Dorset Wildlife Trust’s Marine Awareness Officer Julie Hatcher shares the story of work to monitor the recently arrived furrowed crab.
The wildlife-rich shallows and seashore of Kimmeridge Bay were designated as a protected area under UK law in 2019 and form part of the Purbeck Coast Marine Conservation Zone (MCZ). The intertidal zone (the region between the high tide mark and the low tide mark) in Kimmeridge Bay is the only stretch with this level of protection along the open Dorset coast and an important part of our work at the Wild Seas Centre is to record and monitor the marine life along this coastline.
One such survey focuses on the furrowed crab, Xantho hydrophilus, a native to the south west coast but a recent arrival in Dorset.
Further west, this crab has undergone a population explosion in recent decades, raising concerns about its impact on other long-term residents. First sighted on the seashore at Kimmeridge in 2019, the survey records the population size and any concurrent changes to other crab species on the seashore, including the edible crab, Cancer pagurus, of which there is an abundance of juveniles. Edible crabs move to progressively deeper water as they grow, so the ones found intertidally are the small, immature youngsters.
A team of trained volunteers records the number, size and sex of crabs, along with the habitat
and associated animals. While the population of furrowed crabs is still at a low level, something interesting has been discovered about the edible crabs; out of the 125 recorded, only four were females. Crab experts appear to have no explanation for this gender discrepancy and
further research is needed to solve the mystery.
The need to monitor
Climate change is known to be altering the distribution and survivability of many wildlife species and it is thought that the furrowed crab may be one of these, hence its recent colonisation in Dorset. The effects of shifting distributions and the fortunes of both winners and losers in these changing times are unforeseeable, so monitoring changes and their impacts is vital to our understanding of how we can help.
Of course, the most urgent need is to slow the global temperature increase, which will at least give species more time to adapt. Meanwhile our volunteers will continue to monitor this most difficult of ecosystems and share our understanding far and wide.