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Fiddler Crabs on Phuket Shores, Thailand

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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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Fine Art Landscape and Wildlife Prints

If you enjoyed this article why not check out my other blogs. I write about travel, the environment, marine biology, diving and wildlife. You may also want to check out my photographic prints. These can be viewed on my main site www.colinmunrophotography.com and include landscapes from around the World, people of the World, marine life and other wildlife. The prints are only available through my website, each one individual processed and made to order. This includes fine art giclee prints, limited edition prints and canvas wraps. I use only the best, carefully selected print houses employing the finest papers and printing processes to ensure image fidelity and longevity.

Some of the framed canvas wrap prints on my website. You can also check out my fine art prints and posters.
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Land hermit crabs Coenobita rugosus in Phuket, Thailand

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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.

Fine Art Landscape and Wildlife Prints

If you enjoyed this article why not check out my other blogs. I write about travel, the environment, marine biology, diving and wildlife. You may also want to check out my photographic prints. These can be viewed on my main site www.colinmunrophotography.com and include landscapes from around the World, people of the World, marine life and other wildlife. The prints are only available through my website, each one individual processed and made to order. This includes fine art giclee prints, limited edition prints and canvas wraps. I use only the best, carefully selected print houses employing the finest papers and printing processes to ensure image fidelity and longevity.

Some of the framed canvas wrap prints on my website. You can also check out my fine art prints and posters.

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.

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Needlefish around Thailand’s Andaman Coast

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crocodile needlefish patrol the shallows

I often see needlefish when snorkelling around the Andaman Sea shores of Western Phuket and Peninsular Thailand. Needlefish, or long toms as they are commonly known in Australia, are predators of (mostly) small fish; They inhabit surface waters where their long, slender form and silvery scales render them almost invisible until quite close.  I generally encounter them hanging around the periphery of reefs, or cruising around piers, just beneath the surface, and sometimes in very shallow water, surfing in on small waves to where the water is only a few inches deep. They patrol these areas in small groups of up to a dozen or so, moving like hunting dogs, looking to sneak close enough to an unwary sardine or silverside to pounce. Usually they will suddenly launch themselves forward at speed into the school of small fish, hoping to grab one as they scatter. Off heron Island, on Australia’s Great Barrier Reef, they have been observed to launch aerial attacks; leaping clear of the water, to land amongst the unsuspecting school of bait fish. I’ve never personally observed this and as far as I know it’s never been recorded around Thailand.

A crocodile needlfish patrols the edge of a pier, looking for unwary sardines to form its next meal.

Needlefish have a reputation for being dangerous to swimmers and fishermen. While far from aggressive (they’re actually quite timid and tricky to approach close) there have been a number of injuries and indeed some fatalities caused by needlefish. The problem is that when needlefish feel threatened, they will often leap out of the water; long, low leaps, travelling at speed and covering some distance. I’ve seen speeds of 40 mph through the air mentioned, though I suspect this is little more than a guesstimate.  With their long, thin needle-like jaws it’s easy to see how being unlucky enough to be stuck by one could cause serious injury. In November 2020, a Hawaiian man out sea canoeing found himself in the middle of 30-40 needlefish leaping out of the ocean. Several stuck his canoe, one puncturing straight through the fibreglass canoe hull. In Nha Trang, Vietnam, in 2014, a Russian tourist was swimming when she felt a sharp pain in her neck. She was rushed to hospital suffering partial paralysis.  Emergency surgery removed fragments of needlefish jaw, and teeth, from around her spinal cord.  Fortunately she made a full recovery.  Though incidents can be very nasty they are also, thankfully, very rare. Fatalities are extremely rare, but do happen. In 2018 a young Thai cadet training with Thai navy special forces died after hit him in the neck during a military training exercise.  

