Competition for shelter occupancy between a native freshwater crab and an invasive crayfish

Potamon potamios populations have decreased significantly due to the degradation of its habitat caused by human activities, mainly the use of insecticides. Today, P. potamios is protected by Cypriot law, but not much information is known regarding possible effects on the species from interactions with the introduced and invasive freshwater crayfish Procambarus clarkii. We tested the hypothesis that the invasive crayfish P. clarkii can cause negative effects on the populations of the only indigenous freshwater crab of Cyprus P. potamios, through agonistic competition for shelter. A laboratory experiment on the competition between P. potamios and P. clarkii for securing shelter has been performed. During the experiment, pairs of equal-sized individuals from each species were evaluated for aggressiveness, domination in fights and shelter occupation. Results showed that even though both species are similarly aggressive, the crayfish wins more fights and occupies shelter more efficiently than the crab. Even though at present there is no evidence that the two species are syntopic, these results are indicative of possible negative effects of the alien invasive crayfish on the indigenous crab where they may become syntopic in the future.