What are fiddler crabs?
Fiddler crabs belong to the Ocypididae family of crabs. These are the stalk-eyed crabs mostly found in the intertidal shores of tropical and temperate seas. They get their name from the males having one claw much larger than the other, which they often wave in the air. This waving of the large claw, and the relatively tiny other claw, makes them look rather like they are playing a violin or fiddle, think maybe the fiddle player in a Galway pub, after 12 pints of Guiness (other stouts are available). Why do they wave their big claws in the air? Well I’ll get to that in a bit.
Where are fiddler crabs found?
Fiddler crabs feed by eating sand and mud, and sifting out the organic material. Thus they tend to be found on the margins of mangrove forests and along muddy estuaries and on beaches where rivers enter the sea. Anywhere on the intertidal where there is significant input of organic material. On Kamala Beach, in Phuket, Thailand, where I took these images, fiddler crabs are mostly found at the southern end of the beach. This is because the local klong (the thai word for a canal, or sometimes a small river) flows into the sea here. The klong is rich in organic material, mostly leaf litter and organic material leeched out of the surrounding earth, particularly so during the rainy season, when the klong runs red with suspended soil. When the flowing water of the klong collides with seawater, two forces come in to play. Firstly, the speed of the flow will decline, causing tiny organic particles to slow down, and then sink and settle. The second factor that comes in to play is that salt (in seawater) causes organic material to clump together into larger lumps (scientists love fancy terms, so this is termed flocculation). This flocculated organic material will also then tend to settle out faster. But I’m digressing here somewhat. The end result is that lots of organic material tends to settle out where rivers (or in this case, a klong) hit the sea, producing over time a fine muddy sand that is extremely rich in organic material. In Kamala Beach this contrasts sharply with the northern end, where there is little organic input. The northern end is also more exposed to the south-westerly monsoon winds of the rainy season. This drives more energy on the beach, creating bigger waves, and so washing away any finer organic material deposited on the beach and leaving only larger sand grains and shell fragments. This produces a beautiful white sand loved by tourists, but it’s relatively sterile, so not loved by fiddler crabs. This also helps explains why surfing happens mostly at the northern end of the beach and why local fishermen chose to anchor their boats at the southern end.
Fiddler crabs foraging. What do they eat?
As you can see in the video, the small claws are used to scoop up handfuls (er ..clawfuls) of wet sediment into the crab’s mouth. Here, jaw appendages called maxillipeds sort and retain organic material: diatoms, bacteria and other microscopic organisms. The maxillipeds are covered in bristles (called setae) with spoon-shaped ends. These separate food particles from the inorganic sediment. Now here the females have a huge advantage; they can use both claws to scoop sediment into their mouths (and they do so with the enthusiasm of a toddler given chocolate). The males however, can only use they’re smaller claw. The larger one is pretty much useless for anything other than .. well, waving around really.
So this brings us back to the perplexing question. Why do males have this one larger claw when it is clearly such an impediment to feeding? The answer is two-fold; to attract females to mate, and to fight off other males.
Mating is a complex and tricky issue among fiddler crabs. Males have been found to employ a variety of techniques to try and persuade females to mate. These have been termed ‘gambits’ by scientists studying them. They range from the distinctly romantic ‘standard gambit’, where the male suitor will unseal his burrow early, as the tide recedes, and position himself outside a female’s burrow, carrying a bunch of flowers (okay I made that last bit up). When the female emerges he will engage in some gentle stroking of her shell, before grabbing hold of her and turning her around. If the female decides she likes him she will allow him to position her. If she decides he’s going too far on a first date she will scuttle back down into her burrow.
With their big, stalked eyes, fiddler crabs have pretty good, 360 degree vision. They will often use these rather like periscopes, sending up one or two eyes above the sand or water surface to recce the terrain before venturing out of hiding. I’ve written more about the crab stalked eyes in my blog about hermit crabs, you can read it here.
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