An alert for some brush crabs reported near Fredrikstad in Norway instead turned out to be the first sighting of a related species, the Asian shore crab, a species that has its origins in the northern Pacific. Brush crabs are also an invasive species from the same part of the world, known to have already established itself in European waters.
But the arrival of the Asian shore crab (Hemigrapsus sanguineus) is considered a hazard for the environment as it has a high reproductive capacity, can compete with local species and could have negative impacts on the ecosystem.
According to Vivian Husa at the Institute of Marine Research (HI), it is no surprise that this species has made an appearance in southern Norway.
‘We have been expecting this one, unfortunately, as it has previously been detected along the entire west coast of Sweden,’ she said.
Both shore crab and brush crab can be easily recognised by the square shape of the shell, with three spikes on the side and striped legs.
‘The Asian beach crab is slightly larger than the brush crab, but it is still smaller than a full-grown beach crab,’ Vivian Husa explained.
The brush crab (Hemigrapsus takanoi) first arrived in southern Europe in the 1990s, and has since been gradually spreading northwards, with confirmed reports of it being in southern Norwegian waters in 2020.
Research in recent weeks has shown that they are spreading through Oslo Fjord.
‘This indicates that the brush crab has now become an established species in the entire fjord,’ Vivian Husa said.
Marine researchers have believe that the brush crab has great invasion potential, as it can quickly become numerous where it settles.
The crab is considered to have a relatively small ecological effect, but is omnivorous and can give the regular Norwegian beach crab competition in hunting for feed.
Brush crab appears to have spread throughout Oslofjord. Image: Rudolf Svensen/MUST
