They predate dinosaurs, eat with their legs, and have blue blood that keeps your vaccines safe. Horseshoe crabs are the ultimate survivors of the invertebrate world, looking like they showed up to a costume party in a prehistoric helmet and never left.

Often dubbed 'living fossils' for obvious reasons - their Japanese name translates to 'helmet crab' - these creatures have barely changed in 445 million years. But don't let their Jurassic-era fashion sense fool you: they're not even crabs. They're arthropods, more closely related to scorpions than crustaceans, because evolution has a sense of humor.

Horseshoe crabs come equipped with nine eyes scattered across their bodies and six pairs of legs - but only five for walking. The sixth pair is for crushing clams and worms and shoveling them into their mouths, because why use hands when you can use legs? Their sharp, deadly-looking tail, called a telson, is actually harmless, used only to flip themselves over when they've had a rough day.

Flip them dome-up, and their shells provide formidable defense. But draw their blood, and you'll see it's blue - not red. This copper-based blood is a time machine linking the era before stegosauruses to your last vaccination appointment. The biomedical industry uses it to detect harmful toxins in vaccines, making these ancient creatures unsung heroes of public health.

Horseshoe crabs inhabit North American shorelines from Maine to Mexico, while the related mangrove horseshoe crab (Carcinoscorpius rotundicauda) hangs out in India, Bangladesh, and Southeast Asia. They use beaches as nurseries, like sea turtles but with more legs. Females deposit about 4,000 eggs in clusters in the sand, typically at high tide during a full moon - nature's version of a scheduled delivery. Many eggs become vital food for shorebirds.

Despite surviving multiple mass extinctions, horseshoe crabs now face a novel challenge: humanity. Rising sea levels and coastal development are squeezing their habitat. 'There's a worry that rising sea levels are eroding sandy beach habitat, and once that spawning area on the beach is gone then it's gone,' said Prof Mark Botton, who has studied these creatures for nearly 50 years.

Botton's interest was sparked as a young man when he saw a David Attenborough BBC Life on Earth crew filming horseshoe crabs in Delaware Bay. But he's found the species is unusually hardy. They have an internal cocktail of chemicals for hostile conditions and are adept at finding tiny sandy patches even on concreted coastlines. 'They are very good at finding small patches of sandy habitat even if a coastline has been almost completely concreted over,' Botton said. 'Don't expect them to go extinct any time soon. We shouldn't sell them short or write them off just yet.'

So here's to the horseshoe crab: 445 million years old, blue-blooded, and still flipping itself over with style. It's Invertebrate of the Year season at the Guardian - from 3-12 August, they're profiling amazing invertebrates. Nominate your favorites and vote. But let's be honest: the horseshoe crab probably doesn't need your validation.