Coastal Coral Reef at Cape Tribulation

When the tide goes out near the famous tourist location of Cape Tribulation in tropical Queensland, reef flats and coral platforms are exposed. However these reefs at first sight appear to be mostly dead and some explanation is needed. The short answer is that these reefs flourished more than 5000 years ago when sea level was about one metre higher than it is today. After sea level feel to its present height, the top of the reef was about 80 cm above the level required for coral growth and coral cover was lost from the top of the reef. The mystery is why the reef platforms are still there at all after 5000 years of battering by rough seas from trade winds and cyclones.

Some areas have a wide flat platform

Fissures and porous reef absorb waves and no waves swashed over the top

Emmagen Creek is the best place to observe these old reefs, however almost all sections of coastline near Cape Tribulation have well developed reefs. Corals are still present in the fissures of the reef and on the reef slope. In fact these reefs hold the record for coral diversity on the Great Barrier Reef nd have even more coral species than the best of offshore reefs. In the order of 175 species of coral have been recorded. However most of what is visible is a yellow brown surface riddled with holes and coated with algae and sediment. It is opposite of an attractive coral reef.

In some areas there are shallow enclosed lagoons

The coral matrix is about 7 metres thick and is so porous that swells can flow into the reef. Twenty five metres back from the edge, I could hear the reef beneath my feet breathing, it sound like a sleeping baby. The reef platform is quite strong, although occasional thin sections can break underfoot causing me to lose some skin. Deep fissures surge with swells and even in the middle of the broad reef, I could not see the bottom through the clear waters. I am keen to investigate this underwater world, however it will likely have hazards including jellyfish which often concentrate in fissures and even moray eels and crocodiles which could be inconvenient. In one of the fissures, I observed large fish being attended by a cleaner fish.

A large fish at a clearer station in the middle of the reef platform

Live corals fill the deep fissures

When the corals could no longer grow on the reef platform, encrusting coralline algae took their place. The top 80 cm of the reef platform consists of a coralline algae matrix with embedded chunks of coral and terrestrial rock. I wonder if aborigines carried the rocks onto the reef as it is hard to imagine a natural process doing so. In any case, I consider the reef to be a living reef that is maintained by algae rather than coral.

Pink coralline algae is easily overgrown by brown algae

Some patches of reef edge have hand-sized stones in the reef matrix

Coralline algae are easily grown over by other types of algae and can only thrive when something is grazing the other algae. At Emmagen Reef, the grazing appears to be performed by air-breathing slugs. These slugs (Onchidium sp.) usually cruise the ground in mangrove swamps and to find them as the dominant herbivore on a coral platform is most unusual.

Onychidium slugs grazing on the algae

There are some specialised fish associated with this reef. Peppered moray eels hunt the crabs which scurry along the margins of fissures. Dart fish inhabit small pools. Larger pools have a subset of reef fish including angel fish and damsels. In wide fissures, there are concessional coral colonies and wave washed edges are covered with dense sargassum. Water clarity is limited.

Peppered moray eel under a coral ledge at the toe of the beach

The most interesting thing and the only thing I failed to record were two small fish that bounded over an exposed two metre wide sand bank on the tips of their tails. The fish were vertical and bouncing away like pogo sticks in a motion that was very different from a mudskipper. When they reached a pool, they swam away underwater. This leaves me wondering if they were rock skippers, which are an amphibious blennie known from islands in the Pacific Ocean.

Ships sail close to the coastline here

The Life of Biting Sandflies

Given that sandflies are the curse of the coastline, it is surprising how little accessible information there is about them. Scientists have studied them on the behest of councils to understand where and when they breed, however these scientific studies do not present much of a window into the life of a sand fly. Being of inquiring mind, I would like to find out exactly where they breed and what their larvae look like so that I can study them for myself. In this post, I will attempt to provide a resource for those who want to study the humble saltwater sand fly.

A sand fly drilling into the back of my finger

There are lots of different types of sand fly and the species are identified by the vein patterns and colour patches in their wings. They belong to the genus Culicoides and C. molestus famously inhabits the canals and sandy shores of South-east Queensland. In the tropical north, C. subimmaculatus is known to breed in muddy mangrove environments. I am not sure which species I have investigated.

As sandflies are so small, a microscope is needed for identification of species. For this blog post, I have only used affordable equipment including a magnifying glass and a set of $20 close-up lenses that screw onto the front of my compact camera.

