Playing dress-up with crabs

Some crabs enjoy attaching material to their carapaces to decorate themselves – what are the mechanisms at play?

Lianos, L., Mollemberg, M., Colavite, J., Lopes e Silva, A., Zara, F. J., & Santana, W. (2022). Much more than hooked: Setal adaptations for camouflage in Macrocoeloma trispinosum (Latreille, 1825) (Crustacea: Decapoda: Brachyura). Arthropod Structure and Development, 66.

Sporting a living hat

Several organisms employ the magic of dressing-up to survive, be it by improving their chances of catching prey or to avoid becoming one. Crabs are among them. Some have a particular way of camouflaging: decorating themselves. This way, they mimic their background, or even add organisms that aren’t enjoyable to eat like sponges or algae, to avoid being eaten themselves.

Decorator crabs are from the superfamily Majoidea. They attach to themselves any element or living organism of their environment, be it algae, sponges, invertebrates, or inorganic materials. These organisms often continue to live on top of the crab!

To be able to decorate themselves, Majoid crabs present special hooks, setae (stiff hair), distributed on their bodies, in places such as their carapace. Setae have their distal part curved. How they are distributed is linked to the behaviour of the species.

The starring model

A new study, carried out by researchers from Brazil, investigated how crabs decorated themselves, focusing on the species called Macrocoeloma trispinosum. This orange-reddish crab decorates itself with algae and sponges.

Macrocoeloma trispinosum and its hooks covering its carapace. Copyright Poupin & Corbari.

Hooked on fashion

The researchers obtained crabs from Bajo de Piedras and Boca del Río, Venezuela, and in Ubatuba, Brazil, collected from depths between 0 and 1.5 m. They then analyzed their carapaces through a microscope.
Out of the five types of setae in the study, four were described for the first time. Their names were: hooked, velvet type I, velvet type II, depressa and cattail seta. Velvet type I, type II, and the depressa form the velvet coverage on the crab carapace. M. trispinosum also showed ducts inside the setae, connected to glands made of cells arranged in a circular pattern. These glands might produce a secretion used as glue, something that would help the decorative process. The setae look from translucent to brownish under a microscope.

  • Velvet type I: they cover most of the carapace and ventral surface, and are straight and shorter than the hooked type. They are translucent, so the crab’s carapace is visible.
  • Velvet type II: these are found in the central part of the carapace, in gastric and cardiac regions. They are stronger and longer than type I.
  • Depressa: they are also in the central zone of the carapace, but on the cardiac spines and flat regions of the lateral spines. They are strong, but shorter and flatter than the other types.

Final pose

We’ve known for a while that crabs fancy dressing-up, but there weren’t many studies on how they do it. Previously, The hooked setae are the ones that crabs may use for decoration by hanging a variety of materials on them. Now we know that there are four other types of setae that may assist with costume-making, as they cover the entire crab and release a glue-like substance that helps with the fixation of materials.

Having different types of setae allows crabs to attach various materials to their bodies, which may differ by the season, latitude, and other environmental factors, even for the same species. But in the end, they all help crabs look prettier and live longer!


Cover photo by Andrei Zamiatin.

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