Response of terrestrial isopods, Armadillidium vulgare and Porcellio laevis (Isopoda: Oniscidea) to the ant Tetramorium caespitum: morphology, behavior and reproductive success

Abstract

Summary Little is known about the effects of predatory stress on reproduction in terrestrial isopods (Crustacea: Oniscidea). We studied direct and indirect influences of predatory ants Tetramorium caespitum (Hymenoptera: Formicidae) on the behavior and reproduction of two terrestrial isopods, Armadillidium vulgare and Porcellio laevis. A. vulgare females were more cryptic than P. laevis females in both the presence and absence of ants, but in both species control animals were more cryptic than experimental animals exposed to ants. This likely resulted from increased movement in the presence of ants. Both species also avoided ventilated enclosures containing ants, but control animals not previously exposed to ants remained significantly further away. P. laevis also remained significantly further from ants than A. vulgare, suggesting that P. laevis more actively avoids potential predators. This is consistent with morphological differences between the two species: A. vulgare can roll into a sealed ball whereas the morphology of P. laevis permits swift locomotion. Furthermore, mortality rates of P. laevis were greater than A. vulgare when both were directly exposed to ants in the laboratory, and P. laevis suffered significantly more attacks than A. vulgare under experimental conditions. However, the length of the brooding period in A. vulgare was shorter in ant-exposed females than in controls, whereas experimental and control P. laevis females showed no such difference. This could be an adaptive trade-off for A. vulgare because in the late brooding period the distended marsupium can prevent rolling into a closed ball for protection. The results of this study underscore the relationships between morphology, behavior, and reproductive success.