Measurements of body size include linear dimensions (e.g., body length, body width, and the length or width of some morphological attribute of individuals), body surface area and biovolume, and weight (e.g., wet weight, dry weight, and body mass as ash-free dry weight). The energy content of biomass, measured in energy units, can also be used as a measurement of body size.
Body-size patterns are derived from individual biomass data, although the original data may have been obtained as the body length, morphological attribute length or size, body wet weight, body volume, or cell volume of unicellular individuals. Some conversion is required, because individual biomass cannot always be measured, although body size in general is an easily measurable characteristic of individuals. Indeed, in many cases it is necessary to avoid the destructive analysis required to measure individual biomass, and in other cases individuals are simply too small.
Indirect measurements of individual biomass, where lengths or biovolumes are converted to weights, may make the body-size patterns weaker or harder to detect, depending on the dimensions measured, on the precision of the allometric relationship used with respect to the specific set of data, on the precision of biovolume detection, and on the adequateness of the conversion equations. As regards the weight-per-length allometry, the comparability of the seasonal period, climatic conditions, sex ratio and the reproductive status of individuals, and resource availability all have to be taken into account as major sources of variation. As regards biovolume, the complexity of individual or cell shape, taxon-specific weight per unit of biovolume, and the type of weight unit used (C, biomass) all have to be taken into account in order to minimize the bias introduced by using indirect measurements and conversion factors.
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