A genetic study of different freshwater crab (qabru) populations, involving a total of 40 individuals sampled from Malta, Sicily, southern and central Italy and the Greek island of Corfu, has shed light on the possible dynamics of the arrival of the species to our shores during the last Ice Age.
In Malta the samples were taken from Baħrija, Għajn Zejtuna, San Martin and Xlendi valley in Gozo.
Individuals were sampled with the removal of just one leg, being released back into the wild soon after, with this intervention not expected to have harmed them unduly since crustaceans regularly regenerate amputated body parts after each moult.
The results of the study indicate that, contrary to what was previously assumed, Potamon fluviatile (the freshwater crab) probably reached our shores during the Last Glacial Maximum (LGM), the last Ice Age, which occurred roughly 20,000 years ago in the late Pleistocene era, and during which the mean sea level within the Mediterranean went down by a staggering 120 metres.
As a result of this marine recession, extensive ‘land bridges’ appeared across the Adriatic Sea, so that freshwater crab individuals from the mother populations in the Balkans could ‘invade’ southern Italian regions as well as the Maltese archipelago.
This primary ‘invasion’ subsequently led to a secondary invasion of central Italian regions further north, once temperatures recovered after the Ice Age, confirming that the species generally colonised new lands during glacial periods and increased demographically (in numbers) during inter-glacial periods (i.e. once temperatures recovered).
Another conclusion of the research is the fact that the anthropogenic (human-mediated) introduction of the freshwater crab on our shores is deemed highly unlikely.
This study, conducted jointly by the University of Malta (represented by Prof. Alan Deidun from the Department of Geosciences, along with Arnold and Jeffrey Sciberras) and by the University of Palermo, is being published in the European Zoological Journal, and will be available online in the coming weeks from the journal homepage: www.tandf online.com/toc/tizo21/current.
The study has been supported financially by a grant from the Research Committee of the University of Malta and all necessary permits to sample the freshwater crab, a protected species, were obtained from the Environment and Resources Authority (ERA).
According to Prof. Deidun, the species is subject to multiple threats given its dependence on a perennial source of water, the intensive utilisation of our limited aquatic resources for agriculture and the recent introduction of invasive, non-indigenous crayfish species.