There are probably more incidents that go unrecorded; local artisanal fishermen are generally at much greater risk due to the amount of time they spend in the water or in small boats, especially at night. Lights, often used to attract fish, are known to attract or create panic in needlefish, causing them to leap out of the water. In Papua New Guinea, for example, where fishing at night from small wooden outrigger canoes is a very common occurrence, numerous injuries are recorded every year.  The use of lanterns to attract fish appears to increase the risk. (You can read more about the outrigger canoes of Papua New Guinea in my blog here)

Should you be unfortunate enough to be struck by a needlefish while swimming or wading it is absolutely imperative that you get proper medical attention. Injuries to the arms or legs may seem minor, but the fish beaks can penetrate deep.  Teeth and jaw fragments will also frequently break off, remaining un-noticed within the wound.  In 2015, a passenger on a Caribbean cruise was struck on the nose by a needlefish while wading in waist-deep water. This left what appeared to be only a small wound that healed quickly. Three months later, after persistent nasal problems, a 39mm long fragment of needlefish beak was removed from the man’s sinus, the tip only 5mm away from the left frontal lobe of his brain.  In 2013 a Japanese swimmer received treatment after being struck in the lower eyelid by a needlefish, the wound was cleaned and all fragments thought to be removed. Swelling persisted and a subsequent CT scan found two 25mm long fragments above his eye, which were then surgically removed.  There is also a high risk of infection from such wounds, so treatment with broad spectrum antibiotics is normally recommended after any injury.

This may all make the seas sound a very dangerous place, but it’s worth keeping things in perspective. These events are rare.  Lightning strikes kill thousands worldwide every year. Tens of millions go swimming or fishing in tropical waters every year and suffer nothing worse than mild sunburn. Maybe don’t go fishing at night in a small canoe, carrying a spotlight.

I took these photographs of crocodile needlefish (Tylosurus crocodylus) snorkelling off a beach just north of Phuket Island. I had been hoping to photograph the large schools of sardines that were hiding underneath the long concrete pier there. Unfortunately a plankton bloom coupled with windy conditions and a strong swell made that pretty much a non-starter.  Visibility of less than three metres was not going to produce great images of large schools of fish. In addition, the low visibility and significant swell did not fill me with enthusiasm for diving down under the pier, the legs of which were covered in a thick blanket of razor-sharp oyster shells. I’d learned to my cost than a bump against those was not a pleasant experience. However, gazing down from the top of the pier I could see number of needlefish patrolling the edges and surfing in on breaking waves. So despite the unpromising sea conditions, and the fact that my camera was set up with a very wide-angle lens – not ideal for getting close to timid fish – I decided I was there now and so might as well get in the water.

The first picture was taken along the edge of the pier. I wanted to catch a needlefish in the light, but have the dark shadows under the pier behind it. This sounded straightforward, but, in reality, involved a lot of slow dancing around the edge of the pier, trying to get close, with the sun at the right angle whilst avoiding getting washed against the pier legs. I would carefully line up a shot only to have the fish disappear into the green haze. Eventually I got one that was acceptable.

A couple of crocodile needlefish cruise past me in shallow water.

The second shot was taken close in to the shore, in less than a metre of water.  Here in the shallows the light levels were high, but plankton combined churned up sand to reduce visibility further still.  Half a dozen needlefish continually moved in to the shore, then turned and circled back out, moving in and out of my vision as I bobbed in the waves. I switched everything to manual, including focus given the turbidity of the water, and concentrated on firing off shots as we, fish and I, warily waltzed around each other. After around 30 minutes I decided it was time to get out. The wind was increasing, and so were the waves. If I hadn’t got useable shots by then it wasn’t going to happen.

Fine Art Landscape and Wildlife Prints

If you enjoyed this article why not check out my other blogs. I write about travel, the environment, marine biology, diving and wildlife. You may also want to check out my photographic prints. These can be viewed on my main site www.colinmunrophotography.com and include landscapes from around the World, people of the World, marine life and other wildlife. The prints are only available through my website, each one individual processed and made to order. This includes fine art giclee prints, limited edition prints and canvas wraps. I use only the best, carefully selected print houses employing the finest papers and printing processes to ensure image fidelity and longevity.

Some of the framed canvas wrap prints on my website. You can also check out my fine art prints and posters.
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Pig-tailed Macaques of Thailand

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Pig-tailed Macaques of Thailand
A northern pig-tailed macaque, Macaca leonina, thoughtfully inspects the back of its hand while grooming its fur.
A northern pig-tailed macaque thoughtfully inspects the back of its hand while grooming its fur.

Caught in shafts of light on the edge of the forest, a pig-tailed macaque is wholly absorbed as it inspects the fur on the back of its hand. The behaviour and pose are strikingly human, and reminded me of how much we have in common with our primate cousins.

