The rock pools lining the dynamic basalt coastline of Madeira are a tough neighbourhood. Many of the marine animals inhabiting this area are subject to potential attacks by lurking predators or in danger of being washed away by the incoming tide. To survive, they need to be well-camouflaged against the rocks and light on their feet.
Almost all rocky beaches in Madeira are covered in scuttling groups of crabs that are extremely nimble and nearly impossible to catch (unless ,of course, you’re a sneaky seabird or a stealthy octopus). The colourful Lightfoot crabs (Graspus ascensionis) pick at the freshly exposed algae during low tide together with darker coloured Runner crabs (Pachygraspus marmoratus), exposing themselves to lurking seabirds. Yellow-legged gulls (Larus michahellis) are particular fans of these agile crustaceans but they must be quick; they aren’t called Lighfoots and Runners for nothing! When threatened the crabs quickly dart into rock crevices or rock pools, easily outrunning most of its terrestrial predators.
The crabs, however, have their vulnerable moments. When surprised by the tide the crabs can be washed directly into the ocean and exposed to hungry fish or lurking octopi. Moreover, they are an easy prey when going through a very frequent and inevitable life stage of a growing crab; molting. Like all crustaceans, crabs do not experience linear growth and must cast off their exoskeleton to make room for their growing body. After exiting their original shell, the crabs new exoskeleton is soft, making the strong crustacean an easy meal.
A third, very nimble crab that is frequently seen on the island prefers to graze underwater. The Nimble spray crab (Percnon gibbesi), also referred to as the Sally Lightfoot crab, is a small and extremely fast crab that, like the two previously mentioned species, enjoys grazing on the layers of algae lining the rocks. This underwater herbivore is best recognised through the yellow rings around the joints of its thin legs and, like its terrestrial cousins, is a difficult one to catch. The decimation of Octopus populations on rocky coastlines around the world has enabled this species to multiply. Like most crabs, they have planktonic larvae which are long-lived allowing the species to proliferate far and wide. Spray crabs are even known as the most invasive decapod species to enter the Mediterranean.
So now you know what those crabs which you shouldn’t bother chasing are called. So instead of tripping over rocks whilst trying to outsmart one of nimblest creatures in Madeira, you can spend your time telling your friends a thing or two about them.
By Paula Thake