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Old Man of the Forest, Bornean Orangutan

Old Man of the Forest, Bornean Orangutan

The name orangutan is believed to come from two Malay words, ‘ura? hutan‘ meaning ‘forest people’ or ‘forest men’ (Wayan Jarrah Satrawan) . Apart from humans, they are the only species of great ape (or Hominid, to use the scientific term) found outside of Africa. Three species of orangutan are currently recognised, the Bornean orangutan, the Sumatran orangutan, and the relatively recently described Tapanuli orangutan.

Bugang, a large male Bornean Orangutan, Pongo pygmaeus, Samboja Lodge, East Kalimantan, Borneo. This older orangutan cannot be released to the wild, so now lives in a natural environment on an artificial island within the lodge grounds. Image Colin Munro Photography.
Bugang, a large male Bornean Orangutan, Pongo pygmaeus, Samboja Lodge, East Kalimantan, Borneo. Samboja rehabilitates younger orangutans to the wild. At over 30 years old, Bugang is too old to learn to survive on his own. A fine art print of this image can be seen on my website.

Nowadays, orangutans are found only on the islands of Borneo and Sumatra. That was not always so; at the beginning of the Pleistocene (the geological epoch that started around around 2.6 million years ago) a number of orangutan species were found all the way from central China, through what is now Vietnam, Cambodia, Laos and Malaysia, across east to Sumatra, Java and Borneo. But by the end of the Pleistocene, around 12,000 years ago, some species had become extinct and range of those that remained had shrunk to the two islands. We don’t know for sure exactly why this happened; there are currently two popular theories. We know that during the Pleistocene (often called the Ice Age) the climate cooled, and large areas that had been tropical forest then became more open savannah that did not favour the slow moving, fruit-eating orangutans. Tropical forests survived on Borneo and Sumatra, providing orangutans with a refuge. Sea level fell perhaps 120 metres, thus the Sunda Shelf seabed between Southeast Asia and the islands of Borneo, Sumatra and Java became dry land. As the land warmed, and tropical forest returned to parts of their original range, so sea level once again rose. Borneo and Sumatra became islands, trapping the remaining orangutans. The second theory is that hunting by man, and in particular the development of bone tools such as bone spear and arrow tips. The sharp decline towards the end of the Pleistocene appears similar to the rapid extinction of other species following the arrival of modern man (e.g. the giant lemurs of Madagascar). Almost certainly, both factors played a role. The relative importance and timing of each is still a matter of debate.

The Bornean orangutan, Pongo pygmaeus, is the most abundant of the three species. Despite that it is still considered critically endangered, with only around 100,000 remaining in the wild. Habitat loss, through logging and burning forests, clearance for palm oil plantations and for agriculture, are major factors in this. Individuals being killed through conflict with humans, increasing as natural habitat is lost and orangutans may damage farmer’s crops, is also a major factor. The planned new capital city of Indonesia, Nusantara, currently being built in East Kalimantan, Borneo, is planned as an environmentally friendly city with major reforestation projects being part of the project. However, such a major urban development, with road and infrastructure and perhaps hundreds of thousands more people to feed, also has great risks for the orangutans in the region.

At over 33 years old Bugang (meaning ‘The Batchelor’) is old. Typically wild orangutans will life for 35-40 years. Mature males like Bugang weigh around 75kg and possess large, fleshy cheekpads (or flanges). In Bornean orangutans these have a distinctive forwards curve to them. Bugang lives on one of the artificial islands created at Samboja Lodge, within Samboja Lestari tropical Forest Restoration and Orangutan Rehabilitation Programe, East Kalimantan, Borneo. This project, Bugang is too old to be successfully released to the wild, so lives on one of the specially created islands in a natural environment. The lodge and programes are run by the Borneo Orangutan Survival Fountation (BOS). BOS is a global organisation dedicated to the protection and restoration of tropical rainforest and the survival of orangutans. I visited Samboja, and being very impressed with the work they do there, both in restoration and education of the public. As this image of Bugang would not be possible without the work they do there, ten percent of profit from any sales of this print will be donated to BOS.

Further reading

Stephanie N. Spehar et al., Orangutans venture out of the rainforest and into the Anthropocene.Sci. Adv.4, e1701422(2018).DOI:10.1126/sciadv.1701422

Sastrawan, Wayan. (2020). The Word ‘Orangutan’: Old Malay Origin or European Concoction?. Bijdragen tot de taal-, land- en volkenkunde / Journal of the Humanities and Social Sciences of Southeast Asia. 176. 532-541. 10.1163/22134379-bja10016. Link

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Rantepao buffalo market, Tana Toraja, Sulawesi

Rantepao buffalo market, Tana Toraja, Sulawesi
A young Sulawesi lad shovels buffalo manure, Rantepao Buffalo Market (Bolu Market) Tana Toraja, Sulawesi. Indonesia. Buffalo. Photograph by Colin Munro. Copyright Colin Munro Photography
A young Sulawesi lad shovels buffalo manure, Rantepao Buffalo Market (Bolu Market) Tana Toraja, Sulawesi.

