Sunday, 12 November 2017

The Unreported Importance of 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.

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 the 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

Friday, 22 September 2017

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.

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

Friday, 15 September 2017

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 manner 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 swamp, 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 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.  

Thursday, 24 August 2017

Rafting Sand Crabs

I was on the outer breakwater of the Cairns Marina when I observed that some of the floating mangrove leaves and twigs had tiny sand crabs riding on them. In about a ten minute period, I counted approximately a dozen small sand crabs and was able to photograph many of them. Being close to dark, it was at the limit of the cameras performance to focus on the jostling flotsam and only a few photos were really clear. I thought that possibly the presence of the floating marina fingers or the concrete breakwater may have been providing a novel habitat that the crabs were exploiting in a novel way, however went I visited a remote part of the outer harbour (near Second Beach) mangrove leaves with rafting crabs floated past my boat from a direction where no man-made structures were present. Clearly, this not an irregular behaviour. To my surprise, I even had photos of rafting crabs from almost the same day, one year before.

Sand crab rafting on a mangrove seed (Aug 2016)
Crab rafting on a mangrove leaf (Aug 2017)
Sand crabs (Portunus pelagicus) or blue swimmers as they are also known, have a planktonic larval stage so do not need to raft for dispersal. The one potential reason that I have found for small crabs dispersing using floating mangrove litter is to possibly to find more suitable habitat. The small crabs have a preference for intertidal habitat over subtidal habitat and prefer seagrass beds to open sandy or muddy substrates. Perhaps rafting provides sand crabs that settled in poor habitats a way to chance relocating to a better habitat. A search of scientific papers on the Internet reveals that whilst significant research has been undertaken on planktonic larval dispersal and that little is known about post-larval dispersal. It is known however that the crabs somehow actively select their preferred habitats and are in low densities outside these habitats.

A crab on fine seagrass leaves
The rafting crabs are usually quite small - this one was caught in seagrass - 21 July 2017
Larger crabs can also sometimes be seen swimming at the surface and on the day that the tiny crabs were seen rafting at the outer breakwater, one full size crab which spanned approximately 30 cm from nipper to nipper was cruising back and forth in a patch of light from a street light on the breakwater.   

The tiny crabs often swim from leaf to leaf

Thursday, 13 July 2017

Salt Pans of Wunjunga

Bowen famously has a salt mine at the entrance to the town. Tourist information used to impel people not to be put off by the ugliness of the salt flats and to continue through to the town. Yet in their own way hypersaline environments are beautiful and interesting. The only place I have found a publicly accessible salt pan which dries to a salt crust is at Wunjunga, which is just south of the Burdekin and on the Bruce Highway.  Driving into Wunjunga during the wet is like driving through a lake.

In the wet season, the salt pan is a shallow lake
Mosquito larvae are present in numbers so don't go there at night if you value your blood

A freshwater fish (spangled perch) swimming on its side in shallow water in an attempt to find freshwater
During the wet season when rains are heavy, the salt flats on the inland side of the salt pan complex drain slowly and become temporary freshwater wetlands.

Bird life is intense, so many birds and so many species
Some birds are timid but others will tolerate a careful approach
Closer to the coast tidal influence dominates and signs of freshwater influence are lost. Just before the beach is a shallow basin that is a saltwater lake during the wet season (shown above), and a bare salt pan with a small hypersaline lake in the dry. Large salt crystals form patterns in this lake.

The shallow lake almost completely dries

The bottom of the lake is encrusted with large cubic salt crystals

Delicate petals of salt spread over the surface
On the bare dry silty surface exposed by the drying lake there are unexpected signs of life. Every solid object is encrusted in barnacles, all dead as the lake fills seasonally and not with tidal cycles. A non-tidal saline lake with barnacles is not a common thing. The only other occurrence that I could find was the Salton Sea, a lake in California that is 70 m below sea level. The beach there is actually composed of barnacle shells. At Wunjunga, meat ants range across the bare flats and probably consume the dead barnacles, leaving empty shells.

Barnacles encrust every surface
A large green tiger beetle was running around in a completely bare expanse. Tiger beetles are predators and I assume they specialise in feeding on insects that land on the salt pans. When I tried to catch the beetle, it ran so erratically that it was almost impossible to catch. Tiger beetles can fly as well as wasps but their running skills are even better.

At the edges of the bare flats, succulent Tecticornia shrubs resemble staghorn coral. Their flowers are almost microscopic yellow specks that poke out between the stem segments. After colouring up, the stems shrivel, releasing segments that contain seed.

Tecticornia going purple

A diversity of succulents is present
Small banks of raised ground lie within salt pan and support an array of grasses and succulents. These areas are important to nesting birds and with their light silty soils are rather delicate places. Tread carefully in these places and look for animal tracks.  For such a harsh environment, there is plenty to find if you take the time.

