Spatial differences in growth and sexual maturity between branches of a large population of the squat lobster Pleuroncodes monodon

Abstract

A large population of the squat lobster Pleuroncodes monodon inhabits the narrow continental shelf off central Chile. From a single nursery ground, 2 adult branches extend in a N-S orientation over 3°of latitude, with the northern branch being much larger than the southern one. Squat lobsters from the southern branch are smaller on average than individuals from the northern branch. Two hypotheses were tested: (1) southern squat lobsters are smaller than northern ones because they grow to smaller sizes, and (2) southern squat lobsters are sexually mature at smaller sizes and earlier ages than northern squat lobsters. Using data from 6 research surveys done on the southern branch of the population, we estimated growth functions for male and female squat lobsters and compared them with previous results from the northern branch. Furthermore, using data from 1 survey we estimated a maturity function for southern females and compared it with a previous estimation for northern females. Southern squat lobsters grow faster and to smaller asymptotic sizes than northern ones, reaching sexual maturity at smaller sizes and earlier ages. Moreover, individuals from the southern branch appear to live less (5 age classes) than individuals from the northern branch (8 to 9 age classes). These results imply that physiological processes are significantly faster in the southern branch. Previous cases of the same phenomenon in benthic crustaceans occurred in spatially discontinuous populations, while in this case it occurs between branches connected through a common nursery ground and separated by a relatively short distance. In this population, spatial heterogeneity in population functioning arises without physical discontinuities, but in the context of a branched population. Alternative hypotheses, based on features of the habitat, are posed to explain the difference. Finally, it was found that the ratio between size at 50% maturity and asymptotic size does not differ between branches, in agreement with the hypothesis that this ratio is a constant for a wide spectrum of phyla in the animal kingdom.