Back In August 2025 I filmed this amazing spectacle at Battery Rocks (at the back of Penzance Jubilee Pool) in Penzance.
This phenomenon, known as “molting,” sees thousands of these giant crustaceans migrate from deep water to the shallows. The reason for the mass gathering is safety in numbers. As they shed their tough outer shells to grow larger, they become soft and incredibly vulnerable to predators like seals and wrasse. By clustering together in dense “mounds” or carpets on the sea floor, they create a defensive wall, protecting the soft-shelled individuals in the centre until their new armour hardens. It creates an almost alien landscape just a few metres from the shore, turning familiar swimming spots into bustling underwater cities.
When & Where to See Them
While sightings can never be guaranteed, late summer (particularly July and August) is the prime season for these aggregations. Penzance is a hotspot, but they are also frequently spotted around St Ives harbour and the Fal Estuary.
If you are snorkelling, look for areas with sandy bottoms near rocky reefs, and keep a respectful distance to avoid disturbing their delicate process.