Sandflies were emerging along the receding water line where beach yields to estuary

Finding where sandflies might be breeding is as simple as squatting down and looking for sandflies walking across the muddy surface. I have observed them mainly at midday to early afternoon, usually walking across the surface toward the wind. In the late afternoon, strong trade winds blow across the ground and both insects and other creatures seem to seek shelter. Usually, I wear long clothes for sun protection and the sandflies do not seem to be biting at any rate in the middle of the day so I can study them unmolested. Most sand fly larvae inhabit a tidal band just below the high tide level of ordinary neap tides and above the level dominated by water breathing crabs and worms.  

Diagram of sandfly life cycle - WikiMedia

Studies have discovered that sand fly emergence occurs mainly during the spring tide part of the cycle. However nothing seems to have been written about the actual process of emergence, let alone any photos or videos taken.  Below is my best attempt so far to video emerging sandflies.  The cover image is a dead pupa that floated to top of one of my collected samples. It appears to be in a clear bag, which may be how the wings are protected.

Observing sandflies in the process of emerging is challenging, even when reviewing video footage things happen so quickly that it is difficult to work out what is happening. I believe a lab study will be needed to sort out what is really happening but this is what I think. As the girl sandflies emerge, they get jumped on by the boy sandflies and there is lots of violence. Alternatively the boys are jumping on the girls when they come in to lay eggs, but the first pattern is common other insect groups. I think that sandflies just pop up out of the mud and can fly a few seconds later. However males sandflies fly can in and land within the space of a single video frame at 30 frames per second. Sandflies suddenly appear and disappear in the footage and much higher frame rate is needed to track their movements. All this action occurs within a few minutes of the ground surface being exposed by the retreating tide. Small trumpeter fish (Therapon jabua) enthusiastically patrol the shallowest of waters and probably eat any sandflies that emerge before the tide has fully retreated.

To speculate about what I think that I have seen, sandflies may be cryptic about how they emerge, mainly to avoid the attention of their conspecifics. Even before they can fly, they appear to jump like fleas when spooked. The big question is where do they expand and harden their wings. A children’s program on sandflies claims that they do this in a bubble in the ground but I can’t find any published info that supports this. What I take to be freshly emerged sandflies seem to have smaller wings and the wings appear to expand over a short period. As the sandflies turn around frequently, they can be clearly visible when facing one direction and nearly invisible when they turn around. I am guessing that they emerge almost ready but need a few seconds to grow and harden their wings. They either jump around to avoid their mates or possibly hide their bodies in muddy pools in micro-depressions. Hiding their bodies in water hides their shadow, which is more visible that that the actual sand fly. In most of the cases when I see a sand fly emerge from the muddy water, running the video backward shows that it was already emerged and moved into the water from somewhere else. Most of these behaviours can be observed in the video, if you are prepared to go forwards and backwards enough times.  Furthermore, sandflies may emerge synchronously. There seem to be small patches of intense activity rather than dispersed trickle of newly emerged insects.

A dead sand fly larva beside the tip a dressmaking pin

Finding sand fly larvae and pupa is even more challenging. The one success I had was to put a few spoonfuls of mud into a small clear drinking water bottle and shake it so hard that the larvae would not have known which way was up or down. The larvae were probably killed by abrasion and when the muddy mixture settled, were in the fluffy sediments between the clear water and the heavier muds and sands. Pouring off the water and some of the fluffy sediment produced a dozen dead larvae.  Sand fly larvae have a head capsule, three small thoracic segments and nine larger abdominal segments.

Scientists have added sugar and golden syrup to sandy samples to make the seawater so heavy that the sand fly larvae could no longer swim down to the bottom of the container and floated to the top. I would prefer to find another method. I have poured the fluffy sediment from another sample of mud into a shallow tray to look for movement but none was seen save for springtails and tiny worms. However later, strange tracks covered the bottom of the tray suggesting that sand fly larva were present but cannot be seen.

