One of our most familiar marine creatures has a set of secretive rituals that allow it to grow , says Paul Naylor
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Spiny ( or Common ) Spider Crab
‘ Moulter and Decorator ’ Maja brachydactyla
A large male spider crab with minimal decoration
One of our most familiar marine creatures has a set of secretive rituals that allow it to grow , says Paul Naylor
In terms of fascinating behaviour that you can watch unfold , I think of this crab as the animal that keeps on giving . It ’ s also a large creature and does much of its business out in the open , making it relatively easy to observe . While there are probably many other aspects of its life worthy of study , two that stand out are the
Aggregated spider crabs with varying degrees of decoration way that it moults its exoskeleton and the ‘ decoration ’ process that often follows .
All crustaceans must moult to grow of course , it ’ s the price they pay for wearing a tough , all-encompassing suit of armour that doesn ’ t stretch . It ’ s a common occurrence too ; a shore crab , for example , typically moults 18 times in its life .
The unusual thing about these spider crabs is that rather than hide away to moult , avoiding prying eyes ( and more importantly teeth and claws ), they perform at least one of them ‘ in public ’. The crabs congregate in massive aggregations in shallow water , completely covering or even heaped up on the seabed , to undergo their ‘ pubertal ’ moult . As the name suggests , it is the point at which the crabs become sexually mature and it is also their last or ‘ terminal ’ moult , as they will never shed their shell again .
A thorough look around one of these groups will sometimes reveal a crab just starting to moult , with its ‘ new ’ body beginning to emerge from an opening at the back of its armour . We have been surprised to see many crabs wandering about in this partly-moulted condition and waiting to see them continue has been fruitless but others , fortunately , have ‘ got on with it ’ and been marvels to observe . It is incredible to see them ‘ pump ’ their legs out of their casing like pistons , then emerge backwards from the old armour before the final struggle of extricating their claws . This last step is particularly challenging for males because their claws greatly enlarge at puberty .
To me , the crabs ’ ‘ decoration ’ process , where they attach camouflaging material ( usually seaweed but sometimes sponge or other growths ) to the newly bare armour ,
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