Interesting Life

The Coconut Crab

The coconut crab is quite an extraordinary and awesome beast. Born in the sea, it spends its ‘teenage’ and adult life on land, living in hiding and coming out at night to clean the forest floor from fallen coconuts. Like all sea creatures it breathes through gills that must be kept moist. Indeed, it needs to go back to the salt water now and then, to recharge its salt content.

Because of its marine larval stage, it is found widely spread in the tropical oceans, from Aldabaran (near Mauritius) to Easter Island in the mid-east south Pacific, typically where also the coconut tree grows. But this does not mean that the coconut crab is present everywhere in this large area. Its near-coastal habitat has been raided and converted by humans for tens of thousands of years. Now precipitated by an unsustainable explosion of people, living particularly in the coastal lands, the coconut crab, like many other living beings, is threatened with extinction although this cannot be said with certainty due to insufficient information.

Not so in Niue, it seems, where the coconut crab is still hunted for sustenance, prestige and cultural reasons. When cooked and presented on a dish, it forms an irresistible focus of a celebratory meal, an awesome spectacle. For those not familiar with this animal, a male can grow to a respectable size, its front legs spread apart to one metre, and weighing up to 4 kg. Its claws can snap a person’s finger or toe with apparent ease, and everything about it is as strong as a tank. Its flesh is appreciated as an acquired taste, and when compared to that of crayfish, prawn or lobster, a bit dry and slightly tainted with the sharp flavour that makes other hermit crab meat inedible.
Now that most Niueans (20,000) live in Auckland, the uga (pronounce as oongah) is in high demand there, reason for a profitable export. Not surprisingly, the Niuean Government has been worried about its sustainability, now that this crab becomes harder to catch and smaller in size. Uga reserves have been mooted and restraints on catching methods. Like any limited resource, the uga now needs to be managed and exploited with caution. Its population estimated at just over 150,000 seems quite generous, but this animal becomes 40-60 years old, just enough for a few successful spawnings. With sea surface currents moving away from The Rock, recruitment back onto the island is a chance affair.
The graphs on right show the crab’s size distribution (from 1-65mm thoracic length) for Niue, Vanuatu and Christmas Island. One can see how the uga of Niue is on average much smaller and the really large males have all but vanished.

The graph below shows how males become much larger than females and how they stop growing after about 35 years of age. Sexual maturity is reached at age 6 and because of recruitment problems, females should be allowed to reproduce for at least another 6 years thereafter.

The coconut crab Birgus latro
The coconut crab, also called robber crab, is an animal belonging to the phylum Arthropoda (joint-legged animals like insects and spiders), class Crustacea (like crabs and lobsters), order Decapoda (ten-legged ones), family Coenobitidae (members of a monastic community), genus Birgus (meaning unknown). One family, one genus, one species – it is a very special animal. The coconut crab is not a normal crab but a hermit crab with one large and one smaller nipper.
It is the largest land crustacean, far exceeding relatives like spiders and scorpions in weight and strength. Males and females mate on land and eggs are carried between the pleopods under the abdomen of the female which are much larger than those of the male. When they are ready to hatch, the female returns to the sea’s edge and shakes them off into the water. Here they live for several weeks as zooplankton before settling in shallow water on the shore. In the process many eggs are lost to predation and to the open ocean, which makes recruitment a haphazard affair, particularly for a small island in a wide open ocean, like Niue.
Coconut crabs never live far from the ocean, and very few are found further than 1km away. They live a nocturnal life with long periods of rest for moulting. Their main food consists of rotting coconuts which they are able to break open with their powerful claws. For that reason, they are also easy to bait and catch.

A medium-sized dark-coloured male uga for sale on the market, tethered with strings.

An orange coloured uga for sale at the market.

The main road encircling the island cuts right through uga territory and crossing it risks being crushed or caught. Fortunately for this uga, it was found by the photographer. Here the medium sized female is seen in a submissive pose.

Although this hermit crab has left the sea many years ago, it is still carrying its protective shell. The coconut crab is not the only land dwelling hermit crab.

As a male is turned upside down, curling its abdomen upward, the small male pleopods can be seen, which distinguishes it from the female which has much larger ones to hold the eggs in between.

