Sponge Crab

Image: Oliver Cardona

The ocean... it's a tough place to live. Teeth and tentacles abound. Bone-cracking beaks and blood-burning venom await the unobservant adventurer around every corner. When they're not descending from above or bursting out of the ground, that is.

What we need is armour. Full body armour. Woven from titanium, poison and vengeance.

Image: Bernard DUPONT

Or maybe a hat. A soft, fluffy hat.

They'll never expect THAT!

Image: Hans Hillewaert
Dromia personata

We're looking at Sponge Crabs, a whole bunch of species belonging to the Dromiidae family and found mostly in warm, shallow waters across the world.

They are incredibly bulky as crabs go. Their bodies are almost spherical in shape and their limbs are immensely thick and chunky. They look like they've been carved out of solid rock by someone who isn't actually all that good at carving things out of solid rock.

Some of them appear to be just a little bit fuzzy all over...

Image: Peter Southwood
Shaggy Sponge Crab (Dromidia hirsutissima)

While a few appear downright mouldy.

Image: Invertzoo
Moreiradromia antillensis

The very smallest ones are less than a centimetre (half inch) long...

Video: John Araki
Sleepy Sponge Crab (Dromia dormia) stomping around like a golem. In a hat

While the Sleepy Sponge Crab is the largest at some 20 cm (8 in) across. However big they are, you can see them striding about the place on four, thick legs as they sport their strange, sponge head dress.

Wait a minute... four legs? FOUR? Crabs are supposed to have eight!

Are we too late? Has the soft, fluffy hat failed them and even now their legs are being gnawed upon by some... some... nightmare creature of nightmarish nightmare?

Sorry, but I'm lost for words (aside from the word 'nightmare'). I just can't imagine what manner of hellish death-beast would be unfazed by such a soft, fluffy hat. Unless... a hell-beast... with an even softer, fluffier hat?

IT CANNOT BE.

Image: Ria Tan
See the little legs clinging on to the sponge?

Turns out I needn't have worried. Those four, rearmost legs are still there, they're just bent upwards and backwards so they can be used to carry the sponge close to the crab's body.

Sponges are those strange, barely-animals that exist as a colony of cells squished onto rocks or moulded into various, towering shapes. Sponge Crabs simply walk over to them and cut a chunk off. Then they use their pincers to nip and tuck it into shape so that it'll fit the contours of their carapace nicely.

Image: mathieustewart
Dromidiopsis edwardsi

Some Sponge Crabs seem to do a better job of it than others. And if there's no sponge available, they'll make do with sea squirts or, in one observed case, a flip-flop.

It sounds quite bad for the sponge. Most of us wouldn't like to have bits and pieces of our body ripped off by a crab. But sponges can bear far worse, and they happily continue feeding and growing right there on the crab's back. This can blossom into a long-lasting relationship as both the sponge and the crab grow up together, with the crab performing the odd bit of pruning now and again to keep things under control.

Image: selbst
Hermodice carunculata uses provocatively coloured pincers to tackle a Bearded Fireworm

Sponge Crabs usually spend the daylight hours hidden in nooks and crannies, only coming out at night to feed on meaty detritus. The sponge serves as camouflage, which is nice, but they may well help in other ways too. Many sponges deter their own predators with toxic defences, which... perhaps don't work on at least one crab I know.

Still, who would say no to a living hat that squirts out toxins when it's frightened? If nothing else, it sounds like a great way to collect anecdotes to share with any surviving friends and acquaintances.

Video: mauiprowler
This infuriated Sponge Crab lacks a sponge so you get to see those strange hind legs

One really cool thing about some Sponge Crabs is that they occasionally go mad for some reason. They not only look like angry golems, they behave like them too. They give unrelenting chase to divers and who knows what else, swiping and nipping with their claws. It's as if some guy carved them out of solid rock, animated them with dark magicks and commanded them to patrol the ocean floor and defend buried treasures from prying eyes.

Maybe I played one too many computer games that featured golems, but this sounds convincing to me...

Image: mathieustewart

What are you hiding, Sponge Crab. What are you hiding...