The king of Cornish seas

Every year, thousands of spider crabs congregate off Cornwall in the UK. Underwater photographer Lewis M Jefferies goes in search of the phenomenon.

Words and photographs by Lewis M Jefferies

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.

A common species in Cornish waters, the spiny spider crab or European spider crab, is a species confined to the eastern Atlantic. It was formerly grouped with the Mediterranean species, and the identification remains confused among some. Since 2009, after results of morphological and genetic analysis, they are now identified as separate species. The spiny spider crab is well known for its striking orange and red colouration, spiky long legs, and dagger-like claws that can span up to one metre across. They are believed to be the longest living of all crabs, with some individuals estimated to be 100 years old.

They inhabit a variety of depths, from shallow rockpools to as deep as 750 metres. The sexes are differentiated with males of the species possessing much larger claws than females. Scientists believe that females need to moult and be soft to mate, so these large-scale gatherings could help enable, or be the prelude to reproduction as the newly receptive females would also have plenty of partners to choose from during an aggregation. They go on to lay around 1.5 million eggs per season. Spider crabs are a commercially valuable species and have sweet white meat found in their legs, claws, and body. Despite this, there has never been a large demand for them in the UK as the edible flesh is believed to be more difficult to pick from the shell than the meat of the more popular brown crab. Furthermore, a survey conducted by the Cornish Fish Producers Organisation (CFPO) among chefs and consumers concluded that the name ‘spider’ was putting people off. A rebranding ploy designed to increase the popularity in consumers was brought in, renaming them ‘Cornish king crabs’ – a grand yet fitting title earned from their spiky, crown-like carapace.

Spider crabs thrive in our waters and are listed as a sustainable species to catch. Many of the animals seem to migrate here in the spring and summer months from deeper waters. However, although we are starting to see a few more restaurants and fish mongers serving spider crab in the UK, a staggering 95% of the spider crab that is landed here is shipped to continental Europe where the market for them is larger and more profitable. To prevent overfishing, and sustain the species, all crab landed in the UK has a minimum landing size. Individuals that are too small have to be thrown back, while egg-bearing females cannot be harvested.

Continue reading...

To continue reading this article – and enjoy full digital access across Oceanographic – sign up to Ocean Hub today! More info below…

Keep reading by signing up for an Ocean Hub subscription

Ocean Hub. More than a magazine subscription.
As well the delivery of all new editions, members unlock access to exclusive products, services and discounts, as well as EVERY digital back issue we have published.

Hub package PRINT_ banner_37
Find out more about Ocean Hub subscription
Issue 35
Supported by WEBSITE_sponsorlogos_christopherward

This feature appears in ISSUE 35: BUILDING HOPE of Oceanographic Magazine

Issue 35
Supported by WEBSITE_sponsorlogos_christopherward
Supported by WEBSITE_sponsorlogos_christopherward

Printed editions

Current issue

Back issues

Enjoy so much more from Oceanographic Magazine by becoming a subscriber.
A range of subscription options are available.