A coconut crab is held with ease by its long front legs bent behind the large claws, but getting it there is not that easy.

http://www.fao.org/docrep/field/003/AC281E/AC281E00.htm for an FAO report on the abundance of uga in Niue. (Craig Schiller 1992)

Land crabs
Although much smaller than the coconut crab, the land crabs of Niue are amazing for very similar reasons. They too are born in the sea, making their way up the cliffs when nearing maturity. On land they are threatened by Niue’s many introduced predators, and they too need to return to the sea for releasing their eggs. But it is perhaps their early marine life that protects them from being eaten by rats, cats and dogs.
Unlike the uga, these crabs are adaptive in the way they live and what they choose to feed on. Some even exploit humans by scavenging the roads for road kill and climbing vertical walls to sleep in their attics. They are also attracted to human rubbish.

The purple land crab Geograpsus grayi is the most versatile of all because it has managed to exploit humans, living in their attics and managing to thwart cats and dogs.

Living with humans demands respect, and the male purple land crab does not lack courage to achieve this. Here a feisty one is seen in a threatening pose, not at all afraid of the photographer.

The beige land crab Geograpsus crinipes is not small when compared to a size 8 shoe, and it too is not shy of an unequal fight.

A feisty female beige land crab brandishes her claws in a defensive pose. The road is a hazard area attracting them for the road kill of insects it provides.

The long-legged land crab Discoplax longipes uses its third leg like a blind man his cane, feeling and tasting its surroundings.

Portrait of a long-legged land crab show its pretty colours and relatively small nippers.

giant sea crabs
Under water, large but not quite giant sea crabs can be found. Some fishermen know how to collect these at night and one can find them offered for sale at times. We wonder what these crabs eat and how they manage to survive, as we have not seen any young ones.

Large red edible sea crab

Large spotted edible sea crab

the fruit bat

The flying fox or fruit bat (Pteropus sp = wing-foot) is called the peka in Niue. Unlike most other bat species who live in caves by day, the fruit bat hangs from the outer limbs of tall trees. It forages in the early evening, capable of flying long distances with ease. Collectively they know where and when trees bear fruit, as they also collectively descend on these. So they compete with humans. They are also nice to eat and large enough to make it worthwhile.

In Niue the fruit bat population oscillates depending on the severity of the last hurricane which sweeps them out into the open ocean. So the life of a fruit bat on Niue is precarious with a population of about 150. Active protection is needed to give this small population a rest.

What is so amazing about this animal is not just that it is a flying mammal, and one who flies as well as the best of birds, but that it has survived in small numbers on such a small island with few fruiting tree species. By all accounts, its small population size rates it as threatened, yet it rebounds from severe set-backs such as cyclone Heta. It would be sad to see this marvellous animal disappear.

sea snakes
Sea snakes also have a land-sea connection but one that requires them to lay eggs on land while living in the sea. Males and females mate in the sea but the females return to the land to lay their eggs, preferably out of sight in some sea cave with a sea connection.

Sea mammals such as whales and dolphins break depth and time records for staying underwater while holding their breaths but sea snakes, having the advantage of being cold-blooded, can do even better. It is a marvellous sight to see them swimming lazily and unhurriedly to the surface, from depths to 3o metres, for a short spell at the surface. And then they swim equally lazily down again to continue their sleep or hunt. Like most sea creatures, sea snakes prefer living in calm waters, and one can find concentrations of them in certain preferred places like Snake Gully, two coves south of Avatele Harbour.
During our observations of the marine environment, we took note of pregnant females and baby snakes, and found that both are rather rare. The sea snakes of Niue must therefore live to a ripe old age as they also reproduce slowly.

These sea snakes are poisonous, and a bite typically results in four weeks of excruciating pain, but not in death. One would therefore think that one should treat them with equal respect. But these snakes are not aggressive, and often come really close for a good look. Some even like to be stroked and come back for more. But one needs to remain vigilant as their very short fangs are still placed forward in the mouth, able to penetrate a human skin when biting with a vengeance. However, a thin dive suit gives complete protection.

Underwater these snakes are interesting to watch, how they hunt and how little they fight. They often hunt in groups together with fish that disturb the bottom, like goatfish or trevally.