Tracks that might be from sand fly larvae

My next attempt was to carefully excise a chunk of mud from prime habitat and use a dressmakers pin to flick crumbs of mud from the edges of my muddy sample until I had worked my way through. Small tunnels can be seen and even followed. I found that the mud was bound together by threads that were strong enough that I could feel their strength. The threads appear to be the rhizoids of a minute but advanced green algae and can be present even when there is no noticeable algal growth on the surface.  Threads within the mud are much finer than any green filaments that can be seen on the surface and are present and apparently alive even tens of millimetres below the surface. It is like discovering a miniature sea grass with tiny leaves and roots has been present in an area I have walked over hundreds of times but never noticed. Despite finding tiny worms below in miniature green meadows, I did not find any sand fly larvae. As a check, I added some water and shook up mud that had been processed and on the following day found a single sand fly pupa.

Sand fly pupa and dressmakers pin

Mysterious threads holding the mud together (click to enlarge)

The threads are from a minute and unidentified green algae

Even in close-up (see baby mudskipper), the algal turf can go unnoticed.

My preliminary research is incomplete. I need to find a simple way of obtaining numbers of larvae. The frustrating search continues, however I did discover an unexpected ecosystem of algal turf and its associated fauna including springtails.

Additional sandfly information is as follows. Sand fly larvae are reported to be predators of small creatures, which I suppose means mainly springtails. The larvae breath via small hook gills at the end of their abdomen. Only female sandflies feed on blood. Mudskippers are their main victims although wading birds cop it too. Adult sandflies live only 2-3 weeks. Swarms are often visible in patches of sunlight in the lee of trees or other wind blocking objects. The swarms are reported to be where the boy and girls get together.

Control of sandflies is difficult as they live in estuaries which contain other biodiversity which we do not want to kill with chemicals. Fogging may be used when sandflies are severe but fogging has no persistent effect. Within their muddy habitat, sandflies may be predated by tiny robber flies and hover flies. Rove beetles also swarm over sand and mud surfaces and may be predators of sand fly sized prey.

Miniature sand flat wildlife looks like a tiny African savanna

Imagine taking a photo of the most featureless and lifeless-looking patch of bare muddy sand in a tropical estuary and the photo revealing that even this place was seething with life.  In fact the place was full of creatures that were just too small to be seen when I was standing up.  The video below shows a swarm of springtails (Collembola).  It reminds me of the great migratory herds of Africa seen from an aircraft. A single sand fly near the centre of the frame provides an indication of scale.

I filmed the swarm at about four in the afternoon, in the cooler, cloud-shaded conditions when springtails had emerged after the heat of the day and were were swarming.  Scientific literature reports that springtails are one of the most common organisms on earth and that densities of 100 000 per square metre are common.  As these springtails are on the surface even during the middle of a summer day, I suspect that their yellow colouration is related to protection from UV rays  In some of the photos below, these animals are so small, even with high magnification, they are hard to see so please click on the photos to enlarge.

A muddy sand flat between a creek and the beach

Filming set-up, a compact camera with a screw-on close-up lens

The original photo, each yellow or brown fleck is a springtail

Springtail tracks and holes can be seen in fine mud even when the creatures are hiding

Insect predators were also zipping around on the muddy surface. Large yellow and black striped hover flies fly at high speed just above the surface and probably catch sand flies. Rove beetles, which look like tiny earwigs seem to be the most abundant and roam over the surface and into and out of holes. Caterpillar-like beetle larvae that were considerably larger than the rove beetles were also occasionally seen. Tiny bugs, which are as small as some species of sand fly were also present. In truth it is difficult to know for certain which of these insects are predators as most groups have both predators and herbivores.

A tiny fly (robber fly?) that hunts by sight

Rove beetles often make chambers under stuck down mangrove leaves

A sand fly-sized bug, which runs across the surface at speed

In hot dry weather, a good place to find tiny animals is under a mangrove leave that has become stuck to the surface of the mud or under loose flakes of algae. Sometimes a full ecosystem of predators and prey can be found under a single leaf. On close inspection, there is a rove beetle and hundreds of blue-green springtails in the photo below, all of which were under a leaf.

Rove beetle and blue-green springtails

Some of these insects are not be restricted to the sheltered waters of a small tidal estuary. I have also found the rove beetles on open sandy beaches near the high tide line and on exposed sand flats where the blue soldier crabs march. 

Sand flats at Yule Point

Beetle tracks under the sand, near the work of a sand bubbler crab

A rove beetle beside a sand ball

It is beyond question that these creatures are present in vast numbers but are they ecologically important? Juvenile predatory fish such as whiting seek the shallow waters where these tiny insects live. Sand fly larvae are also predators and I wonder if springtails are their main dietary item. Certainly, the tiny fauna would consume algae and help to prevent purification of surfaces with excessive algal slime. More research needed.

