We’ve written about crabs before here at Nature Matters, in our blog Feeling Crabby. But today’s subject, the horseshoe crab, wasn’t mentioned in that article. Why? Because it’s not really a crab at all!
Horseshoe crabs get their name from their carapace, or shell, which is in the shape of a horseshoe. This led to the mistaken belief that they were crabs – crabs have shells after all. But in fact, horseshoe crabs are more closely related to spiders and scorpions that to their namesakes.
Unlike spiders or crabs, horseshoe crabs have tails. They use these to steer when swimming and they can also ‘flip’ themselves over with their tails if they get stuck on their backs. They have 5 legs (spiders have 8 and crabs have 10), the front 2 of which they use like hands to grab prey – usually small fish or crustaceans.
When it’s time to breed, horseshoe crabs venture out of the sea and onto the beach. Males select a female and climb on to her back – sometimes one female has several males climbing on her. She digs a hole in the sand and lays as many as 60,000 eggs which the male fertilises. Why so many? It’s because only a very few of the eggs will grow into adults. Many are picked off the beach by birds before they hatch. Those that survive have to crawl to the sea where they are vulnerable to predators because of their size. Even as adults they are prey to sea turtles and sharks.
One peculiar thing about horseshoe crabs is their blood – it’s blue! Ours contains haemoglobin, a molecule containing iron which carries oxygen for us. This iron makes our blood red (think of red rust – that’s just iron oxide). Horseshoe crabs use a different molecule, haemocyanin, which contains copper. It’s the copper which makes the blood blue. Their blood is also useful for detecting bacteria – sadly for the horseshoe crab! We’ve taken to ‘harvesting’ them and extracting their blood.
It’s not just their blood we take either. We use them as bait for fishing, and even as fertiliser for crops. We’re also ‘developing’ their coastal habitats which could become a great threat to horseshoes crabs’ breeding. They will only ever lay eggs on a beach so it’s impossible to breed them in captivity. If they become extinct in the wild they will be gone forever.
Horseshoe crabs have been around for 450 million years. They saw the dinosaurs come and go. After all this time on planet Earth, horseshoe crabs could become extinct because of one species – humans.