Freshwater Crab – the story of its arrival on the Maltese islands
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A total of 40 individuals of different freshwater crab (qabru) populations sampled for a genetic study, from the Xlendi valley in Gozo, and in Malta at Bahrija, Ghajn Zejtuna and San Martin in Malta, as well as from south-east and north-west Sicily, from southern and central swathes of peninsular Italy and from 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.
The University of Malta has said that the individual freshwater crab 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.”
This study, which was 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 from in the coming weeks from the journal homepage.
The University said that 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, and during which the mean sea level within the Mediterranean putatively went down by a staggering 120m.
It noted that as a result of this marine recession, extensive ‘land bridges’ appeared across the Adriatic Sea, such that freshwater crab individuals from the mother populations in the Balkans could ‘invade’ southern Italian regions as well as the Maltese archipelago.
The University went on to say that this primary ‘invasion’ subsequently led to a secondary “invasion” of central Italian regions further north, once temperatures recovered after the Ice Age.
It said that this confirms 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 such research worthy of note is the fact that the anthropogenic (human-mediated) introduction of the freshwater crab on our shores is deemed highly unlikely,” the University said.
Prof. Deidun said that 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.
The study has been supported financially by a grant from the Research Committee of the University of Malta and it said that all necessary permits to sample the freshwater crab, a protected species, were obtained a priori from the Environment and Resources Authority (ERA).