Big hairy yellow stripe hermit crabs seem to graze algae and not hunt for springtails.

Leaf-cutting Snapping Shrimps

Snapping shrimps, also known as pistol shrimps make more underwater noise than any other marine animals. Look up why snapping shrimps make loud popping noises and you will almost certainly find some dazzling claims such as “While hunting, pistol shrimps open the hammer part [of their nippers] and then release it, allowing it to snap into the other part. This release is extraordinarily fast and results in an enormously powerful low-pressure cavitation bubble. The bubble that is created serves the purpose of stunning a prey. How does that work? The bubble shoots out at a speed of 62 miles an hour, reaching a temperature of 4700 degrees Celsius for a very brief period of time. This temperature is actually close to the temperature of the Sun.” Indeed, it is the holy grail of videographers to capture this ultra high speed phenomenon as the clips listed below show.

BBC Earth Unplugged - Slow motion pistol shrimp attack hits 4000⁰C! - Slo Mo #14

National Geographic - Amazing Pistol Shrimp Stun "Gun" | World's Deadliest

However with my $400 dollar superzoom camera, I have filmed a completely different reason for snapping. I can find no trace of what the video below shows in the scientific literature and this may be the first time that this information has ever been reported.

If mangrove snapping shrimps use their large nippers to cut up mangrove leaves so that the leaves can be pulled down into their burrows and consumed, this makes the mangrove snapping shrimp the marine equivalent of leaf-cutter ants. A radical claim such as this must be supported by a lot of evidence.

I became suspicious of the standard explanation for snapping shrimp behaviour when it occurred to me that having a bright orange warning flag in the middle of an ambush predator’s lethal weapon does not make sense.

Giant snapping shrimp are large and powerful but not very aggressive

Ecological observations did not add up either. There were simply too many snapping shrimps in some swamps. On some nights, the noise from snapping shrimps is a dull roar, like giant raindrops falling on a tin roof. At low tide, these areas are completely cratered with snapping shrimp holes. As for preying on shrimps, crabs and fish, I have watched the tide flow into snapping shrimp areas and it carries with it very few potential prey. Other types of shrimps have eye-shine which is easily seen in torch light and even the tiniest fish can be seen when they move.

Stilt mangrove swamp near the seaward fringe

The entire floor of this area is pitted with snapping shrimp holes

The mangrove swamps that fringe the open mudflats of Trinity Bay are flooded by seawater that crosses at least one hundred metres bare sand and mud flats. Not that many small shrimps and juvenile fish want to cross such a space - so they hide in creeks. Many mangrove areas are more than 500 m from the seaward fringe of the mangroves and are flooded by seawater for less than 6% of the time, yet snapping shrimps are still abundant. The idea that snapping shrimps are mainly predators was starting to fall apart.

Snapping shrimp holes in stunted Ceriops backswamp near a salt pan

So far, I have discovered that snapping shrimps rarely leave their holes or even come to the entrances of their burrows. At low tide, there are occasional snaps. Snapping is most intense in the evening and when the outgoing tide has exposed the mangrove floor. Waters continue to drain from the forest floor for minutes to hours when the tide recedes and leaves are sometimes transported in the thin film of receding water. Snapping shrimps can be observed grabbing these drifting leaves and pulling them into their burrows. Very occasionally, it is possible observe a leaf being cut up on the surface. 

Snapping shrimp capturing a mangrove leaf

Observing the snapping shrimps is difficult as during the day, the sky reflects on the waters surface, making visibility through the surface quite low. At night, a powerful light is needed for photography and this usually suppresses snapping shrimp activity.  Once I turned my light out after scanning an area for several minutes and was greeted with roar of snapping shrimp pops. Snapping shrimp may also occasionally respond to snapping noises by making a pop themselves. Popping up my camera flash makes enough noise to trigger the occasional response.

The one time that snapping shrimps are quiet is at high tide. Bream and other snapping shrimp predators can be seen swimming through the milky waters and it is too dangerous for shrimps to be active. On the incoming tide, the shallow film of moving water that snapping shrimps need to catch floating leaves becomes too deep and too swift only sixty seconds after the incoming tide has arrived and they fall silent seconds later.

