Size dependence of offspring production in isopods: a synthesis

In isopods, parental care takes the form of offspring brooding in marsupial pouches. Marsupial brooding was an important step towards the origin of terrestrial lifestyles among isopods, but its potential role in shaping isopod life histories remains unknown. It is here considered that marsupial brooding imposes costs and creates a temporary association between the survival of mothers and that of their offspring. Integrating findings from different life history models, we predicted that the effects of marsupial brooding set selective conditions for the continuation of growth after maturation, which leads to indeterminate growth, and the production of larger offspring by larger females. Based on this perspective, a study on the size dependence of offspring production in the woodlouse Porcellioscaber was performed and the generality of the results was tested by reviewing the literature on offspring production in other isopods. In P.scaber and almost all the other studied isopods, clutch size is positively related to female size. Such dependence is a necessary pre-condition for the evolution of indeterminate growth. The body mass of P.scaber differed six-fold between the largest and smallest brooding females, indicating a high potential for post-maturation growth. Our review showed that offspring size is a rarely studied trait in isopods and that it correlates negatively with offspring number but positively with female size in nearly half of the studied species. Our study of P.scaber revealed similar patterns, but the positive effect of female size on offspring size occurred only in smaller broods, and the negative relation between clutch size and offspring size occurred only in larger females. We conclude that the intraspecific patterns of offspring production in isopods agree with theoretical predictions regarding the role of offspring brooding in shaping the adaptive patterns of female investment in growth, reproduction, and the parental care provided to individual offspring.

How coexistence may influence life history: the reproductive strategies of sympatric congeneric terrestrial isopods (Crustacea, Oniscidea)

Patterns of allocation between reproduction, survival, and maintenance are what we call life history. By investigating the life-history strategy of sympatric species, we may understand how they are able to coexist, as different strategies are expected to evolve in species that occupy similar niche space. Terrestrial isopods are a group in which multiple species frequently inhabit the same area. Notably, they are usually infected by Wolbachia Hertig, 1936, a notorious manipulator of the hosts’ reproductive processes. Thus, the aim of this study was to analyze the investment in reproduction in three sympatric and closely related species of terrestrial isopods: Atlantoscia floridana (Van Name, 1940), Atlantoscia inflata Campos-Filho and Araujo, 2015, and Atlantoscia petronioi Campos-Filho, Contreira and Lopes-Leitzke, 2012, only the latter being infected with Wolbachia. We showed that the presence of the bacteria seems not to affect the fitness of A. petronioi, because there was no clear difference in the reproductive output of infected and noninfected individuals. On the other hand, we observed that the three species possess alternative life-history strategies; that is, they differ in how much they invest in maintenance (body size) and reproductive effort. Such differences probably facilitate the species coexistence, reducing the competition among them.

Demography of some non-native isopods (Crustacea, Isopoda, Oniscidea) in a Mid-Atlantic forest, USA

Introduced species dominate the terrestrial isopod fauna in most inland habitats of North America, including urban landscapes. These non-native species are often very abundant and thus potentially play a significant role in detritus processing. We monitored isopod assemblages in an urban forest for a year to examine the relationship between surface activity and abiotic environmental factors, and to analyze reproductive characteristics that might contribute to their successful establishment. Using pitfall trap samples we recorded five species, two of which, Trachelipusrathkii and Cylisticusconvexus, were highly abundant. We determined size, sex and reproductive state of each individual. Surface activity of both species reflected variability in abiotic stress factors for isopods, such as soil moisture and soil temperature. Early spring the main trigger was soil temperature while later in the season increasing temperature and decreasing soil moisture jointly affected population dynamics. Activity significantly correlated with soil moisture. The temporal pattern of sex ratios supported the secondary sex ratio hypothesis. Males dominated the samples on the onset of the mating season in search of females. The pattern was reversed as females searched for suitable microsites for their offspring. Size independent fecundity decreased as conditions became more stressful late in the season.

Population size structure and sex ratios in some woodlice (Crustacea: Oniscidae) from southern Africa

The population size structure and adult sex ratio were recorded for four indigenous and one introduced species of terrestrial isopod from southern Africa. Interspecific variation was considerable with either discrete or continuous distributions indicative of the production of separate cohorts or continuous recruitment. Intraspecific variation was also considerable particularly in species such as Aphiloscia vilis which can be found in diverse habitats. Sex ratios were consistently female biased, a result consistent with observations made on temperate species. These observations, and a consideration of sexual dimorphism based on body mass, suggest that phenotypic plasticity may be an important tactic in the life histories of tropical woodlice and that in some populations the potential exists for strong sexual competition and complex mating systems.

Reduced survival and body size in the terrestrial isopod Porcellio scaber from a metal-polluted environment

Terrestrial isopods (woodlice) may show trade-offs in life history parameters when exposed to toxins. We have shown previously [Jones and Hopkin (1996) Functional Ecology 10, 741-750] that woodlice which survive to reproduce in sites heavily polluted with metals from an industrial smelting works do not alter their reproductive allocation. This study investigates whether there are differences in the survival and body size of Porcellio scaber from these same populations. Specimens were collected from eight sites at different distances from the Avonmouth smelter, UK. The sites represented a gradient of concentrations of Zn, Cd, Pb and Cu in the woodlice, from background levels to a grossly contaminated sites close to the smelter. In laboratory trials, the number of days survived by starved males showed a significant decline with increased concentrations of Zn in those animals. The maximum size of both sexes declined significantly from the least to the most polluted sites. The most polluted sites had significantly fewer large animals. The cost of detoxifying assimilated metals appears to be reduced energy reserves and smaller body size.