A sea snake lazily swims for the surface to take a deep breath.

Sea snakes clustering together for a mid-day snooze in a sheltered place. They always hide their heads.

giant moray eel
The giant moray eel (Gymnothorax javanicus) should not exist on Niue because it is such a large animal, requiring proportionally large prey, of which there is very little. The giant moray eel is the largest of moray eels, reaching a length of 2.5m and a weight of 40kg. It is not an animal to mess with. The photos show a giant moray living at Tepa Point in shallow water, which exceeds all expectations and may be over 100 years old and double its maximum weight . It is truly a giant amongst giants.

Its discovery is actually one of embarrassment for the photographer, who while inspecting the location of the anchor, dangled his legs unknowingly right in front of it. The buddy, not being able to alert him with the eel in her way, was extremely worried. But the eel did not strike. With its yellow-brown colour, it actually looks green under water and it lives in a hole at 5m depth, sufficiently deep to protect it during severe storms. The photos we took do not do justice to the size of this eel, a miracle to behold only for those who find it. Typically for Niue, another large giant moray but much smaller, lives almost next door, and must be of the opposite sex to be tolerated so nearby.

The eel and other giant organisms make us wonder whether gigantism (being exceptionally large) is a feature of small and remote islands. Animals who are capable of living thriftily while growing old, can become very large because, although food is scarce, there is no competition. As the waters around Niue are so clear, and life not very dense, the threat from disease is also small. So animals can grow exceptionally old and large.

A very large giant moray eel (Gymnothorax javanicus). These photos were taken with a 50mm lens which does not distort perspective.

The 8cm blackspot cleanerwrasse (Labroides pectoralis), here upside down, gives an idea of the size of this moray eel as it is as long as a hand is wide.

giant sea cucumbers
Continuing with giants, we are awed by the gigantic sea cucumbers found around Niue. These are exceptionally large for their kind and they also survive hurricanes unscathedly. Sea cucumbers are known to live from detritus which is dead plant and animal plankton raining down from above. But Niue has precious little of it, at least not enough for these large animals. We suspect therefore that they are capable of scraping the algal scum which is highly productive and nutricious

The ananas urchin (Thelenota ananas) is up to one metre long with thousands of tube feet to hold it firmly to the rock. An animal like this is many years old, which is necessary to reproduce itself.

Frontal end of the ananas sea cucumber. The thorns on its skin are not hard and sharp but bouncy, able to fend hard objects more effectively than hard armour. The ananas sea cucumber is a hardy survivor of hurricanes and can grow very old. We found very few young ones.

This species is comparable in size to the ananas cucumber but has a square profile with flat sides.

Frontal part of the square sea cucumber. Its coloration makes it almost invisible on the coral rock.

The harmonica sea cucumber (Opheodesoma australiensis) is not massive like the giants mentioned before, but thin-skinned and hollow. Sleeping by day as a small blob inside a small burrow, it comes out at night by inflating itself and becoming over one metre long.

The mouth of a harmonica cucumber has ten sticky mops with which it cleans detritus from the rock’s surface. In doing so, it keeps the rock surface free from obscuring dust, thus providing a vacuum cleaning service to its environment. This species may well differ from the one shown on left.

giant clams and mixotrophs
Giant clams (Tridacna gigas) can grow exceptionally large, as individuals measuring one metre across, have been seen elsewhere in the world. The giant clam has highly prized flesh which threatens its existence. Not surprisingly it has disappeared from many places. In Niue they are still reasonably common, but real giants may indeed be rare.

The giant clam is a filterfeeder which passes water over its gills not only to breathe but also to filter fine phytoplankton for nutrition. It supplements its food intake by cultivating plant cells (zooxanthella, symbiotic dinoflagellates) inside its sun-exposed tissues. We discovered (see the DDA chapter) that this works only when also symbiotic (together-living) decomposers are cultivated on its tissues. With these three different types of organism united in one (called mixotrophs= mixed feeders), the giant clam maintains a micro ecosystem capable of recycling its own wastes. In doing so, it lives extremely thriftily. The decomposers furthermore enable it to tap into a nutrient source (slush= incompletely decomposed organic molecules) which is abundant in the blue waters around Niue, enabling this species to grow relatively fast. real giants may indeed be rare.