In the soft muds of the seaward edge of an accreting stilt mangrove swamp, snapping shrimp appear to outnumber crabs. Only in the infrequently flooded and very saline Ceriops swamps do numbers of snapping shrimp fall. The video in this post was filmed in a Ceriops swamp. Sometimes a few snapping shrimps even persist around the edges of salt pans. Middle areas which have a mix of mangrove species seem to have equal numbers of snapping shrimp and crab burrows. This unreported and extraordinary abundance would make snapping shrimps one of the most important components of the mangrove ecosystem.

Two tiny crabs can be seen but the glossy mud surface indicates that crab grazing is insignificant

My work in identifying the species of mangrove snapping shrimp is ongoing. They belong to the genus Alpheus. Alpheus strenuus and A. cf. lobidens are the leading contenders and are found from Australia to at least South East Asia. American mangroves have similar species.

Mangrove snapping shrimp live in burrows that resemble a the branches of a stag horn coral. Several sloping burrows that radiate out from a central chamber. The central chamber can be as much as 0.5 m below the surface and possibly even deeper. Tracing tunnel systems with ones fingers in semi-liquid mud has its limitations. The burrows also sometimes intersect the burrows of animals that I would not like to grab such as mantis shrimp. I have traced several burrows in newly colonised mangrove swamps where the snapping shrimp burrows are first generation and have not become part of an interconnected network of burrows. Most burrow systems have approximately a dozen entrances. Craters may form where part of the branched burrow system has a roof collapse. Originally, I thought the craters were by design and were to increase the surface area water within the burrow mouth to improve oxygenation.

Collapsing tunnels in sandy areas reveal the tunnel structure created by a single shrimp

Usually one or two small snapping shrimp is recovered together with a goby. It appears that even mangrove snapping shrimp have gobies as friends.

Goby and shrimp are rarely observed, the shrimp was touching the goby before the photo.

This goby was restricted to this hole for at least consecutive five days due to very low high tides.

When the tide goes out the goby become a prisoner within the snapping shrimps tunnel complex. In Cairns, which has two tides per day and tides are moderately large, six days can pass between tidal flushing of the seaward zone. That is a long time to be confined to a muddy hole the size of a tea cup. At night, a goby can be seen in one hole in about 50. Only once have I seen a snapping shrimp antenna running over the back a goby and I wonder if the relationship between shrimp and goby is as tight as the relationship is for reef living species.

Goby and shrimp obtained from tunnels in a mangrove creek bank.

There is much more to this discovery that I can report and many of the details remain shrouded in mystery. There is much more work to be done

Kingfisher spearing into a tree

If I told you that mangrove kingfishers fly into trees at full speed to make a nesting hole, you would not believe me, so watch the video.

This mangrove kingfisher (Todiramphus chloris) is nesting in a dead Sonneratia tree deep within a tall stilt mangrove swamp. At intervals of approximately one minute, the kingfisher flew into the tree with a run up of approximately 10 m. The bird was in a frenzy to make the nest hollow as quickly as possible. A female was watching. Mangrove kingfishers are quite large at about half the size of a kookaburra, so it is quite surprising that they can survive such an impact, let alone repeat it. In the video, the kingfisher passes through the field of view in a single video frame. I saw about six hard runs at the tree.  Between runs, the kingfisher flew around the tree, landing on stilt roots, calling and displaying to the female.

The tree that the kingfisher was chiseling away at also has a story.  It belongs to a species that only colonises the seaward edge of mangrove swamps.  However as the mangroves prograded, it became stranded and then overshadowed by stilt mangroves (Rhizophora spp.) and died.  Beetles drilled into the tree and spread fungal spores which grow into the white fungal coating that the beetle larval eat.  Tasty beetle larvae attract striped possums and this tree was cratered with striped possum pits when I first recorded the tree in June 2017 for the striped possum post.  From my experience in locating fig parrot nests, I know that there is a window of time between when the tree becomes soft enough for a bird to make a hollow and the tree developing vertical cracks which pipe water down inside the timber or the tree simply falling over.  This window probably lasts for several months only.  Mangrove trees also tend to be quite thin and it is would be hard to find one with sufficient diameter for a nest.  Sometimes a kingfisher will hollow out a tree that is too small and will drill clean through the tree or create cracks that allow chunks of wood to fall away.  Looking at the hole, I suspect the female will reject it.  The mouth of hollow is just too wide and it appears that part of the roof of the hollow has fallen away.

