Searching For Spider Crab Aggregations

Spotting awesome wildlife behaviour is one of the highlights of any dive, but it's not often you get to witness a natural phenomenon like these spider crabs joining together in a large aggregation. 


The Search


In July 2020, Collette and I were invited to Pembrokeshire to go and search for mass gatherings of spider crabs. Local divers had been reporting sightings of these crabs, but they can leave again as quickly as they arrive so we were quick to jump in the car and drive down to Wales. The aim of our mission was to first find, and then video and photograph any crab gatherings we could find.


On the day we were blessed with a warm, calm morning which was ideal for shore diving. We expected to find the crabs no deeper than 12m, and fairly close to shore. On our first dive, we spent a fruitless dive of just over an hour swimming along the edge of the reef, peering out onto the sandy sea bed looking for any signs of crabs - moulted shells, live crabs, anything really! We didn’t find any, but that didn’t put us off.


Between dives we spoke with more locals, who said our best chance of spotting the crab action would be to return at high tide near to dusk. We only had one cylinder of air remaining each, so we skipped a second dive at lunchtime and instead decided to meet up again that evening. That decision worked out pretty well!


The Crabs


Returning to the water as high tide arrived, we set off in a similar direction. We didn’t come across anything for the first half an hour, but as we turned the dive and headed back to shore, we spotted something on the sea bed a little way away. Swimming closer revealed a moving mass of spider crabs, all huddling together with safety in numbers the name of the game. We then spent the next half an hour of our dive watching these crabs and recording some video to remember the experience.


The crabs join together in these aggregations to moult their shells. When they have shed their old shell they’re much softer, and therefore vulnerable to predators. By getting together in large numbers, sometimes much larger than the example we found, they give themselves the best chance of survival and with time their new shells harden and the crabs can return to a more solitary life.


This dive will certainly live long in my memory, as it’s the first time I’ve ever witnessed this animal behaviour up close. In my log book I recorded that ‘found a big crab cluster and we exited the dive at dusk. Very cool!’ which I think sums up the experience perfectly.