Wednesday, 21 June 2017

Striped Possums in the Mangroves

Near Cairns, most of the mangrove swamps appear to have a good population of striped possums. The possums leave distinctive marks on dead trees, when they chew holes into the timber to get at the beetle grubs inside. Sometimes the possums leave great pits in the sides of trees. Mangrove timber is very hard and I often cannot even mark the sides of the pits with my fingernails. It must take the possums a great deal of effort to get at the beetle larvae or witchety grubs (moth larvae).

Striped possum feeding marks
Dead mangrove at end of boardwalk with possum damage
In June 2017, I was able to make a video of a striped possum (Dactylopsila trivirgata)  breaking open the bark of a mangrove tree on the Cairns Airport Mangrove Boardwalk.

During the day, the mangrove forest is revealed to be a dense, 20 m tall stilt mangrove forest. This forest has no connection to any terrestrial areas, not even stranded beach ridges, so the possum must live in the mangroves entirely. This makes me wonder how it obtains freshwater and enough variety of food to survive. The possum I observed appeared to be fairly small for a striped possum and was so hungry that it completely ignored me even though I was standing about 4 m away with a bright light.

Mangroves beside Jack Barnes Memorial Boardwalk
This tall stilt mangrove stand is about 250 m wide and 750 m long 
This particular trip into the mangroves taught me quite a lot. Firstly, there are very few flying insects in the mangroves at night (other than bloodsuckers). There also seemed be very few spider webs. Bright eyes revealed occasional wolf spiders on tree trucks. Wolf spiders jump on prey, rather than using webs. Only one sleeping bird was observed and one large Papuan Frogmouth.

Podargus papuensis
Papuana frogmouths are large and have red eyes
It is counter intuitive for a place with such exuberant vegetation to be so devoid of wildlife. However, on reflection, very few insects can eat mangrove foliage and even then, they usually only take a few bites, so it makes sense that the terrestrial food pyramid is virtually absent. Mangroves have a detrital food web that is based on plant material that has died and been reprocessed by bacteria and fungus into a less toxic form. Normally mangrove leaves which have fallen partially decompose and are then consumed by crabs and shrimps. 

Perisesarma messa feeding on fallen mangrove leaves
In a way, the striped possum is also part of a detrital food web. It feeds on beetle larvae that burrow into dead timber and spread the spores of fungus, which grows and provides food for the beetle larvae. When watching the video of the striped possum, it almost looks like the possum is either drinking or feeding on the fungus lining of the beetle tunnels in addition to feeding on the beetle larvae. Perhaps someone should research this someday.

Striped possum feeding marks
Marks created by striped possum in video

Saturday, 6 May 2017

Most Beautiful Fish in the Mangroves

Sometimes floating mangrove leaves are really fish in disguise. Batfish take pretending to be mangrove leaves to an extreme. This post shows a Round Batfish (Platax orbicularis) that I found on the Cairns waterfront, in North Queensland, Australia.

A round batfish beside a floating mangrove leaf 
During April, the surface waters of harbour are alive the juvenile fish and mangrove-leaf-like batfish are suddenly present. They lie on their sides and almost drift passively with the current, making only slow motion movements to capture small food items.

The fish's face would often come out of the water when it was feeding

The fish would only be vertical when turning

Mangrove leaf mimick
So great was the fish's faith in its disguise, that I could almost poke a camera in its face.
In the few hundred metres of the Cairns waterfront, perhaps half a dozen are present. Naturally, I would prefer to be searching for fish in pristine mangrove wilderness, but my experience is that the marina is by far the best place to see fish that mimic mangrove material. Mimics seem to be more abundant in the outer estuary which has large tidal flows. Mangrove flotsom mimics are difficult to find in the long mangrove creeks where the same water moves back and forth within the creek instead of being flushed and replaced with new water on each tide. As the marina is at the mouth of the harbour and the floating concrete fingers trap floating objects, ideal conditions for observing mimics are present. Still, it is no coral reef and the pursuit of mimics needs a long attention span and time to waste. Many species of juvenile fish are only present for a few weeks of the year as they grow quickly and move to new habitats

Batfish begin life in the mangroves but live on coral reefs when they are mature. Where they come from before they become 'mangrove leaves' is not clear. I think that they first appear as the small dark fish that lurk in the shadows under the floating fingers of the marina. The dark young batfish swim vertically and are hard to photograph from above. Perhaps they mimic dark mangrove detritus, which sometimes swirls around in eddies below the surface. Fins turning orange is the main clue that these are juvenile batfish.

In Cairns Harbour, there are two species of batfish that pretend to be mangrove leaves, the Longfin Batfish (Platax tiera) and the Round Batfish (Platax orbicularis). There are also a several other fish that pretend to be bits of floating vegetation including sea grass, green leaves or pieces of bark. Perhaps, there are about 20 vegetation mimicking fish species in total.

Long-finned bat fish swimming around floating concrete fingers

Platax tiera juveniles

Below are some (suitably licensed) photos from the net that show how the batfish change after they leave the mangrove environment. Sometimes the adults return to the mangroves and can be seen swimming around the mangroves by snorkelers.

Longfin batfish in transition to adult form

Platax tiera
Mature longfin batfish