Molting, Growth and Reproductive Strategies in the Terrestrial Isopod, Armadillidium Vulgare

Abstract

The relationship between molting, growth and reproduction was studied in the terrestrial isopod, Armadillidium vulgare. Reproduction and growth are tied together in three major ways: (1) Among reproductive ♀ ♀ the growth rate is less than that of comparable nonreproductive ♀ ♀. This is because both the wt gain at ecdysis (initially HO) uptake, but ultimately replaced by biomass) and the frequency of molting are less for reproductive ♀ ♀ than for nonreproductive ♀ ♀. This reduced growth associated with reproduction is most pronounced among the smaller ♀ ♀; (2) The estimated total energy devoted to both growth and reproduction among reproductive ♀ ♀ is about the same as the energy devoted only to growth among nonreproductive ♀ ♀ (3) Among individual reproductive ♀ ♀ there was an unexpected positive correlation between growth and reproduction: ♀ ♀ producing larger (size—specific) broods also gain more weight. This contrasts markedly with the clear reduction in growth in the comparison between reproductive and nonreproductive ♀ ♀. It is suggested that this apparently anomalous result may be explained by differential mortality. In natural field populations, large ♀ ♀ produce 2 broods/season and small ♀ ♀ produce only 1 brood/season. Using a simple model relating ♀ size to the number of offspring produced in either 1 or 2 broods, this difference in reproductive output between large and small ♀ ♀ can be explained as an adaptive response to the constraint imposed by the trade off between growth and reproduction. The predicted size at which individuals shift from a one—brooded to a two—brooded strategy is nearly identical to the observed size at which this change in reproductive output occurs in a field population. Using a simple model, the analysis of reproductive strategies is extended to variable environments. It is suggested that reproductive strategies optimizing fitness in a variable environment may destabilize the population's resistance to environmentally imposed disturbances and result in large numerical fluctuations.