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Biology

Marine Halloween: Crabs Dress Up To Trick-or-Treat

Halloween brings to mind the scary, the spooky, the ghoulish and the creepy, but this year, let’s look at another key Halloween tradition:  dressing up.  On All Hallows’ Eve, people dress up in costumes and knock on neighbours’ doors with a threat: give me candy or suffer the consequences of a prank.  In the ocean, some animals ‘dress up’ to trick their predators or go unnoticed by their treats (prey).  There are countless examples of marine life with built-in camouflage, but these creeping crabs rely on outfitting themselves with materials and even living animals to get into the Halloween spirit year round.

Pom Pom Crab or Boxer Crabs

Quarter-sized crabs in the genus Lybia dress up as cheerleaders.  They lack big powerful claws to defend themselves, but they have tiny pincers for something else—carrying around living, stinging pom poms, hence their name pom pom crabs (aka boxer crabs).  The ‘pom poms’ are actually sea anemones that harbour stinging cells called cnidocytes that pack a powerful one-two punch.  Cnidocytes consist of a tightly coiled thread attached to a stinging barb that shoots out when triggered, and injects venom into predator or prey.  By waving their anemones from side to side in their Indo-Pacific homeland, pom pom crabs are cheerleaders from hell to potential predators.  While it is still a mystery as to how these crabs get their pom poms in the first place, anemones are not trivial accessories.  Pom pom crabs have yet to be found without their trusty sidekicks and a recent study in the lab observed that a pom pom-less crab will fight other pom pom crabs in an attempt to steal an anemone.  Spookier still, if a pom pom crab only possesses a single anemone or a single fragment of one, pom pom crabs can split what it has in two pieces.  The anemones do not die and come back as ghosts.  Given time, each freshly ripped anemone piece will regenerate completely.  While this sounds like a haunted life for the anemones, they benefit by dining off the crab’s scraps and traveling the seas in claw class—a huge benefit considering anemones are usually sedentary.

 

Carrier crabs

What are the most important characteristics of a good gladiator costume?  A shield of course!  As their name suggests, carrier crabs or porter crabs in the family Homolidae carry things.  In fact, their hindmost pair of legs are specially modified for this purpose.  Compared to their other walking legs, the last pair are smaller, and point up in such a way allows them to hoist up unwilling animals such as spiky sea urchins.  By using other animals as a shield, they suit themselves up for best chances in the gladiator ring of life or death: the ocean.  If sea urchins are in short supply, they can also ‘wear’ jellyfish, sea slugs and less delectable treats to put off predators from eating its more tasty crustacean self.  For the ultimate disappearing trick, carrier crabs can burrow into the sand with their hostages exposed which would be bone-chilling, if urchins, jellyfish or sea slugs had bones.  

 

Decorator crabs

Some species can disappear optically but species of crab in the Majoidea family have a different strategy: costuming.  Little hooked bristles on their shells, much like Velcro allow decorator crabs to stick things onto itself.  It takes a lot of work though.  First, they snip materials such as seaweed down to size, then carefully wedge scavenged flair into their bristles–read more about when they start dressing up here.  They even accessorize with living organisms such as anemones and sponges!  The end result?  They’re dressed to the nines in a perfect replica of their environment, or rather, they become their environment.  What’s a better costume than that?  In order to grow, crabs molt their shells along with their adornments, leaving them horrifyingly exposed.  But that doesn’t mean their hard costuming work has gone to waste.  If they’ve survived until now, their outfit must be working, so they pick off materials from their old, molted shell and reuse them on their new one.  

 

Halloween Moon Crab (Credit: Bhny)

And while this crab is more coastal in nature than strictly marine, check out the Halloween moon crab (Gecarcinus quadratus).  This land crab lives in sand dunes and mangroves and has evolved a natural, festival, Halloween colouration.  Pretty scary, eh?

 

Want more traditional Halloween-themed ocean animal reading?  Check out previous posts on oceanbites featuring Creepy ocean critters and Creepiest Looking Creatures.

Megan Chen
I graduated with a Masters of Coastal & Marine Management from the University of Akureyri in Iceland, and am currently working at the Smithsonian Institution’s National Museum of Natural History in Ocean Education. I am interested in smart and feasible ocean solutions, especially in fisheries management, and the incredible adaptations marine life has come up with. In my spare time, I like to stargaze, watch talks on random topics and explore different corners of the world.

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