It’s moulting season in the world of Spider Crabs, so I’ve been heading out to Battery Rocks in Penzance several times recently to capture this fascinating natural event. What happens beneath the surface during this short window of time is nothing short of remarkable.

On my first visit, I spotted plenty of smaller crabs moving about the rocky reef, each cautiously going about their business. But on my most recent snorkelling session, the scene had completely transformed. Hundreds of discarded exoskeletons littered the seabed like ghostly remains, and clusters of enormous Spider Crabs had gathered together, easily the largest I’ve ever seen in Cornwall.

This mass gathering happens every year around late summer into early autumn, when the crabs shed their hard outer shells to allow for growth. During this vulnerable stage, they often group together in huge numbers for safety, creating an extraordinary sight for anyone lucky enough to witness it.

Among the rocks and seaweed, the seabed becomes a maze of movement, legs, claws, and shells scattered in every direction. Some crabs are freshly moulted, their new shells still soft and pale, while others stand guard, waiting for the right moment to begin the process themselves. The whole scene feels almost prehistoric, a reminder of how much life unfolds just metres from shore.

It’s moments like this that make Battery Rocks such an incredible place to explore. You never quite know what you’ll find beneath the waves and sometimes, nature puts on an amazing show that I feel very privileged to witness.