The giant clam (Tridacna gigas) exposes its tasty mantle to sunlight such that its symbiotic plant cells can produce the food it needs. During the day it is very skittish towards any movement but at night it does not flinch for the dive torch.

The giant clams have beautifully coloured lips from yellow to dark brown, green to purple and even metallic blue as shown here. Fish love to nip the fleshy lips, reason why it reacts quickly to the slightest movements, but where are its eyes?.

The sun-loving corals have been known for a long time, to possess plant cells (zooxanthella) in their tissues. Now we know that they must also have symbiotic bacteria (decomposers) on their skins. This triple combination allows them to tap into the half-decomposed organic molecules of which there is far more than phytoplankton or nutrients. By decomposing the slush, the plant cells receive the nutrients they need but also carbon dioxide and hydrogen ions for faster growth. This triple combination allows sun-loving corals to grow rather fast.

Many corals look dull brownish because of the brown plant cells in their transparent bodies. This super closeup shows a star coral with its polyps recessed between its spiky skeletons.

This coral with small polyps has its gardens of plant cells in between its protruding skeleton mounds. Such coral polyps are no longer capable of catching food.

One year’s growth of an Acropora stagshorn coral is impressive.

Most of this bushy Acropora coral was grown in only one year but the smaller table corals around it are also one year old.

The very clear waters around Niue have been measured at 40-80 metres of visibility, which is extraordinarily clear. As phytoplankton (plant plankton) absorbs light for photosynthesis, it follows that very little phytoplankton is found in the deep blue sea around Niue, typically one tenth of that found around New Zealand. Because zooplankton (animal plankton) lives from phytoplankton, it too is ten times less abundant, which means that jellyfish cannot exist in these blue waters. Yet, we found a jellyfish happily swimming around but it looked rather strange. Could it too be a mixotroph, living from sunlight rather than from zooplankton? We think so. It has a strange crest which enlarges its surface area, and if one looks closely, its entire skin is brownish as if it accommodates zooxanthella. Furthermore, its tentacles are not made for catching and stinging zooplankton, but rather for absorbing more sunlightand for cultivating microbes.

A strange jellyfish is able to survive in waters with insufficient food for jellyfish. (Cephea sp.)

Tentacles ending in flat discs are unsuitable for catching prey. (Cephea sp.)

urchin-opia
Urchinopia consists of two names butted together: sea urchin and utopia (an imagined perfect place), which we thought appropriate for the Alofi side of Niue. For some reason, hurricanes strike mainly from the north to north-west. They push high waves in the direction they travel, which also become long and deep. Unobstructed by islands or currents, the waves then break on the shore, causing extreme damage to the underwater environment. They effectively lay the environment barren, creating what is known as barrens, down to 70m depth. Thus life with sensitive tissues, which includes corals, are wiped out periodically.

Hard-skinned animals suffer less so, and not at all if they can hide in the many crevices and small caves (Heta didn’t even break sea urchin spines!). An army of sea urchins is most suited to survival, as they are not only hard skinned, but also dig their neatly fitting burrows. To the occasional visitor, <em>urchinopia</em> is not immediately visible until a night dive reveals the exuberant night shift of a wide variety of sea urchins, feather stars and sea cucumbers. The sea urchin should in fact be Niue’s national symbol as it comprises the most abundant part of its sea life.

It is not just by survival strategy that the sea urchin dominates the hurricane side of the island, but also because of its feeding habit. With its five teeth it scrapes whatever is loosely attached to the rock, including young plants and coral spat. It thus prevents the recovery of corals, shaping the environment to suit itself. Where urchins graze, only a scum of fast growing algae can survive, and it so happens that these are very productive due to their small size. They may also live in symbiosis with decomposing bacteria, the subject of next chapter.

This algal scum is also the main food source for surgeonfish (tangs) and parrotfish, of which there are many individuals and species. Urchinopia is thus also tang-opia.

Niue has a large variety of sea urchins much as it has a large variety of coral fish. Some have very short spines, some have thick blunt spines, some have long needle spines, some have a combination of short and long spines, and there are undoubtedly many more.