In preparation for an upcoming trip to Borneo and Sulawesi I’ve started going through some of my old images from previous visits. I took this photograph a little over ten years ago, in the Tana Toraja region on the island of Sulawesi, Indonesia. This is Rantepao buffalo market, known as Bolu market. In years gone by, Tana Toraja highlands were known for their trade in slaves, guns and coffee. Fortunately nowadays only the coffee trade remains. The Toraja people are known for their cave cemeteries high on cliff faces. Funerals of wealthy individuals involve the slaughter of numerous buffalo and pigs, buffalo considered essential to assist in the journey to the afterlife. A small buffalo may sell for around 5 million Indonesian Rupiah (~300USD). As in other parts of SE Asia the rarer white buffalo are much prized and will fetch many times this sum. This young lad will probably be in his late teens now. On that day he did not have the best of jobs, shoveling buffalo dung while wearing flipflops. So if you’re having a bad day today, think of him. It’s been ten years last in either Borneo or Sulawesi, it will be interesting to see how much life has changed in the intervening years.

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Fine Art Prints and ready to hang Canvas Gallery Wrap prints

So Here’s the sales pitch. If you like my stories and images you might be interested in viewing my canvas wrap prints, fine art prints, posters, all available direct from the photographer (that would be me) stock images (and more stories) on my main site: www.colinmunrophotography.com

I’ve just added this photograph to my collection of canvas gallery wrap prints for sale. Canvas wrap prints, also known as ‘gallery wrap’ are where the canvas is stretched over a wooden frame (known as stretcher bars) and wraps around. Thus the image goes all the way to the edge. They arrive ready to hang, with no additional framing required. My canvas prints are archival quality (not to be confused with cheap, mass produced canvas prints) using state of the art pigment ink technology. Tests indicate such prints are fade free for 100+ years out of direct sunlight.

The Pink Whipray Pateobatis fai

The Pink Whipray Pateobatis fai

The Pink Whipray aka Tahitian Stingray Pateobatis fai (formerly Himantura fai).

A pink whipray (Pateobatis fai) lies quietly in shallow water, having previously thrown sediment up into the water column, allowing it to settle on its back to provide camouflage.
A pink whipray (Pateobatis fai) lies quietly in shallow water, having previously thrown sediment up into the water column, allowing it to settle on its back to provide camouflage.

The pink whipray, aka Tahitian Stingray, is a large stingray, around a metre wing-tip to wing-tip, often found in shallow bays and lagoons. It’s preferred habitat appears to be sandy or sand and coral rubble seabeds. It occurs widely across the Indian Ocean and Western Pacific, from West Africa to Western Polynesia. Like all stingrays they do have a venomous barb near the end of their tail. The venom is not generally considered dangerous to humans, but it can be painful. Equally the barb is long and sharp, so is capable of inflicting a nasty wound. Pink whiprays are generally not at all aggressive, and so the risk is mostly if the animal feels threatened, thus chasing or cornering it is not a good idea. Sometimes they can be seen swimming with the tail raised almost vertical, possibly as a warning.

Pink whiprays feed mostly on crustaceans; crabs, prawns and shrimps; but will also take molluscs. In some locations they will gather in feeding groups in very shallow water (less than one metre). This has created a tourist attraction, especially in parts of French Polynesia.

This continues on my main website, where more information about pink whiprays, and more images, can be found. https://www.colinmunrophotography.com/pink-whipray-pateobatis-fai/

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Laos Stories

Laos Stories

I’ve recently returned from ten days in Laos, a truly fascinating and stunningly beautiful country. This is the first of what I hope will be a short series of blogs about Laos. One of the poorest in Southeast Asia, it is leapfrogging into the 21st Century in fits and starts. Laos has many things holding back its development. It has no coastline, so all trade must pass through its surrounding neighbours. The majority of the country is mountainous, still clad in dripping tropical forest, and much of the land has still to be cleared of cluster bombs and other unexploded ordinance (UXO), a deadly legacy of the USA’s ‘secret war’ between 1963 and 1974, when they dropped over 260 million bombs on Laos, making it the most bombed, per capita, country on the planet. Still, six decades later, people are killed or lose limbs every year through old UXO. Laos has some of the worst roads on the planet, but more modern high speed rail than the United States. Most transactions, from hotel booking to hiring an electric motorbike are still done in cash. With the low value of the Laos kip, the smallest banknote you can you can usefully carry is the 1000 kip note, which is worth about 5 cents (US) at the time of writing. Laos is the only country I know where I can become a multimillionaire after visiting an ATM in the morning, and have blown it all by mid-afternoon.

