A s a young teenager learning to scuba dive in 2001, I remember one of my first open water dives off a pebble beach in east Devon. We made entry into the water from shore and proceeded offshore to gain depth. At first, we swam over a largely featureless, sandy seabed until a large shadow came into view, and trailed strangely off into the distance. As we neared the dark shape it became clear that this was not a change in the topography of the seabed into a rocky reef or bed of seaweed, but a writhing mass of individual animals. At just 13 years old, I had never seen creatures like this before. Huge, red animals that resembled spiders, with large, oval, crown-like shells adorned with spikes, and ten long appendages, the two frontal ones of which were tipped with menacing claws and a flash of white down the front edge that made them look like samurai swords.

The scene was like something out of a science fiction film, hundreds of creatures gathered, scuttling along the seabed, and clambering over one another. Suffice to say, that dive was a memorable one, and over the following years, as my passion for diving, marine life and photography grew, I was eager to see this spectacle again, but this time, with camera in hand.

Fast forward 20 years and I’m on the northwest coast of Cornwall. It is peak holiday season, and the beach is packed, full of tourists that have flocked from all over the country to spend their holidays at one of the most popular places in the county, St Ives. This picturesque Cornish town is famed for its beautiful golden sands, for its clear turquoise waters, for surfing, art and coastal scenery. It is below the waves here, just metres from the bustling town and beaches, that this most remarkable natural spectacle takes place every year. I had been anticipating the event all year after hearing that this was a reliable location, and for some weeks had been looking around for any signs that a gathering was starting to happen, both underwater, and above from local water users.

Then in mid-August the news I was hoping for came through: a local surf photographer based in St Ives let me know that he had spotted a very large group of crabs around 300 metres from the beach. With the fair weather and good water visibility set to change the next day, it was now or never. Equipped with my freediving gear, underwater camera and flashes, I drove an hour from my hometown on the south coast, and battled my way through the hordes of tourists on the beach to get to the water. Timing my visit around the low tide, it was just a short swim out to the location. I started to search around. I first spotted a tell-tale trail of empty crab shells drifting around in the current.

Then, as I got closer, a dark, familiar looking scene stretched out before me. I’d found exactly what I had come to see: 200 or more spider crabs carpeted the sandy seabed below, amassed at around six metres depth. Freediving down, my excitement rising, I was met with an incredible scene that resembled an alien invasion. An enormous mass of red spider crabs had rallied together, and in their efforts to remain part of the group were stacked two or three deep in places. Sand had settled atop their carapaces and glistened; the beautiful turquoise St Ives waters rippled around them as the warm summer sunlight filtered through from the surface. Individuals climbed over each other, yet they all appeared to stay as one unit moving slowly and purposefully along the seafloor.

As UK waters warm during the late summer months, these hefty crustaceans head to the shallows in large numbers to grow. Like all crabs, their bodies are enclosed in a hard, unexpendable shell. To mature and grow, they must break out of it. As the crabs extract themselves from their old shells and expand their soft new flesh to grow, they are vulnerable to predation for a few days until their exoskeletons harden. Gathering in large numbers helps individuals protect themselves from potential predators and gives them the best chance of survival. Often the newly moulted crabs can be found in the middle of the group protected by the others around them.