The Alofi barrens look barren without any indication that corals might have grown here. The coral rock is formed by an encrusting coralline alga, affectionately known as pink paint.

The pink crustose coralline algae are grown over by green scum, here enthusiastically grazed by tiny grazing snails (4mm). Small scratches are made by urchins and the bigger scratches by surgeonfish. Parrotfish make much larger and deeper scratches.

A robust black urchin has left its burrow (on left) to graze the rock directly surrounding it. Competition for burrows is fierce and each sea urchin knows precisely how to return home, even from three metres away.

Rivers of coral sand are flowing from urchin burrows. Although much coral sand is produced by parrotfish, sea urchins also contribute large.

The smallest of needle urchins with the sharpest spines are the most dangerous for divers. But this variety of needle urchin has large white hollow spines to enlarge its size and these blunt spines make it diver-friendly.

A fragile long-spined needle urchin, entirely undamaged by cyclone Heta. How do they do it?

To the occasional observer, a sea urchin may not seem like an interesting animal. But we have gained respect for their enormous diversity and capabilities. The fact that they are absent during the day, coming out at night, has equipped them with an uncanny homing instinct, always finding their way back to where they belong.
We’ve seen robust black urchins spawning at night with masses of white spawn oozing out as if the whole animal contained spawn only. And one night we happened upon the short-spined or golfball urchin, shown in the photos below. Typical of Niue, there was a whole lot of them in a variety of colours. On closer inspection, these urchins seem woolly because of the thousands of tubefeet. Long ones ending in a flat suction disc for locomotion and holding, and shorter ones (pedicellaria) in between ending in a three-pronged claw for snapping at intruding organisms and for stinging predatory starfish. For the first time we witnessed an urchin with three-coloured tubefeet.

A short-spined urchin with white spines and tube feet and purple pedicellaria in between.

Super detail of the urchin on left shows long white tubefeet and fields in between with thousands of purple pedicellaria.

A short-spined urchin holding stones and a heart urchin test on its back.

Closeup of the urchin on left shows tri-colour tubefeet in green, brown and white. In between the rows of tubefeet, purple fields of thousands of pedicellaria.

During a night dive we encountered this strange looking sea urchin. In between its short green spines, it has pedicellaria looking like flowers on stalks. These can snap shut by way of defence, but they also greatly enlarge the urchin’s surface area for absorbing sunlight. This flower urchin is also called the toxic urchin (Toxopneustes pileolus) because these three-pronged flowers can deliver excruciating stings.

This strange looking sea urchin has strange pedicellaria, the defensive tubefeet located in between the sucky tubefeet. These may serve to absorb more sunlight, like a radio dish for radio waves. The dish folds such that its three-prong snapping action remains for defence.

Flower-like pedicellaria can work like catch bags or to extend surface area to catch sunlight. They can also sting. Notice the urchin’s green colour and spines. This image is an enlargement of part of the previous photo. (Toxopneustes pileolus)

amazing feather stars
Feather stars are some of the most fragile seastars, their arms long and narrow, branched and studded with numerous almost invisible tube feet. With these tubefeet they catch very small planktonic particles, both phyto- and zoo-plankton. The tubefeet then pass the catch as in a bucket chain from one to the other towards the mouth which is at the end and centre of all arms. Unlike other sea stars which have fixed numbers of arms, featherstars are born with only few arms, acquiring new ones as they grow. So the relative age of a featherstar can be inferred from its ‘bushiness’.

To the occasional diver, a featherstar is observed as a static item attached to a coral fan or hidden inside the cracks of the rock face. Unlike other sea stars they appear to move less and stay put more. But this may be a false impression, as we observed some exceptional behaviour of these mysterious animals.

We’ve wondered how featherstars suddenly appeared at the tops of the reefs before the sun had completely set, until we witnessed a late one hurrying along from its hideout to the top of the reef. It moved along by pulling itself forward in rowing movements with its arms and it moved along so well that it could do 5 metres a minute. One late nightdive we literally caught one from mid water, happily swimming with all its arms in a rowing movement.

A baby featherstar with 9 arms found underneath a small stone. 9 arms may well be their starting formula.

A young yellow featherstar with 13-14 arms out only by night, here curling its arms in the presence of zooplankton attracted by the photographer’s torch.

