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Fiddler Crabs on Phuket Shores, Thailand

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.

A male fiddler crab, Gelasimus (Uca) vocans, where the much larger claw can be clearly seen. Kamala Beach, Phuket, Thailand

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?

A short, 30 second clip showing fiddler crabs feeding on Kamala Beach. It also illustrates the difference between females and males.

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.

A male fiddler crab feeding on muddy sand on tidal flats, Phuket, Thailand. Colin Munro Photography — A male fiddler crab using it smaller, more dextrous, claw to shovel sand into its mouth. its larger claw is useless for this task.

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.

A male fiddler crab wades through running water where the klong (a canal or small river) flows across the intertidal flats. Kamala, Phuket, Thailand. Colin Munro Photography — A male fiddler crab wades through running water where the klong flows across the intertidal flats.

A short clip of a male fiddler crab emerging from hiding

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.

Fiddler crabs will often send up one eye – rather like a periscope – to check for any potential threats before venturing out of hiding.

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Land hermit crabs Coenobita rugosus in Phuket, Thailand

Many years ago, my undergraduate thesis was on hermit crabs, and I’ve had a fondness for them ever since. Hermit crabs are the great survivors. They live in some of the harshest environments, pounded by surf, exposed to searing heat and desiccating winds, and somehow able to find sustenance on the most barren of beaches. Possibly the most common species seen around the shores of Phuket is the land hermit crab Coenobita rugosus.

A land hermit crab, Coenobita rugosus, walking just above the waves at high tide, on a beach in Phuket, Thailand. — A land hermit crab, *Coenobita rugosus*, walking just above the waves at high tide, on a beach in Phuket.

How many species of hermit crab are there?

There are thought to be over 800 species of hermit crab. Fifty five different species of hermit crab have been recorded from Thailand’s shores and surrounding seas (McLaughlin, 2002). This can make indentification tricky. Hermit crabs fall in to three distinct families: the Dioginedae, the Paguridae and the Coenibitidae. The coenobitids (like this guy, pictured above) are the real land hermit crabs, often venturing some distance from the sea. So if you are beach-combing in the tropics, they are the ones you are most likely to see.

How do you identify Coenobita rugosa?

In the tropics (and I’m primarily writing this about Thailand), any hermit crab found high up on the shore, at or above the high tide mark, or inland of this, is likely to be a Coenibita species. However, to complicate matters there are actually three Coenibita species found on Phuket shores ((McLaughlin, 2002; Bundhitwongrut et al., 2014) all occurring in more or less the same habitat. Tricky! These are C. rugosus, C. violascens and C. brevimanus. The best way to identify Coenibita rugosus is by looking closely at the top of the outside edge of the end joint of its left claw. A series of small, linear tubercles can be seen on C. rugosus, but not the other two species. Incidentally, it is thought that these tubercles are involved in generating sound. Land hermit crabs will ‘chirp’ by stridulating, that is rubbing body parts together to produce a sound (as cicadas and grasshoppers do).

A number of small, linear tubercles on the outside edge of the end of the left claw distinguishes *Coenobita rugosus* from the other *Coenobita* species occuring on Phuket shores.

How do land hermit crabs breathe when out of the water?

A big problem for any animal primarily designed to live in the sea is how to breathe in air. The gills used by fish and most aquatic invertebrates to adsorb oxygen underwater don’t work very well on dry land. The gills of crabs are composed of many very thin plates (lamellae) each with a central shaft rather like a feather. Oxygen is absorbed and carbon dioxide expelled across the surface area of these plates. However, when out of water, these lamellae collapse directly on top of one another, greatly reducing the available surface area for gas exchange. Most coenobitids (such as C. rugosus) have adapted by having much smaller gills, they spend little time in the water, and with these gills somewhat adapted for breathing air. Around the gill is what is known as a branchiostegal lung. This is essentially gill tissue that is more adapted to absorb oxygen from air rather than water. Branchiostegal lungs are not unique to hermit crabs, other crabs that spend significant amounts of time out of the water, such as fiddler crabs, also possess them. Such are the wonders of evolution, each group has developed a different design of branchiostegal lung; there are many different ways to acheive the same end it seems. The coenobitid hermit crabs have also developed a second method for gas exchange. Protected inside their mollusc shell home, the upper surface of their thin-skinned abdomen has become highly vascularised, with gas exchange taking place directly through the skin. To further improve this the skin has become deeply wrinkled, increasing surface area. This has been termed an abdominal lung.

What do land hermit crabs eat?

What do they eat? Pretty much anything. They consume quite a lot of plant material, including fallen leaves such as acacia and beach cordia. They are also partial to a bit of carrion, crabs or fish washed up. Cannibalism too, is on the table if the opportunity presents. Smaller C. rugosus hermits need to be wary of getting too close to their bigger brothers.

Senses

The current evidence suggest that Coenobitid hermit crabs primarily use their sense of smell for food detection. The studies also suggest that their ability to detect smell was limited to water soluble molecules, and greatly enhanced in humid conditions. Here in Phuket a lack of humidity is rarely a problem!

The stalked eyes of Coenobita rugosus. Their elongated shape can be clearly seen. The dark, elongated shape of the pseudopupil can also be seen.

Many hermit crabs also have pretty good eyesight. This is probably more related to defense and avoiding predators than feeding. Crabs have compound eyes; each eye is actually made up of a great number of hexagonal light sensors, each with their individual lens and cornea. These individual light sensors are known as ommatidia. Like fiddler crabs and ghost crabs, Coenobita rugosus and its relatives have eyes on stalks and eyes that are rather longer than they are wide. This vertical stretching of the eyes means that the angular change between each individual ommatidia is less in a vertical plane than in a horizontal, thus they have better optical resolution for objects, and changes in light, vertically than horizontally. This is probably an adaptation for predator detection; most predators likely to be bigger and casting a shadow or darker shape from above. If you look carefully at the eyes you will notice what appears to be a dark stripe near the centre of each eye. This is known as the pseudopupil. It represents the ommatidia that are more or less pointing straight towards you, and thus no light is being reflected back. As explained above, the angular change between ommatidia is less vertically than horizontally, so the pseudopupil has the shape of a vertical line. In humans, and other vertebrates, the pupil contracts or expands as light levels change, allowing more or less light into the eye. But crab eyes don’t have real pupils; this begs the question, how do they deal with changing light levels? The answer, in fiddler crabs at least, has been discovered only recently. When light travels through the lens of an ommatidiait is focussed on a column of dense, tiny finger-like projections (microvilli) containing photosensitive molecules. For crabs that are active both in bright sunlight and around dusk, these microvilli change size, growing or shrinking, and so dramatically altering the amount of light they capture.

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Selected references.

Farrelly, C. A., and Greenaway, P., 2005. The morphology and vasculature of the respiratory organs of terrestrial hermit crabs (Coenobita and Birgus): gills, branchiostegal lungs and abdominal lungs. Arthropod structure and development, 34.

McLaughlin, P. A., 2002. A review of the hermit-crab (Decapoda: Anomura: Paguridae) fauna of Southern Thailand, with particular emphasis on the Andaman Sea, and descriptions of three new species. Phuket Marine Biological Center Special Publication 23(2): 385–460.

Bundhitwongrut, T., Thirakhupt, K. and Pradatsundarasar, A., 2014. Population ecology of the land hermit crab Coenobita rugosus (Anomura, Coenobitidae) at cape Panwa, Phuket Island, Andaman Coast of Thailand.