Relative growth and specific growth rates in crustaceans

Typically, studies of relative growth focus on the slope of a double logarithmic regression for two measured body parts. This slope represents the relationship between the specific growth rates of the two parts. To better understand processes of relative growth, workers have directly calculated and compared specific growth rates. However, it is not clear that these calculations have meaning in crustaceans where growth is discontinuous. Here it is suggested that growth can be regarded as continuous over an entire molt cycle, and this suggestion is tested empirically. Shell-living Pagurus longicarpus hermit crabs were grown in one species of shell. Half the individuals were then given shells of a larger species, while half were given shells of the same species. Both indirect (using logarithms of distance measures) and direct (using per molt cycle calculated values) comparisons of specific growth rates indicate that test individuals exhibited reduced growth of the right chela relative to the anterior carapace, while control individuals did not. Direct comparisons of specific growth rates, however, yield significant differences with much smaller sample sizes and provide greater insight into the process of relative growth.