An old featherstar with more than 23 arms.

An old featherstar ‘running’ from its hideout to the top of the reef, by pulling itself forward with its sticky arms. This goes rather fast at the speed of one metre in less than twenty seconds.

This is the only kind of featherstar permanently out by day and night. It may well be a mixotroph, able to convert sunlight into food but it must also have a secret defence because it would so easily be eaten.

Super closeup of previous featherstar shows flat side branches, suitable for catching sunlight. The very fine tubefeet are hardly visible. Its pink colour may well come from symbiotic algae.

mysterious hermits
The number of hermit crab species on Niue is unexpectedly high and among them we found some weirdoes. Hermit crabs are related more so to lobsters rather than crabs. Their bodies have adapted to living inside shells. If shells are not available, they will live inside the holes made by date shells in the solid coral rock. As hermit crabs grow, they need progressively larger shells. Should these not be available, they will stop growing and mature at a smaller size.

In order to fit inside a shell, a hermit crab has made some adaptations. Its body protection is lost and its body curves and corkscrews according to the inside of a shell. Its rear two pairs of legs are permanently allocated to holding the shell so that it appears as if they have only two pairs of legs and two claws. In most hermits one claw is larger than the other to cover the entrance of the shell and to be used as pliers. The other claw is more for cutting. But there exists a family of hermits with claws of equal size. Most hermit crabs are flexible feeders, living from whatever they can get.

The anemone hermit (Dardanus gemmatus) collects anemones until all of the shell is occupied. These anemones are carried along and they thrive by sweeping their tentacles over the terrain, catching whatever. Some anemone hermits have a shell consisting of anemones only, which grows in size with them. So they won’t need to look for bigger shells. We suspect that Dardanus does something similar.

Scientists studying the evolution of shells, discovered that they did so in an arms race with their predators. The cowrie shell is an extreme fortress against crab predation: it is smooth and rounded, and its entrance narrow and heavily fortified. Intrusion by crab claws has become impossible, thus the shell has also become useless for hermit crabs. Not so. The letterbox hermit (our name) has adapted its size to fit through the slit, then bend backward, extend its eyes, and even lift and carry the heavily fortified shell. So amazing!

The anemone hermit, also called the bug-eye hermit for its large eyes (Dardanus gemmatus), gathers anemones on its shell. When it needs a bigger shell, it carefully transplants these.

The letterbox hermit crab is so flat that it can slide through the narrow slit of a cowrie shell. It has very long stalked eyes in order to be able to look out from beneath the massive shell.

wandering mushroom coral
If one were told that corals could wander around, one would be deemed insane. But the mushroom coral is not attached and defends itself against being cemented by other organisms. This coral has a flat top and lamella like those in a mushroom, beneath. Its size and shape are such that it can be moved and uplifted by heavy storms to be deposited on top of other corals the right way up. This prevents it from being shaded out by fast growing corals.

Because of its long shape, this mushroom coral is also known as slipper coral. The shaded side is shown here with ridges and lamella as underneath a mushroom head.

Two slipper corals showing a smooth top and ridged bottom. Hydrodynamic properties make this coral land the correct way during storms.

pipefish
Niue does not have sheltered lagoons where fragile organisms can live. So don’t expect any of these here. But there is this little miracle, Avatele harbour (pronounce as avasele), which is very shallow and only the size of a swimming pool. But at our first swim there we counted 24 different species of fish and this number has been increasing steadily. It is such a wonderful place for snorkelling if only the fresh water on top did not create a fuzzy mixing layer. So one needs to dive under this layer to see clearly.

Even so, one would not have expected a small colony of network pipefish (Corythoichthys flavofasciatus) to live under the protection of the boat ramp. These small and thin fish are so well camouflaged that one may be forgiven not to notice them. Pipefish are related to sea horses, but rather than a closed pouch, they have two open folds underneath the male’s abdomen to hold the eggs until they hatch. By caring for their young, pipefish increase their chance of successful recruitment.

The network pipefish has a network pattern on its body, giving it a disruptive coloration necessary for its cryptic life.f047532: the right-hand picture shows a male network pipefish with its brood pouch half-way its body.