The kingfisher hollow, photo taken at night when kingfisher was away.

Water mouse (Xeromys myoides), a predator of crabs

One of the most mysterious and elusive mangrove animals is the water mouse (Xeromys myoides). They mostly live in mud nests in the mangroves and come out at night to prey on small crabs. They occur in the Northern Terrritory from Darwin to Arnhem Land and in Queensland from Cannonvale, near Mackay down to the Queensland border. The gap between the NT and QLD populations is about 4000 km in round numbers and possibly twice that long for a small rodent that can’t swim across wide rivers. During the previous twenty years, many zoologist have searched long and hard to find water mice in the great gap between the known populations and no water mice were ever found until now.

Records of water mice, Atlas of Living Australia

On 20 June 2017, I was walking through the mangroves in Cairns at night when I saw an interesting rodent with a white underside running around on the floor of a stilt mangrove (Rhizophora) swamp. I saw enough to suggest that the rodent was a water mouse but could not get a photo of the mouse as it disappeared through the dark tangle of roots. Six days later, one of these creatures came right up to me even though I was following it with a powerful torch. This time the camera was ready and as a result water mice have been officially recorded for Cairns, which is 500 km north of the next closest record as the seagull flies.

The first photo of a Cairn's water mouse, about to escape into a crab hole

Perhaps one of the reasons why water mice have been so hard to locate is that it is hard to form a good search image for them. Most of the available information is in text form, which not quite the same value as a photograph, especially for a visual thinker like me. There are also many other creatures that leave similar signs to water mice. Adding to the complication, water mice are apparently not present in every seemingly suitable mangrove swamp either and nobody knows why. In this post, I will try to present a clearer picture of what to look for.

Water mouse, also known as false water rat (photo: wikimedia)

Water mice occur as far south as the Queensland border and occur in very different environments to the places where they have been found in Cairns. In southern areas, they were first studied at Stradbroke Island, which is a giant sand island. At low tide, freshwater seeps from the ground so I thought that water mice were limited by freshwater availability. In these areas, the mice made strange mud nests in clumps of sedges. On the Noosa River, also in southern Queensland, they made mud nests that were referred to as termitaria-type nests. Several years later, water mice were found on Curtis Island, near Rockhampton. Curtis Island is in a dry region where mangroves occur as a band between the salt pans and the sea and there is unlikely to be much groundwater seepage. In this area, they lived in the Ceriops mangroves, which are mangroves that prefer higher and more saline environments.

The Mackay region is believed to be the species stronghold, however the mangroves in Mackay are challenging place to search. Huge tides flush away the relatively high rainfall of Mackay and make the mangrove environment a more saline environment than it ought to be. Stunted mangroves often occur in low woody thickets of robust trees that difficult to push through let alone walk through. Some swamps in river mouths or near grassy plains can even have an understorey of succulent herbs and grass! Given the known habit in southern Queensland, the swamps with grassy understoreys and adjacent grassy flats with signs of freshwater influence were the obvious place to search, however data collected over the last twenty years suggests that water mice do not like that habitat.

Mangroves with grassy understory, Dunrock near Mackay (water mouse central)

In Cairns, the tidal range is smaller than Mackay and the remains of crabs that have been preyed on by water mice are less likely to be swept away. Mangroves thrive on the higher rainfall and are taller and are easy to move through. However, water mice are not the only crab-eating rodent and care has to be made when identifying both rodents and the signs they leave. Rakali (Hydromys chrysogaster), which are approximately six times larger and a formidable predator dominate the swamps. I thought that rakali might also prey on or fight with water mice and limit their distribution. There must be some kind of habitat partitioning however where water mice are present, rakali are also almost certainly present. Introduced black rats are also a common predator on crabs in mangrove areas. Some other native rodents such as Melomys also venture into the mangroves. Water mice apparently do not climb trees to escape, like the rodent below.

