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 flies 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. Mangrove kingfishers are quite large and are of similar size to an Indian Myna, so it is quite surprising that they can survive such an impact, let alone repeat it.  

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

Saturday, 29 April 2017

The Tortured Geology of the Mackay Coast

Many stony beaches near Mackay have an exceptional diversity of rocks. Even bedrock exposures reveal a complex series of rock types. How did the regional geology become such a complex mixture? 

Varied geology of Mackay Region
Diversity of rocks on shore of Taffy Island (Click to enlarge)
Firstly, the geology in other parts of Queensland is usually not like this. In Far North Queensland, the grand landscapes have large areas of uniform rock types such as basalt, granite and metamorphic rocks formed from hardened deep sea sediments. Stony beaches usually have rocks of similar geology.  There is nothing like the confusion of rocks seen on the Mackay Coast, where there can be several wild swings in geology within a space of a few metres. A single small island or bay may have more than half a dozen different rocks types and these rocks can be interbedded or mixed up in complex ways.

Breccia on beach at Cape Hillsborough
Breccia and basalt at Cape Hillsborough
Despite the crazy geology, the coastal landscape of the Mackay Coast has an almost painfully low topography and the geological wonders are pocket-sized rather than awe-inspiring tourism grade. The places shown in this post are mostly accessible by car and the remainder are close enough to reach in a small boat on a good day.  There are also a few islands that can be walked to on a low tide if you take care not be stranded by the huge tides near Mackay.

Vocanic ash features in Mackay region, near Whitsunday Islands
Red Cliff Island near Seaforth
The Mackay Coast and nearby Whitsunday Coast were shaped by the largest slicic igneous province on earth. Rather than issuing floods of basalt, the volcanoes of this igneous province were explosive and produced mainly volcanic ash. The Whitsunday Islands are mostly formed from water-laid deposits of volcanic ash which were compressed into stone. Time has filled the submerged calderas of the volcanoes that produced the Whitsundays with sediment and has hidden them from view. The positions of the calderas are inferred by changes in rock types on islands and the mainland.

Geology of Cape Hillborough, hardened volcanic ash island
A hollow island formed from fused volcanic ejecta
Recent scientific papers reveal the power of volcanic blasts from this province.  Some were so powerful, that sand-sized zircons were blasted up so high that they came down in Western Australia. In total, the volume of material discharged is estimated to have been approximately 1.4-2.5 million cubic kilometres. That would be enough material to cover all of present day Australia to a depth of more than 300 m. 

Geological feature in Central Queensland
Funnel Mountain (344 m, viewed from Taffy Island) is formed of volcanic deposits have been protected from erosion by a hard sandstone cap.
Just two years, it was discovered that the world’s longest continent chain of volcanoes starts at Cape Hillsborough (active ~32 million years ago) on the Mackay Coast. The Cosgrove volcanic chain as it is now known runs through inland Central Queensland, then disappears under a section of very thick crust, before re-emerging in Southern Victoria, approximately 2000 km away.  Volcanic chains are the result of a continent drifting across a plume of magma, known as a hot spot.

Geological tourist attraction
Metasediment cliffs on Outer Red Cliff Island
The volcanic episode that created the Whitsundays was associated with the separation of Australia from Antarctica and the relatively unknown other continent in our region, Zealandia. In addition to volcanism, the Mackay Coast has had intense cycles of rifting (stretching) and compression. Compression has tilted many of the originally low angled volcanic ash deposits to much steeper angles.  This tilting can be seen in the cliffs of Outer Red Cliff Island.   In other areas, solid masses of igneous rock have been cracked in every direction by the stresses.  I imagine that the process was similar to how ice is cracked by a moving glacier. Weathering along these irregular cracks may be the reason behind the jagged boulder beaches we see on the Mackay Coast today.

Rifting led to the creation of the granitic islands off the coast. Magma filled expanding rifts deep within the earth's crust and then cooled into granite. Subsequence cycles of compression resulted in faulting which uplifted and exposed the granite.  Where the surface sank due to faulting, basins were formed that accumulated sediment. New sedimentary rocks were formed including sandstone. Some of these rocks have also been uplifted.

Granite island near Mackay, Central Queensland
Foreground: Flat Top Island showing granite outcrop

Geological history of Central Queensland
Wedding Cake Rock (uplifted sandstone), Newry Islands
Unfortunately most of the Mackay Coast’s dramatic volcanic history has eroded away and only traces remain. Cape Hillsborough is part of the side of the only remaining coastal volcano. The crater has been lost to erosion and subsidence. The next nearest volcano is the Nebo Volcano which is 70 km west of the town of Sarina. There are however many volcanic features on the Mackay Coast that are associated with side vents and dykes. Most of the features of Mackay Coast and nearshore islands, including Taffy Island, seem to be a product of dykes.  Dykes occur where magma has forced its way into a crack in the overlying bedrock or compressed sediment. Heat from the cooling magma cooks the surrounding rock into a harder stone that can resist erosion and it is these rocks that comprise many of the nearshore islands.

Taffy Island
Taffy Island is located near Freshwater Point, Sarina. It is one of the few islands that I have been to rather that a specially selected example. At the eastern end of the 300 m long island is a dyke and the rocks in the contact zone have been cooked to produce much harder rocks. Moving away from the contact zone, the ground reverts softer rocks formed from volcanic ash. Volcanic ash breaks down to clay and much of the volcanic ash has turned into clay with the passage of time.  A single basalt rock, which might have been a basalt bomb lies on the beach in the middle of the island. As the nearest volcanoes are approximately 80 km away, the bomb possibly flew this distance to reach its current position.  Strong trade winds and high waves continue to shape the island today. Here is the geology of Taffy Island in photos.

Metamorphosed fine-grained hard rocks with a dull red surface protect the eastern end of the island.

Running through this area is a dull grey stripe of igneous rock, which is the dyke that many have metamorphosed the surrounding softer rock.

In areas that received less heat, the volcanic ash was only slightly hardened.

A brightly coloured rocky islet beside Taffy Island seems to be created from ash consolidated by heat. 

Basalt bomb
A single and quite large possible basalt bomb was present on the shoreline of the island.  Behind the basalt is the soft rock derived from ash that makes up most of the island. 

This is what a fresh basalt bomb looks like (Photo USGS)

The western end of the island has cliffs of unconsolidated clay and stone

Critically endangered littoral rainforest, Central Queensland.
Windswept grassland and horizontal metre-high rainforest attest to the strength of the winds
I am investigating the regional geology to understand the distribution of fauna and flora in the landscape, which is my profession.  This post has been prepared to provide an entry point to other resources which provide more detailed and peer reviewed information.  Background information used to prepare this post has been drawn from a number of sources including: