Tank Tuesdays 6

Common Spider Crab (Maja brachydactyla)

Today’s article puts the spotlight on the European spider crab Maja brachydactyla. Our resident spider crab moulted recently so we thought it would be interesting to have a look at this process. We also wanted to describe a little about its status as a fishery species around the UK, even though very few people eat it in this country!
We’ve had our spider crab since September last year. He is quite happy scuttling around the tank, doing crabby things. Disguising himself as a rock, moulting his exoskeleton and snipping at the anemone. Wearing a scallop shell like a hat. Just living that crab life. This is one of those marine species that could really make a convincing alien. Wafting water through your face, with a set of feathery mouth appendages will do that to you. You’d be surprised though at how quickly a spider crab’s crusty appearance grows on you. He’s one of our favourites in the tank now, so we make sure to give him plenty of mussels as a special treat.

The spider crab enjoying some mussels

M. brachydactlya is found commonly around the South-West of Britain and Ireland, as well as spreading into parts of Scotland. It is found as deep as 50 meters below the waves, where it inhabits rocky and coarse sandy areas. With long spindly limbs, it’s easy to see why it’s called a spider crab. Its other common name is the ‘spiny spider crab’ which stems from the spiky spines that grow out of its carapace. These spines are indeed very sharp and painful to handle; and are designed to protect the crab against predators! They also have two well-developed spines between their eyes, giving the impression of horns.
The carapace of M. brachydactyla can reach a max diameter of 20 cm, which means that it’s the largest species of spider crab around the UK. To reach this impressive size the spider crab must moult its hard exoskeleton several times, before it reaches sexual maturity. At this point it will stop growing and look for a mate. The moulting process is very impressive and is a great illustration of how an invertebrate species’ biology differs from ours. It starts with the crab reabsorbing calcium carbonate, which is an essential shell material; giving it a tough quality. The absorbed calcium carbonate is used to form a soft thin shell beneath the old one. Next the crab secretes enzymes, separating the old exoskeleton from its skin beneath. It then puffs itself up by absorbing water, causing it to swell like a balloon. This expands the crab inside its old shell pushing against it from within. Eventually it’s old shell splits, pulling apart at a seam to the rear. The crab then reverses out of its old exoskeleton, out of the newly opened seam and pulling its legs along with it. The shell then begins to harden over the next few days and will be back to normal within a month. I imagine it’s a very satisfying process for the crab.

We recently discovered that our spider crab had moulted. One morning it seemed like we had magically gained a new, much bigger crab. But of course, on closer inspection the smaller crab was just the old shell; which remains very intact. Unfortunately, we missed the moulting process itself but it’s still very interesting to compare the sizes of the crab post-moult and its old exoskeleton; as we can see from the images. The shell of the crab after it had moulted was also very soft, with the spines being a lot less sharp than usual.
In the wild this species forms dense aggregations called mounds where the crabs moult together. Up to 50000 individuals have been observed climbing over each other to moult and breed. From diver’s reports, this is a regular occurrence along the Devon coastline and close by to our lab in Torbay. The actual frequency of these giant crab clusters though is hard to find.
In comparison to the size of the carapace M. brachydactyla has relatively small and elongate claws, which are more specialised for picking molluscs out of their shell. This is opposed to the crushing action of the common shore crab Carcinus maenas or the edible crab Cancer pagurus, would do.
Unlike our snake locks anemone this species has distinct sexes and will migrate to deeper waters to reach their breeding grounds. Our spider crab is a male, as we can tell from the narrow triangular pleon or abdomen (see below), folded under the crab’s body. In comparison a female’s pleon would be much wider (see below), allowing her to brood eggs safely under the abdomen. This sexual dimorphism is apparent in many different types of crab. For those that enjoy crabbing or rock pooling keep an eye out!

Younger spider crabs live between 5-15m. After growing up fully and completing their “terminal moult” they will migrate to 50+ meters. Once they have reached their breeding grounds, they will begin courtship rituals to find a mate. It is quite common for crustaceans to show courtship behaviours, as it acts as a method to assess mate quality.
M. brachydactyla is caught in UK waters for the edible white meat found within its legs and claws. They are mostly fished for around the South-West of England and Ireland. In 2016, 350.95 tonnes of live spider crab were landed in the UK (ICES, 2018). Most of the catch (over 95%) is exported to mainland Europe where there is a demand, particularly in France and Spain. It’s difficult to access accurate information on export weights but using a simple calculation with 95% of catch exported, this would be around 333 tonnes of spider crab exported in 2016. This isn’t even that high in comparison to previous years where in excess of 1000 tonnes of spider crab were landed in the UK, presumably for export. It’s difficult to assess the sustainability of the fishery as no stock assessments have been performed. The only current management practices are a minimum size and the return of berried – egg-bearing – females.

Although this is another species of spider crab this is a good clip to show how spider crabs moult (credit to Into The Wild)

Official Nominal Catches 2006-2016. Version 07/08/2018. Accessed 07/05/2019 via http://ices.dk/marine-data/dataset-collections/Pages/Fish-catch-and-stock-assessment.aspx ICES, Copenhagen.

This article was written by Jamie Mathews and Sam Hickling

Tank Tuesdays is a series of mini articles describing the marine life here at ARC Marine’s aquaria; and their interactions with our artificial habitats.