One of the favorite critters I love to watch during night dives are decorator crabs. They come in so many different beautiful and funny colors, shapes and decorations. Many times I have wondered how they decide which costume to wear. From funny “hats” to pink girly dresses they always seem to find a way to entertain divers. Pretty sure that was not was nature had in mind by giving then the ability to decorate their bodies with stuff from their surroundings.
Decorator crabs belong to the superfamily of spider crabs
The decorator crabs are not a single family but they belong to many different species of the family of the spider crabs (Majoidea). This is a very diverse group with around 900 species while not all of them are decorator crabs.
75% of spider crabs decorate themselves
During a phase of their lives 75% of spider crabs decorate at least part of their body. So that makes (digging out math skills) 675 spider crab species with the ability to decorate themselves.
Decorator crabs attach materials from the environment to their body
While most camouflage involves body colors and shapes the decorator crabs have mastered the camouflage by attaching other organisms or materials to their body to hide from predators.
Decorator crabs use hooks to attach the materials
To do so they are using stiff hair-like hooks called satae. The more hooks a crab has, the more it can decorate.
Decorator crabs use their mouth to get the material ready for decoration
Decorate according to the habitat or choosing the habitat according to the decoration?
Some decorator crabs have preferences in materials to decorate themselves with. For example materials which are easier to handle, to cut and to carry around. It is not entirely clear if the crabs choose their decoration according to their habitat or choose the habitat according to the decoration.
The decoration might make the crab unpleasant or toxic for predators
While the decoration is often used to camouflage in the environment to avoid being seen by predators, some crabs decorate themselves with organisms that are unpleasant or toxic for its predators. This can either be chemically noxious, like toxic seaweed, stingy like an anemone or a material that makes them taste like something other than crab.
Some crabs decorate themselves with organisms that are unpleasant or toxic for its predators https://ctt.ec/5fJ8X+ @morefundiving
Many decorator crabs are nocturnal
And will be sitting still somewhere during the day not being seen thanks to their camouflage. At sundown they start to move around to feed. This is the reason why the best time to observe them is during night dives. Not to scare them away it is recommendable to use red light while watching them.
Hultgren, K. & Stachowicz, J. (2011). “Camouflage in decorator crabs: integrating ecological, behavioural and evolutionary approaches”. In Martin Stevens & Sami Merilaita. Animal Camouflage. Cambridge University Press. ISBN 978-0-521-19911-7. (PDF)
Facts about Spider Decorator Crab (Camposcia retusa) in Encyclopedia of Life. Retrieved April 20, 2016 from http://eol.org/pages/2982793/details#distribution
Decorator Crab (2016). In Wikipedia. Retrieved April 20, 2016, from https://en.wikipedia.org/wiki/Decorator_crab