A farmer and her buffalo, near Luang Prabang, Central Laos.
Kam la and her buffalo

The lady pictured above is Kam la. She and her husband, Boon Tham, are farmers in Central Laos, a little outside the former capital of Luang Prabang. I passed them on my small hired motorbike as they were walking along the new highway that connects Central Laos to Northern Thailand, so I stopped to have a quick chat. Their buffalo had a sudden urge to go for a stroll early that morning. They had finally caught up with her almost at the shores of the Mekong and now faced an eight kilometre plod back home.

A small herd of domestic buffalo, Luang Parabang district, Laos.
A small herd of domestic buffalo, Luang Parabang district, Laos.

Domestic water buffalo (Bubalis bubalis), aka domestic asian buffalo, are descendents of the wild water buffalo (Bubalus arnee) native to Indian and Southeast Asia. The domestic variety are still commonly used for meat, milk and ploughing rice fields. The wild variety is now pretty rare; the few remaining populations are small, widely scattered and most are in decline. habitat fragmentation is a significant cause of decline. Like so many other wild species with domesticated relatives, the junglefowl and domestic chickens, and Scottish wildcats and domestic cats being obvious examples, they are also suffering from inbreeding with their domestic counterparts. Domestic water buffalo are generally smaller than their wild counterparts, with proportionally shorter horns. Wild water buffalo have the largest horns of any known bovid. Unfortunately, despite the large areas of wilderness, no known populations of wild water buffalo exist in Laos.

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Painting with shadows. Poor Knights Islands underwater.

Painting with shadows. Poor Knights Islands underwater.
Blue maomao Arch, Poor Knights, New Zealand. Schooling blue maomao congregate close to the walls of the underwater arch. Copyright Colin Munro Photography. www.colinmunrophotography.com
Blue Maomao Arch, Poor Knights, new Zealand. Schooling blue maomao congregate along the sheer walls of the arch

Photography is painting with light, but it’s equally the absence of light. It’s about creating a mood, an emotional response, with the balance of light and shadow; it’s using shadow to draw one’s eye to the light. Technically, this image may be too dark, it’s ceratinly not evenly lit. But that’s the point; what’s aesthetically interesting is rarely what’s technically perfect. Blue Maomao Arch, Poor Knights, new Zealand. Schooling blue maomao (Scorpis violacea) congregate along the sheer walls of the arch, feeding on the plankton rich currents that are entrained and accelerated through between the walls of this long tunnel.

This image was shot in early 2020. I used a Full frame Nikon camera, 20mm Nikon lens, in an Aquatica housing. Although I had a flash (strobe) attached to the camera I chose not to use it for this shot, rely purely in the natural light penetrating the arch.

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Yaku sika deer, Yakushima Island, Japan

With a little spare time right now I’ve begun re-organising my stock image library, sorting through older folders that were created before my current system was created. This is something of a daunting task; from 2014 alone, I have around 4 terabytes of images. Still, it refreshes my memory of many places that have become a little hazy in my mind.

yaku sika deer, Cervus nippon yakushimae, Yakushima Island, Japan. Colin Munro Photography. www.colinmunrophotography.com
A young yaku sika deer, stag Cervus nippon yakushimae, resting in a forest glade, Yakushima Island, Japan.

Occasionally I come across images I’d almost forgotten about. This is a yaku sika deer, in a forest on Yakushima Island. Yakushima, Kagoshima Prefecture, is a small, mountainous island, less than 30km across, in Southern Japan. It forms part of the Ryukyu (Nansei) island chain that separates the East China Sea from the open Pacific proper. It’s known for the beautiful cedar forests that cover much of the island. Yaku sika (Cervus nippon yakushimae) are a subspecies of Sika deer endemic to Yakushima Island. They are relatively unafraid of humans, they haven’t been hunted on the island for over 50 years, allowing me to get fairly close to this young stag resting in dappled sunlight in a forest glade.

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Why do fish have stripes?

Why do fish have stripes?
Five-lined snapper (Lutjanus quinquelineatus) aggregate under an overhanging wall, Similan Islands, Andaman Sea.

