Smooth box crabs (Calappa calappa) are also known as red-spotted crabs, shame-faced crabs, and giant box crabs. They belong to the Calappidae family of crustaceans and are found in the intertidal zones of the Indo-Pacific region, including the Hawaiian Islands. The behavior of the male smooth box crab in this video clip may be portrayed as chivalrous as he sweeps his ‘wife’ in his arms and whisks her to safety. However, if you look more closely at the mating behavior of this species, you will find that his intentions are not quite so honorable. Some may even describe him as somewhat possessive!
How Do Smooth Box Crabs Mate?
Box crabs can only mate when the female is molting (shedding her old shell).
©Jesus Cobaleda/Shutterstock.com
Crabs reproduce sexually, so sperm from a male crab must fertilize an egg from a female crab to produce offspring that are genetically different from both parents. Yet, crabs are not exactly built to make mating that easy! They have a hard shell (carapace) that measures up to around six inches and stretches downwards to protect their legs. They also have large front claws. Both of these features can get in the way when they are trying to mate.
Therefore, most crabs only mate while they are molting. This is the process of shedding their hard exoskeleton so that they can grow. Crabs outgrow their shells just as we outgrow our clothing. Immediately after shedding its old shell, the crab is very vulnerable to predators as the new shell is soft at first. Females in this state are receptive to mating but also require protection—which is where an eager male enters the scene.
Protection or Imprisonment?
Female crabs that are about to molt release pheromones that attract the males. In the video clip, you can see a male rushing into a group of females and grabbing at all of them—even if some are already being held by another male. He then keeps her in his embrace, which can last several days, until she molts and he can mate with her. He transfers his sperm by inserting his gonopods—sexual organs that resemble antennae—into her two genital pores, called gonophores. Mating is thought to be a prolonged process in this species.
After mating, he guards her for a day or so longer to keep her safe until her new shell has hardened, thus ensuring the safety of his offspring. He then releases her and heads off in search of a new mate. These crabs are not monogamous; soon, he will have a new female in his clutches.
Hiding Under the Sand
Hiding in the sand is a box crab’s defense mechanism. The rear of their carapace is curved downwards, and they use this to push themselves downwards and backwards into the sand, which they are able to do remarkably quickly. Their eyes and respiratory organs are located on the top of their head, allowing them to see and breathe while hiding under the sand with their mate.
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