How does benthic living affect body volume, tissue composition, and density of fishes?

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Abstract
Tissue composition, tissue density, and body density of two benthic fish, plaice (Pleuronectes platessa) and father lasher or shorthorn sculpin (Myoxocephalus scorpius), and a benthopelagic fish, the whiting (Merlangus merlangus), were measured. Since the fish were not identical in absolute size, data on composition were normalized by dividing body and tissue volumes by the cube of the total length of the fish. For body volume, this normalized volume (volume × length−3) is analogous to the condition factor (mass × length−3), and hence is called the volume condition factor. Normalized tissue volumes are called relative volumes. Volume condition factor was 13 L∙m−3 for lasher. Plaice and whiting have compressed body cross sections compared with that of lasher, which were associated with lower volume condition factors of 8.1 L∙m−3 for plaice and 9.1 L∙m−3 for whiting. Plaice are highly compressed to reduce drag and facilitate station holding on current-swept substrates. The reduced relative volume of plaice was associated with reduced visceral volume, which may correlate with reductions in rates of energy and material processing. This may represent a cost of low-drag forms for benthic fish. Whiting were neutrally buoyant, but carcass density was 1061 kg∙m−3. The densities of plaice and lasher were 1069 and 1074 kg∙m−3, respectively. Tissue densities and composition were variable among the three species. Benthic species showed neither consistently high tissue densities nor consistently larger proportions of high-density tissues. Therefore, benthic fish do not appear to maximize body density, which could improve passive station-holding ability in currents. Density may not be maximized because of higher locomotor costs when fish swim off the bottom. Increasing density may not be essential, since other factors, such as high friction coefficients, could have larger effects on station-holding performance.