These unknown rodents were also foraging in the mangroves 

Water mice prey on smaller grapsid crabs, which tend to have flat rectangular bodies, usually grey in colour. Grapsid crabs often live on the lower part of trees or in simple burrows that go straight down into the ground. Sesarmid are larger crabs that have a squat cylindrical body shape (scone-shaped) and often live in mud igloos which the crabs build on the forest floor. Remains of sesarmid crabs that have been caught by a rakali look as if the crab has been blown up. In contrast, water mice leave a tidier scene. Typically, there will be a bright white upturned carapace which is often close to a scattering of legs and nippers. The best evidence that crab remains are from water mouse predation is an intact segmented breast plate, which is the undersurface of the crab (pers comms Tina Ball). As the carapace is small and can potentially be moved by tides, finding the carapace with the other parts provides stronger evidence that the water mouse consumed the crab at that spot. Empty crab shells with the legs still attached are probably moulted exoskeletons rather than remains of predated crabs.

Small, thumbnail sized empty crab shells are the main sign of water mouse presence

Rakali eat larger crabs and make a mess of them

A cleanly removed breast plate is a good water mouse sign

Water mice are too small to leave trails of footprints, except in places where the ground is quite soft and where it remains undisturbed by other creatures. Such conditions are rare in mangrove swamps and I have only seen footprints where mud had dried hard soon after the prints were made or in soft wet mud that had yet to be disturbed by the traffic of crabs and snails. Rakali leave prodigious numbers of footprints. Rakali have long webbed hind feet with toes of different lengths. Their front feet leave large star-shaped prints with the span of a 50 cent piece. Bandicoots, wallabies and a variety of other terrestrial wildlife including other small rodents also get around in the mangroves and leave prints, so footprints provide unreliable evidence.

Possible water mouse footprints (front and rear)

Water mice live in mud nests which they construct. Depending on the surrounding environment, these nests can be easy to see or they can be almost impossible to distinguish. Mud lobsters, sesarmid crabs and even mangrove ants create large mounds of mud that are shaped like water mouse nests and these other mounds can be so numerous, they even outnumber the mangrove trees. Suitable mud for nest building may be one of the environmental parameters that water mice require. The nests also need to be located near the high tide line as the water mice probably do not like deep or prolonged submersion. Whilst water mice feed in stilt mangrove swamps, they prefer to make nests in areas which are tidally inundated less often.

Possible nest pointed out by Tina Ball

A hollow filled with mud, with crab remains on top is very likely to be a nest

A possible nest showing the horizontal entrances, crab remains were in the tree hollow as well.

The nests have a variety of forms and many other animals make similar mud nests so it is hard to be certain whether the nests belong to water mice. Crab igloos often have vertical chimney style exits whereas water mice tend to make horizontal exits which are supposed to be more oval-shaped than crab holes which are round. As water mice are a listed threatened species (listed as Vulnerable), breaking nests open to see the occupants would be an offense under the Nature Conservation Act. The only alternative is to look for crab dinners which the mice consumed on or in their nest or to set up a camera trap to photograph the water mice as they come and go.

Water mice also make mud ramp nests, but this mud ramp is likely to be a crab house

Sesarmid crab igloos around the base of Ceriops mangrove trees

My working theory is that the most productive feeding areas for water mice are the drier mangrove forests, where grapsid crabs are more abundant than sesarmid crabs. Look for where the ground is flat and full of small crab holes and avoid areas where the mud is covered with crab igloos or the ground is intensely churned by subterranean creatures such as mud lobsters. Proximity to freshwater is not a requirement as the places they have been found are unlikely to have potable freshwater during the dry season. In Cairns, water mice were found deep in mangrove communities that are isolated from the landward fringe and terrestrial vegetation. Water mice seem to be in most of the mangrove swamps I have searched, however water mouse sign is scarce in luxuriant mangroves be they swamps of Ceriops, Rhizophora or some other species. Sign was most abundant in harsh saline back swamps that may be seasonally become quite fresh during the summer wet season. These places tend to be mosquito infested, even by mangrove swamp standards. Some of these places are also subject to deep and violent flooding when creeks within the catchment run deep and water mice apparently climb up high inside hollow trees to survive these conditions. Water mice seem to hunt mainly along the boundary between the Ceriops and Rhizophora mangrove zones where tides are still a regular event and crabs are usually active.

Water mice hunt in stilt mangrove swamps as well as Ceriops swamp

As a threatened species, I will not tell exactly where I found the water mice but those who are interested can contact me via this site. I would also like to thank Tina Ball from the Queensland National Parks and Wildlife Service, for investigated my sites in the field and via camera trapping. She has was able to officially confirm the presence of water mice and to help me sharpen my knowledge of this mysterious creature.