Measuring fluctuating asymmetry of the terrestrial isopod Trachelipus rathkii (Crustacea: Isopoda, Oniscidea)

Studies on fluctuating asymmetry (FA) in soil invertebrates, particularly in isopods, are almost absent so far. As a result, methodology of measuring FA in isopods is little known. However, the ecological importance of these animals in decomposition processes and the fact that FA in soil invertebrates might serve as an indicator of environmental stress, e.g. soil pollution, make the topic worth for investigation. Our results revealed that woodlice are potentially able to signal environmental stresses via FA. We compared six traits (length of three segments of antennae, two segments of the seventh legs and the number of ocelli) between two populations (Hungary: Hajós, Blaskovics-puszta) and genders of the species Trachelipus rathkii living under different environmental stresses (i.e. changing humidity). Asymmetry was significantly higher in the Hajós population, where the soil moisture has changed dramatically for the last years. Asymmetry was lower in Blaskovics-puszta, where the habitat has remained constant during the last centuries and the population had time to adapt to seasonally changing environments. Gender did not have an effect on FA. Traits differed significantly from each other, thus it is advisable to use several traits simultaneously to detect FA. FA was higher in the a1 segment of the antennal flagellae than in the ischium of the pereiopod and ocelli. We also studied the effect of regeneration of lost body parts on the FA values. Asymmetry in the treated groups was significantly higher than in the control ones. This calls the attention to the problem of outliers, which may be caused by physical harm and can lead to artificial results.