Five-lined snapper (Lutjanus quinquelineatus) aggregate under an overhanging wall, Similan Islands, Andaman Sea. There are a number of closely related, blue lined snapper, that occur, mostly on coral reefs, in the tropical and subtropical Indo-Pacific. The common bluestripe snapper (Lutjanus kasmira) and the Bengal snapper (Lutjanus bengalensis) all have similar patterns and overlapping ranges, including the Andaman Sea. Differentiation is not always easy, and the academic literature has at times been confusing. Modern genetic techniques are helping clarify relationships, to a degree at least. Perhaps a more important question, and one harder to answer, is why very similar species that appear to occupy similar ecological niches, and with broadly overlapping ranges, actually exist. What is the driver for speciation, genetic and reproductive isolation? This is a harder problem to investigate because studying the behaviour of highly mobile species inhabiting deeper waters on offshore reefs is difficult, time consuming and expensive. Colour patterns involving horizontal stripes is a feature that has evolved separately in many species of fish, especially schooling fish (e.g. fusiliers, snapper). Various hypothesis have been proposed as to why horizontal stripes might be advantageous. It may be difficult for a potential predator to identify individual fish in a tightly formed school, all horizontally striped, as the stripes may be more obvious than the outlines of individual fish (often called the confusion effect). It may be difficult to judge the speed of horizontally striped prey as swim past if they are swimming in the same orientation as the stripes are aligned.  Others have suggested that that stripes help co-ordinate movement in schooling fish; as the school changes direction so does the orientation of the stripes, providing a strong visual cue.  Zebrafish are used in many laboratory studies, partly due to their ease of rearing and fully sequenced genome. Working with striped and unstriped zebrafish morphs, scientists found that striped individuals showed the strongest tendency to form schools, and would preferentially form schools with individuals that most closely resembled themselves. How much of this behaviour transfers from zebrafish, fish that inhabit the shallow streams, ponds and rice paddies of South-east Asia, to large schooling fish on deep, offshore reefs. Who knows? Deeper offshore reefs don’t easily lend themselves to manipulative experiments where confounding factors can be controlled. Observational studies can be conducted in large public marine aquaria, but extrapolation to truly natural behaviour must be done with care. It is likely that much of our understanding of behaviour and interactions in such difficult habitats will continue to be opportunistic, conducted from recreational dive boats, with some coming from big-budget natural history documentary filming, such as the BBC’s Blue Planet series, where the money and expertise exists to overcome some of the major logistical problems. There is so much we don’t know about the how and why of the ecology of such reefs, and that situation is unlikely to change anytime soon.

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Photography under surf

Photography under surf

Photography and video, free-diving, in shallow surf, for me at least, presents some of the most challenging situations in photography.

A crocodile needlefish (Tylosurus crocodilus) menaces a school of sardines.

The visibility is generally terrible, rarely more than two metres and often less than one. The waves and shallow water make staying underwater difficult unless massively overweighted (which brings its own problems when you need to breathe). Waves will unexpectedly sweep you up on to oyster and barnacle covered rocks – which tend to leave you looking like you’ve had a fight with a cheese grater, or worse still, may smash the plexiglass dome port of the camera housing. Passing waves occasionally pick up and then dump a truck-load of sand, reducing visibility to zero, to be followed by a washing machine of bubbles as the wave breaks.

Waves sweep sand up of the seabed, depositing them in a rain of bubbles and glittering sand particles as they crash against the rocks.

In these conditions auto-anything becomes pretty useless. Exposure is all over the place and the water is full of all sorts of crap flying past you (or you flying past it). So manual exposure and manual focus is really the only way to obtain useable images. Underwater flash cannot be used either; with so much suspended sand in the water that would act like tens of thousands of small mirrors bouncing light back into the camera.

Miniature sand storms swirl around the rocks as the waves sweep past.

But despite all that, the rewards can be well worth it. Often, just in the shallows is where the action is. Especially close to sunset. These are schooling sardines, swirling around in around a metre to a metre and a half depth, just of the shore, just as the sun was touching the horizon. Crocodile needlefish (Tylosurus crocodilus) cruise around the edges of the school like hunting dogs, looking to pick of a straggler that gets separated from the main body, whilst small schools of small-spotted darts (Trachinotus baillonii) zip in and out picking up planktonic crustaceans and fish fry.

At sunset, sardines sweep past across submerged rocks, only centimetres below crashing waves.

Photographing in these conditions will never yield the clean, sharp, colourful images so loved by advertisers and photographers alike, but they do record a reality not often featured in underwater photography magazines or glossy brochures. They also, I hope, capture the atmospheric look of turbid, energetic waters. For me, if I get in the water and the fish are there (there are times I get in the water, buffeted by waves, but see little) and I get out of the water with no major lacerations and my camera gear still intact, then the evening was a success. If I end up with useable images, that’s a bonus.

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Fiddler Crabs on Phuket Shores, Thailand

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

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.