Decorator crabs: dressing up to avoid attention

There are over 700 species of decorator crabs that cover their bodies with a wide range of unexpected ornaments. But these crustaceans are not dressing up to impress or get noticed.

spider crab
This spider crab is covered with all manner of objects and marine creatures

Although decorating is a trait that is only normally associated with humans we are by no means the only animals that like to accessorize. In fact there are several fashionable animal groups that can incorporate other objects or animals into their appearance. But behind humans the next best decorators are probably crabs. With over 700 species including spider, harlequin, moss, little seaweed and toothed crabs all belonging to the family Majoidea, the decorator crabs are one of nature’s trendiest dressers. But unlike humans and some other animals the goal for these crabs isn’t to catch someone’s eye but rather to avoid it completely. So just what are these crabs ‘decorating’ with and how and why are they doing it?

Who are you wearing?

Much like humans the real question in crab fashion isn’t necessarily what you are wearing, but instead who. That is because most decorator crabs find that the best outfits actually consist of other marine creatures. Among the hundreds of different species of decorators there are almost as many other species who are used as decorations. Including seaweeds, corals, sea fans, sponges, seagrasses, anemones, sea urchins, bryozoans & hydrozoans (types of colonial organisms often mistaken for algae or corals) and even other small crustaceans. However just because these organisms are hijacked by these decorators doesn’t mean they don’t get anything out of it. In fact the relationship between crabs and their ‘accessories’ can be highly mutualistic helping plants and animals to move over large distances and reduce their chance of predation.

sponge hat
This crab has attached part of a sponge to its carapace in an attempt to blend in with its surroundings

Pulling off the look

So these crabs are able to attach just about any marine organism smaller than themselves to their shells. But how is it that they make them stick? Well decorator crabs have specialised hooked bristles along its body known as ‘setae’that act very much like Velcro. The crab simply selects what it wants to attach (this is easier said than done as decorator crabs can be very picky about what they wear), picks it up and places it on the bristles which wrap around the object and hold it in place. Some crabs have just a small covering of bristles over the certain areas they want to decorate whereas others like the moss crab are covered in them from head to claw. The only downside is that when the crabs moult their carapace to grow, they also lose all their decorations for a brief period of time. Check out decorator crabs selecting and attaching their decorations in this excellent video by PBS Deep Look.

Dressing discrete

It is easy to understand how these crabs have got their names, but once you realise why they are dressing up the term ‘decorator’ doesn’t seem to really do them justice. That is because they’re not showing off or trying to attract the attention of a potential mate, but instead to hide away and protect themselves from predators. This is the reason seaweeds and colonies of bryozoans and hydrozoans are such a popular choice. But for some species with particularly colourful habitats pieces of corals, sea fans and sponges work much better. As long as they don’t move too far away from whatever they are trying to mimic then it is usually very effective. So instead of ‘decorator’ crabs perhaps stealthy, disguised, camouflaged or impostor might be a more accurate name.

sea fan
This crab uses part of a sea fan that perfectly matches its spindly legs

Animal armour

But some crabs have gone further than just trying to disguise themselves with their decorations by instead using them to bolster their defences. A great example of this is the use of anemones, urchins and hydrozoans, all of which can give any potential predators a nasty sting if they get too close. As well as this using special selected toxic seaweeds can leave a nasty (and potentially fatal) taste in the mouth for anyone that tries to take a bite. By decorating their already heavily armoured shell with these extra fortifications makes these crabs an off-putting option for many of their natural predators.

This collimorph crab is named after the type of coral-like creature that grows on it

Gone fishing

On the other hand a select few decorators have flipped the concept on its head and decided to go on the offensive. One such species is the boxer crab that can attach anemones and urchins to their claws to use as weapons. However some crabs have taken advantage of a particular decoration to acutely fish for prey. That decoration is the hydrozoan Hydrichthella epigorgia which like corals is made up of lots of tiny polyps that work together as one collective organism. But unlike most hydrozoans its polyps do not have feeding tentacles but instead sticky polyps that catch microscopic food and slowly transfer them to its ‘mouth’. Some particularly cunning crabs have taken advantage of this by covering their front legs in H. epigorgia to create a pair of sticky fishing rods.

fishing crab
This spindly crab has its legs covered in sticky hyrdozoans

Creative crustaceans

Given all this information about decorator crabs and the many different animals accessorizes they use, as well as the unique and creative ways they utilize them, it has become quite clear that this is much more than just a fun gimic. Decorating is a highly selective, complex and varied behaviour that shows us there is a lot more to these crustaceans than first meets the eye. Resigned to the seafloor spending their days walking sideways from rock to rock it is easy to assume that there is not a lot going on with these simple invertebrates. However I would argue that they are actually a lot more advanced and intelligent than we perhaps give them credit for.


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