Decorator crabs are ingenious and often unwittingly entertaining in their choice of adopted camouflage gear. LISA COLLINS has an eye for them
THAT IS SOME QUIFF! Stylishly coiffured and tinted purple and blue, the creature in front of me sported a look of which any rock & roller would be proud.
It was some time ago and I was on a night-dive – only my second ever – on the house reef at Papua Divers Sorido Bay in Raja Ampat.
I had not been particularly impressed with my first night-dive, which had been in the Caribbean. The reef had been close to shore and had very little life. However, this was something else.
Following my torch-beam and trying not to think about what was lurking in the blackness, I was amazed by the critters coming out to feed and play at night.
Scanning over the corals, I couldn’t believe my eyes as an anemone appeared to be crawling over the reef. Moving closer, I noticed that it wasn’t actually the anemone moving, but a crab underneath that was carrying the anemone on its back.
Stopping on top of a large piece of coral the crab posed for me, as if it were actually Elvis on a stage, while it fed on tiny crustaceans.
This was my first encounter with a decorator crab, one of those fascinating creatures that use different items to camouflage themselves.
There are more than 900 different types of decorators, and some 75% of these belong to the Majoidea family.
Seeking out a variety of normally sedentary living plants and animals such as sponges, algae, seaweed, coral and anemones, the crabs attach them to their bodies, legs and backs with hook-like hairs or setae. It’s a bit like Velcro.
Larger crabs may also have adapted point legs to help hold the decorations in place.
The crabs choose suitable garb depending on their surroundings, aiming to match the two to perfection.
I have found it extremely hard to detect which parts of a decorator crab are actually the crab, and which the camouflage.
ELVIS WAS A FAIRLY LARGE decorator crab at around 10cm long. I have seen miniscule ones such as orangutan crabs, which adorn themselves with algae, to one that was almost the size of a dinner-plate, which had adopted the guise of a shell covered in several small anemones.
I have seen several of this type, often very colourful because they have added decoration in the form of small pieces of sponge.
One small decorator crab I came across in the Philippines had decorated its whole body with a dark red sponge. The shape of the crab was still evident, but it seemed to be wearing a spongy jumpsuit. Sponge being a living animal, I knew it would grow larger and disguise the crab’s body more.
When either a crab grows and needs to shed its shell, or its attached decoration grows too large, the creature will delicately remove it piece by piece, replacing it again when its new shell is hard enough, or with trimmed-down decoration if its chosen animal or plant has grown too big.
Crabs often choose poisonous or stinging animals to add further protection to their camouflage. Anemones are a prime example. They are thought to live symbiotically with the crabs, offering protection in return for food supplied by the crustacean.
ONE THING’S FOR SURE – crabs are very much like humans, in that some are way better dressers and far more stylish than others. I have seen my fair share of messily decorated crabs, and some stunningly attractive, well-tended and colourful ones.
Again in the Philippines, I saw several upside-down jellyfish undulating across a sandy bottom on one dive. On closer inspection, I noticed that they were being carried on the backs of decorator crabs. One crab was completely engulfed by the jellyfish, and virtually invisible when it wasn’t walking.
On another night dive, we had to be careful not to get too close to the bottom as hundreds of bright orange and pink sea-urchins came out of the reef and sand to feed at night. Scurrying around the bottom, they congregated in large groups.
One of them was slightly away from the rest, and seemed to be bobbing up and down. On closer inspection, it was sitting on the back of a decorator crab.
It must have been very heavy, because the crab would take a few steps, lifting itself and the urchin up high, then slump down a few steps later, like a weight-lifter between lifts.
In the Caribbean, yellow-line arrow crabs seem to be everywhere. I had not realised that these were a type of decorator crab until I saw a couple with seaweed attached to their probosces.
If the seaweed is not of the correct consistency to stick to their carapace, the crabs will chew it to make it more adhesive.
Decorator crabs are found all over the world from temperate to tropical waters. The UK has many, one example being the large spider crab.
Lembeh Strait in Indonesia seems to be a mecca for decorator crabs of all sizes, shapes and appendages. On one dive, my guide beckoned me over to a piece of branching soft coral covered in delicate filter-feeders. He kept pointing at a part that protruded slightly more, but for the life of me I couldn’t make out what he was trying to show me, though I took a few photos anyway.
Back on the boat, he told me that it was a decorator crab. Even looking at the photograph it is difficult to tell which part of the sponge is actually the crab.
One of the strangest decorator crabs I have seen was on a night dive off Anilao in the Philippines. Scanning the black sand, I saw a large piece of cellophane floating along the bottom.
Conscious of conservation, I try to collect any large pieces of rubbish I see under water if I can store it in my BC or wetsuit easily.
AS I WENT TO GRAB THE CELLOPHANE, it moved away from me very quickly, stopping several metres away. Again I approached, and the same thing happened. Moving round and coming at it from a different direction, I was astonished to see that it wasn’t just a piece of rubbish, but was mounted on the back of a large crab as decoration. It looked just like a wedding veil!
I thought it looked so funny that I laughed out loud, and got a mouthful of sea water!
Some divers consider crabs boring because they see them quite often, but I find them fascinating, and look forward to finding ever new and more ornately decorated and camouflaged crabs whenever I go on a dive, day or night.
Next time you see a piece of rubbish or an odd-looking sponge, check to make sure it hasn’t got legs!
Appeared in DIVER September 2016
