Out of the Blue: In praise of the humble crab
Written by Elaine Hayes, Plymouth Sound National Marine Park Interim Chief Executive
On the face of it the crab would not be considered a particularly loveable animal but you only need to look at iconic films such as The Little Mermaid and Finding Nemo to realise that these animals are not just invertebrates but a veritable feat of nature. With 8 legs two pincers and a tough shell to navigate around the oceans, they are truly tough critters. But we shouldn’t be surprised, crabs have been roaming Planet Earth in one guise or another for over 200 million years, long before the dinosaurs roamed the planet and ever since! So resilient are they that they survived the mass extinction that wiped out the dinosaurs. Today there are many, many species of crabs around the world – on land, in the sea and in freshwater – some even live in caves.
Crabs are the preferred prey of a huge array of species – from the ones you would expect like fish such as cod, to sea birds such as cormorants and shearwaters to some very, unexpected species including turtles, snakes and otters and humans too! Crabs are most vulnerable as larvae and juveniles when they make a tasty morsel indeed, but today crabs face other threats and I will go into that later.
So back to the indestructible crab – there are over 6,700 known species in the world today – with just 62 found here in the UK. Nonetheless, that could be a lot of pinches! Obviously, I can’t talk about every variety in the world so I am just going to pick out a few favourites. I am going to start with one I call the perpetual renter – the easily recognised hermit crab. You can find these on any beach In the UK. They have a soft body which would make them very easy to eat. So to protect themselves they hang out in the unwanted shells of molluscs such as whelks, topshell and winkles. As they grow they simply find a new slightly larger shell – crawl out of one and into the next. How cool is that?
From a relatively small species to the biggest in the world which is the Japanese Spider crab. I definitely wouldn’t want to meet one of these whilst out on a swim- these are huge and can be nearly 4 m across from claw to claw – yes I did say 4m! They look very similar to their UK cousins which thankfully are much smaller! The Japanese spider crab is considered a delicacy and is very vulnerable to overfishing. Mainly a deep water species it can live for up to 100 years!
Now I want to focus on two species that are critically endangered – the first is the Singapore freshwater crab – It Is one of the 100 most endangered species in the world with only a few hundred animals left. Populations are very isolated and do not interbreed. Efforts are being made to save them but with such low numbers this may, sadly, be impossible.
Another vulnerable species I would like to highlight is the Chinese tri-spined horseshoe crab. This species faces so many challenges! It is the largest living horseshoe crab in the world and is overfished, taken for biomedical research and is at risk from pollution and habitat loss.
The better news for UK species is that leaving aside the St Piran’s crab, which is incredibly rare and has only been found in Cornwall, currently all our species abundance is okay. There is a great deal of pressure on all species through fishing and this is compounded by the looming threat that climate change poses. Increasing water temperatures will favour the species that prefer warmer waters, driving our native species further North. Rising temperatures, increasing levels of carbon dioxide and importantly dilution effects caused by storm events will also impact all crab species. As always if we are to live with nature and not in spite of it we the humans who have only been on the planet for 300,000 years should try a bit harder not to kill off species that have been here for over 200 million years – enough said!