The Tetragonal Fiddler Crab (Gelasimus tetragonon) lives in the tidal zones of mangrove forests and muddy marches of the Indo-West Pacific and the East coasts of Africa. These burrow-digging crabs filter out detritus from the sand and mud during low tide.
Males of these semi-terrestrial crabs exhibit an extreme form of sexual dimorphism, which is defined as a condition where sexes of the same species exhibit different morphological characteristics. These crabs have one claw that is disproportionately larger than the other, and this claw is called a “chela”.
The smaller claw is used to scoop up a portion of sediment from the substrate, which is then brought to the mouth and sifted through. After anything edible is retrieved, the sediment is replaced in the form of a little ball, as seen in this videoclip.
The oversized claw or chela is used to fight other males over mating and territorial rights. By waving it around, this large claw is also used to attract a female. Female fiddler crabs choose their mate based on claw size and the quality of the waving display. An impressive and vigorous display of claw size and waving performance demands a male in prime health condition, suggesting that the male will help produce viable offspring.
Like all crustaceans, fiddler crabs undergo several moults in their lifetime as they shed their shells and grow. If a male loses his larger claw, for example, in a fight with another male, the smaller claw on the opposite side will begin to grow larger, and the lost claw will regenerate into a new normal-sized claw. The regenerated claw remains small, while the previous small one grows into an oversized claw over a period of several moults. The male fiddler now has a new chela or big claw on the opposite side.
Ironically, the claws that are best for fighting other males do not match up with the claws best suited for waving. So to impress a female, one must be a graceful waver, and if he’s not, then he must be able to fend off other graceful wavers. This trade-off is an evolutionary enigma.
After mating, the female fiddler crab carries her eggs in a mass on her abdomen. She retreats into the comfort of her burrow and patiently carries the precious offspring for two long weeks, sheltered from the outside world. Once the gestation period is over, she emerges from her burrow and braves the vast expanse of the open ocean, releasing her eggs into the receding tide. The tiny larvae, still in their planktonic stage, will roam the vast ocean for another two weeks before finding their new home on another intertidal zone, mudflat or mangrove forest.
The extremely wary crabs duck back into their burrows at the slightest detection of movement. To film their natural behaviour, we observed a single individual as it roamed around its burrow. Once we had an idea of how this specimen would behave, we positioned our camera in front of its anticipated path. The camera was then controlled remotely using a dedicated app on a smartphone. It took some time before this terrestrial crab gathered enough courage to venture out of its burrow.
For another in-depth description about Fiddler Crabs please go to our vlog post 172 or click on this link: https://www.beyondscuba.com/post/an-orchestra-of-east-african-fiddler-crabs-cranuca-occidentalis
and vlog post 39 or click on https://www.beyondscuba.com/post/waving-fiddler-crab
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