Changes in body chemical composition were compared among groups of fiddler crabs (Uca pugilator and U. pugnax) freshly collected from the field or offered diets containing various amounts of nitrogen in the laboratory. Specific growth rates of experimental crabs were calculated
based on estimated changes in total body nitrogen during the experiment. Crabs offered a diet enriched in nitrogen (casein) had higher organic contents (mg ash-free dry weight˙g DW−1) than either crabs offered a non-supplemented diet or field-caught conspecifics. A positive
relationship between specific growth rate and organic content, based on the amount of nitrogen in the diet, was found for crabs in the laboratory. A comparison of values of organic content of field-caught crabs with those of crabs offered the diet supplemented with nitrogen suggests that U.
pugilator and U. pugnax do not grow at maximal rates in nature.
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