Science - April 15, 2013

Exotic Asian shore crab conquers Zeeland

Invader chases away young indigenous crabs.
Effect of climate change still uncertain.

‘Tenacious’ little crab from Asia pushes out local crab four times its size.
Until the end of the last century, the European shore crab ruled the roost along the Dutch coast. But that has changed. The exotic Asian shore crab from the Pacific ocean around Japan, China and Korea, has partly pushed it out. Exit local crab then? Probably not, thinks PhD student Anneke van den Brink. Not even if climate change effects continue.
New Zeeland born Van den Brink loves 'those strange crustaceans with their sideways motion and waving claws,' as she puts it in the foreword to her thesis. 'They are intriguing because considering their small size they punch above their weight. They are dynamic and tenacious and I like that.'
Niche
Van den Brink studied the reproduction of the crabs, their population dynamics and their vigour as an invasive species. Part of her study is about the Asian shore crab which was first spotted in the Dutch delta and around Zeeland in 1999. The crab is most likely to have arrived in these foreign parts in ballast water or a ship transporting shellfish. This tiny crustacean (an adult specimen is only a couple of centimetres in size) managed to claim a place for itself in no time. How did it manage to displace a shore crab four times its size?
According to Van den Brink, the Asian shore crab found precisely the right niche for itself on the rocky sea bed of the Zeeland delta waters. Here, the tiny newcomer usually only had to deal with young local crabs, generally about the same size, which take shelter among the rocks. The young crabs probably surrendered to the more aggressive adults of the exotic species. The fact is also that the indigenous shore crabs were already in decline before the arrival of the exotic species. No one really knows why that was, says Van den Brink.
Warmer seas
The shore crab is not likely to be driven out completely, though, according to Van den Brink. It can survive in many habitats and has been a successful invasive species itself in other countries. Van den Brink: 'I think in the end they will both survive but the European shore crab in smaller numbers, perhaps.' And what if the sea warms up even more? 'It is hard to say what sort of effect that will have on the interaction between the species. Rising temperatures make physiological processes go faster. Reproduction goes faster then too, which can lead to a larger population. But that applies to both species.' 

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