponent of our equation should be -0.33; hence, the surface area hypothesis is not supported. Our result is essentially the same as that predicted by demands of physical support and muscle biophysics (McMahon, 1973), which seems paradoxical since aquatic organisms are supported by the bouyancy of water.
Total oxygen consumption increased with increasing body mass; however, mass-specific oxygen consumption decreased with increasing body mass. Though the exact causes of this relationship remain unknown, its pervasive effects are well understood. A larger goldfish uses more total energy per unit time. Smaller goldfish must, therefore, eat a greater proportion of their body mass as food, and probably must spend more time acquiring food. This relationship holds true for a diversity of groups of organisms, including insects, mammals and birds (Lasiewski and Dawson, 1967; Stahl, 1967; Coelho and Moore, 1988).
Literature CitedCalder, W.A.III. 1984. Size, function, and life history. Harvard
University Press, Cambridge Massachussetts, 431 pp.
Coelho, J.R. and A.J. Moore. 1988. Allometry of resting metabolic
rate in cockroaches. Comparative Biochemistry and Physiology 94A:587-590.
Stahl, W.R. 1967. Scaling of respiratory variables in mammals.
Respiratory Physiology 22:453-460.
Lasiewski, R.C. and W.R. Dawson. 1967. A re-examination of the
relation between standard metabolic rate and body weight in birds. Condor 69:13-23.