Variation of morphometric traits in populations of an invasive carabid predator (Merizodus soledadinus) within a sub-Antarctic island

Invasive predators may change their own trophic conditions by progressively displacing or reducing diversity and abundance of native prey. As food quality and quantity are two main factors determining adult body size in arthropods, alteration of the available resources may thus affect predators' morphology. The flightless carabid beetle Merizodus soledadinus was accidentally introduced to Iles Kerguelen in a single site in 1913. Its successful spreading process has been monitored over the long term, providing an exceptional research opportunity with multiple snapshots of similar colonized sites mostly differing by the residence time of M. soledadinus. To test if M. soledadinus' morphology is correlated with its residence time in each habitat, we measured nine morphometric traits in five populations. We detected significant morphological differences: individuals from the first colonized site were the smallest, whereas individuals from the most recently colonized site were the largest. Our study also highlighted among-site variation in sexual dimorphism of the last abdominal sternite: its length differed between sites for females, but not for males. We discuss this diminution of M. soledadinus' size in the light of both a priori (development under diet restriction, survival) and a posteriori (intrapopulation competition, cannibalism) effects on growth and development.