In my experience, it takes three good looks at a decorator crab to truly understand what it is. The first look is when you acknowledge that present before your eyes is something living. The second look is when you start to realise there are many different living entities coming together to take on the identity of a decorator crab. By the third look, your mind is blown into pieces, from the sheer lack of believability that comes with witnessing these unusual animals.
Decorating for us humans, transcends time periods and cultures. Whether it is adorning ourselves, objects, or places, decoration is a form of expression, reflection, and a representation of our identity and personalities. Here we see that there are non-humans too who are known to decorate with purpose — and these creatures are marine crabs.
Why would crabs want to embellish themselves? We may never know all the reasons behind why they decorate their shells, if celebrations are involved, or whether there are individual differences in taste and style among crabs. What we do know is that the type and extent of decoration assembled by these crabs could mean all the difference between life and death. The twist here is that all the elements of decoration that are assembled on a decorator crab are alive.
Crabs in general are extremely diverse, and occupy numerous habitats on the ocean floor in warm and cold temperatures. Irrespective of location, and sometimes size, crabs are a fine meal for any predator that can manipulate their crusty exoskeletons. To address their vulnerability, decorator crabs have taken their anti-predator adaptations to a whole new level, by mastering the art of deception. Contrary to one of the reasons humans decorate (to be seen and marvelled at), the reward for decorator crabs is in going unnoticed.
Decorator crabs escape being attacked by predators (such as an octopus or strong-jawed fish like a triggerfish or wrasse) by masquerading as life forms that would not interest them or would actively deter them. The toppings on a crab’s exoskeleton could include numerous types of filamentous algae, calcareous algae, notably Halimeda; and, or animals such as hydroids, anemones, sponges, and zooanthids among many more we are still discovering.
In a nutshell, the construction of a disguise involves picking up an animal or plant and placing it on the body. A closer look will reveal that this is quite an elaborate endeavour. It is achieved through a careful study of the ecology of its neighbours and the microhabitat it lives in. Understanding what types of algae can be cultivated on the body and using it to blend in with an algal meadow. It requires keen knowledge of animals that are not only useful for camouflage (like anemones) but also can be used as defensive, toxic weapons against predators. The crab must have a zen-like calmness to remain absolutely still when blending in with a stationary sponge; or must master a practiced sway to match the surrounding hydroid colonies. These are some of the many traits seen in a determined decorator crab that has survived the test of time and predation.
There is an impressive diversity in the techniques evolved by different decorator crabs to hold their masterpieces up on display. Several species of decorators in the Majoidea family have hooked hair (or setae) lining their carapace that hold their live accessories with the power of velcro action. Crabs from families such as Dromiidae and Dorippidae have one or more inverted pairs of legs at the back of their carapaces, used to literally carry around their accessories — sponges, urchins, and sometimes a colony of coral.
Dardanus anemone hermit crabs on the other hand live inside empty snail shells which form the substrate needed to assemble their defensive decoration. An anemone cannot be coerced into leaving its existing home. It takes some stroking and patting on the part of the crab to make the anemone loosen its grip and latch onto its shell. A hermit crab can often convince multiple anemones to comply and get on board for life on its shell. In a sense this is a symbiotic relationship where the anemones are provided a stable substrate to live on, gathering additional food from discards left by the hermit crab in return for providing defensive stinging and camouflage services to its host.
Decorator crabs do have to pay their share of the price for the benefits they receive from using the living world around them. There is significant physical effort involved in picking up, cultivating, and carrying around their decorations with them all the time. They must be healthy and find adequate food to fuel the energy requirements of weight lifting. Last but hardly the least important of all is that as crustaceans, these crabs will have to shed their old exoskeletons as they grow bigger. In the case of the hermit crabs, they must procure a larger snail shell as well. This would effectively mean that decorator crabs have to reassemble their disguise from scratch each time, or smartly repurpose and transfer participants from their existing team onto their newly moulted bodies. Sometimes I wonder whether decorator crabs have their own version of Extreme Makeover or Better Homes and Gardens for the latest in live accessories and camouflaging tips, to keep up with the constantly changing trends in evolution!