Does the largest chela of the males of three crab species undergo an allometric change that can be used to determine morphometric maturity?

Abstract

Abstract The allometry of the largest chela of male crabs has often been assumed to undergo a change at the pubertal moult, i.e. the moult to maturity, and could therefore be used as a basis for determining the size of males at morphometric maturity. As initial plots of the logarithms of the length of the propodus of the chela and carapace width (CW) or length (CL) of male Portunus pelagicus, Hypothalassia acerba, and Chaceon bicolor revealed no conspicuous change in allometry, we used an information-theoretic approach and a range of models to explore, in greater depth, whether the chelae of these species did undergo such an allometric change. The candidate models were linear, quadratic, cubic, and broken-stick models, and broken-stick and two-line-segment models with logistic transitions between line segments. There was strong evidence that the largest chela of male P. pelagicus undergoes a subtle change in allometry at 82.0 cm CW, which is only 6.4 mm less than that at which 50% of males attain physiological maturity. Further, as the 95% confidence region of this estimate of size at morphometric maturity overlaps that for physiological maturity, the sizes at which morphometric and physiological maturity are attained by male P. pelagicus are similar. Because the estimate of the carapace length at which allometric change in the chela of male H. acerba was very imprecise, there was far less convincing evidence that the allometry of the chela of this species undergoes a conspicuous change at a certain length. The allometry of the chelae of male C. bicolor changed progressively and continuously with body size, and did not change abruptly at a particular size. In view of the morphometric results for H. acerba and C. bicolor, it would be advisable to base management plans for conservation on the carapace lengths at which 50% of the male crabs of these two species attain physiological maturity, i.e. 68.1 and 94.3 mm, respectively.