This is an adult male northern pig-tailed macaque. Until quite recently pig-tailed macaques were considered one species, Macaca nemestrina, with the northern pig-tailed classed as a sub-species. However studies conducted in the early 2000s (e.g. Gippoliti, 2001) looking more closely at anatomical and behavioural differences, determined that the differences were so significant that they should be considered two distinct species. The southern pig-tailed macaque retained the original scientific name (M. nemestrina) while the northern pig-tailed macaque (previously the sub-species M. nemestrina leonina) was elevated to full species level and given the new scientific name Macaca leonina. However this change remained scientifically controversial until quite recently, and only in the past few years has it become fully accepted. The most recent (at time of writing) research on speciation of pig-tailed macaques in South East Asia, using analysis of differences in mitochondrial DNA proteins to determine molecular clock timelines, suggests that northern pig-tailed macaques separated from their southern brethren around 1.7 million years ago, and from the Siberut macaque of Western Indonesia only just over one million years ago (Abdul-Latiff and Md-Zain, 2021). Both northern and southern pig-tailed macaques occur in Thailand, with the boundary between ranges of the two species believed to lie in the Krabi region of Southern Thailand. I photographed this guy on the island of Phuket, which is very close to this boundary. The lighter colour fur, white flashes above the eyes and a thin red line running from the corner of the eye towards the ear, mark him out as distinctly a northern pig-tailed.

A northern pig-tailed macaque (Macaca leonina) looks straight ahead as it emerges from the shadows of the forest.
A northern pig-tailed macaque caught in sunlight with background in deep shadow. It can clearly be identified as a northern pig-tailed by the conspicuous white flashes above its eyes and the deep red lines running from the corner of each eye towards the ears. Phuket, Thailand.

Pig-tailed macaques are denizens of lowland and hill rainforest through much of South-East Asia. However in many locations their natural habitat is disappearing rapidly as rainforest are cleared to make way for palm oil and rubber plantations, rice paddy fields and urban development. Because of this northern pig-tailed macaques are classed as a vulnerable species by the IUCN. They are primarily fruit eating, but will also take leaves, birds eggs, insects and caterpillars, and are not averse to raiding palm oil and fruit tree plantations. Along with their relatives the crab-eating macaques (Macaca fascicularis) pig-tailed macaques have learned to live alongside humans. In Phuket the inhabit the forests and mangroves (in the case of crab-eating macaques) fringing towns and villages, especially along the eastern shores, and will often congregate in tourist areas drawn to handouts of bananas. The habituation to humans brings numerous problems. The monkeys develop a taste for the easy pickings of tourist handouts and the risks of infection and injury due to bites or scratches from fearless monkeys is significant. But for me it is the similarities between us that are the most fascinating; the strong social bonding, the way a youngster will play with a leaf or a discarded plastic drink bottle just like a small child, or the way an adult will stare at the back of his hand as if in deep introspection, and for all we know maybe he is.

References

Gippoliti S. 2001. on the taxonomy of Macaca nemestrina
leonina Blyth, 1863 (Primates: Cercopithecidae). Hystrix It J Mamm 12: 51–54. doi:10.4404/hystrix-12.1-4171.

Muhammad Abu Bakar Abdul-Latiff, Badrul Munir Md-Zain. 2021. Taxonomy, Evolutionary and Dispersal Events of Pig-Tailed Macaque, Macaca nemestrina (Linnaeus, 1766) in Southeast Asia with Description of a New Subspecies, Macaca nemestrina perakensis in Malaysia. Zool Stud. 2021; 60: e50. Published online 2021 Oct 8. doi: 10.6620/ZS.2021.60-50PMCID: PMC8685347

About these images

I took these shots late afternoon, on the edge of some forest on the eastern side of Phuket Island, Southern Thailand. The macaque had just climbed down from a tree and was perfectly lit by the low-angled sun, while the forest behind was in deep shade. The images were taken with a Nikon full-frame DSLR; the full size image is around 80 megapixels.

Fine Art Landscape and Wildlife Prints

If you enjoyed this story why not check out my other blogs. I write about travel, the environment, marine biology, diving and wildlife. You may also want to check out my photographic prints. These can be viewed on my main site www.colinmunrophotography.com and include landscapes from around the World, people of the World, marine life and other wildlife. The prints are only available through my website, each one individually processed and made to order. This includes fine art giclee prints, limited edition prints and canvas wraps. I use only the best, carefully selected print houses employing the finest papers and printing processes to ensure image fidelity and longevity.

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