Fauna on Floating Mangrove Debris

A few days ago, I was cruising along a rock coastline near Cairns when I realised that many of the floating objects had a passenger. Photographing these passengers is ridiculously hard as they hide on the other side of the object when you approach and everything is vigorously bobbing up and down. In smooth weather, one also has to consider that Irukandji jellyfish are likely to be present. These small jellyfish which are as clear as ice are occasionally lethal. Irukandji and their relatives hunt for copepods at the water surface in glassy weather. Irukandji jellyfish kill by inducing so much pain that people can and do die. That would not be a nice experience on a remote coastline and I fear these creatures more than sharks and crocodiles.  Tourists should go on organised tours to minimise the risk of jellyfish sting.

Rocky coastline near Cairns

I did manage to photograph a couple of creatures that one would not expect to be floating around the sea on bits of mangrove. The first creature was a sea hare and the second a crab.

Sea hare on a stilt mangrove propagule

Crab on a floating branch about 1 km from land

I really do not know why animals ride on floating objects. It would seem that the risk of being taken out to sea or getting washed up on a beach would be so high that it would almost suicidal. It obviously takes these animals some deliberate effort to find a floating object. For example, I have never found a sea hare any near a mangrove swamp, they are an animal of the sub-tidal zone. Likewise the crab has chosen to ride the branch. It is unlikely that the crab just happened to be on the branch when it started floating.  Most of the animals that associate with floating mangrove material also have planktonic larval stages so they do not need mangrove help to disperse.

One day I would like to do a proper job of photographing mangrove hitchhikers because there are so many of them. Many actually mimic common floating mangrove debris including orange leaves, specks of bark and even thin green eel grass leaves. The only thing that is not mimicked to my knowledge are mangrove flowers.   

Giant Semi-terrestrial Crabs

Some of the subjects I would like to write about take years to track down. Finding out about them was like solving a mystery where I get one clue at a time. Nocturnal animals in particular fit into this category as finding them and learning about their habits can be very difficult. Sometimes I do not know which animal makes the signs that I am observing so it is very hard to organise the clues I am observing into records. For this reason, special attention needs to be be given to recording animals which are rarely directly observed. I am writing my own software to make the task of piecing such information together easier. It was actually the creature described below that gave me the idea to do so.

The mystery burrow

Not far from home and close to the path to the beach, I found two large burrows that were much larger than usual and I had no idea what creature made them. Each time on my way past, I would watch the holes for signs of life. Just once, I saw a stilt mangrove dropper slowly disappearing down the hole, so now I new what it ate. One day the hole was closed over and remained so for all of the cooler months, reopening only quite late in the year. A second year passed without laying sight on the creature. The hole closed over again. I bought an inspection camera with a 7 m cable and eased it down the hole. Without the ability to steer the camera, it quickly buries into the sandy side of the twisting tunnel and no images were forthcoming. I resisted the urge to just dig the creature up. In late 2015, I caught my first glimpse of the creature, a huge black boxy crab about 15 m from me-too far to see any detail. Gradually, I eased out of the crab's view and ran to grab a camera but the crab was gone when I returned. A few months passed and I was just walking to the beach and cast a glance in the direction of the holes and there it was. This time, I stood frozen, which induced the crab to freeze and I asked someone else to run for the camera. In the weak light of late afternoon and with a superzoom camera at full length, I finally got enough detail to find out what this creature was. It was a giant oceanic land crab (Cardisoma carnifex), which can be 125 mm across the carapace – making it the second largest crab in the mangroves after the mud crab.

Repairing burrow after heavy rain. (Click to enlarge)

Same crab - close up.

There was no record of this species on the Australian mainland in the Atlas of Living Australia, although there is a brief description of the species being present in a book on crabs by by Peter Davie at Queensland Museum so they are known from the mainland. It is certainly not common. Despite searching similar habitats through the area, I found no other similar holes although I did have some near misses with some very large feral pigs whilst exploring some remote backswamps. Apparently these crabs are present in select places around the Cairns/Port Douglas area. I had privilege of claiming the first mainland entry into the Atlas of Living Australia!

The species is one of the dominant life forms on many Pacific and Indian Ocean Islands, where they are an important source of food for local peoples (Lairo seems to be the recognised common name). Yet despite its importance, very little accessible information is available about its ecology. Some additional information is available from the following link. http://www.oocities.org/ericdemuylder/gecarcin.htm

Update February 2016
Two years ago, there was single crab hole, last year there were two holes becoming three by the end of the year.  Now there are twelve holes in the colony.  I am pretty sure that the crabs have only a single hole as sometimes I see the crab above at its nearest neighbour as the same time.  I have not found any other colonies, even in places where the crabs have been reported by others.

Natural habitat of the Pet Hermit Crab

Many pet hermit crabs come from the Northern Territory in Australia.  Terrestrial hermit crabs have a planktonic larval stage in the sea, so they are not the sort of animal that is easy to breed in captivity.  As a consequence most pet hermit crabs are wild collected.  At least 30 000 crabs are sold to the aquarium trade every year according to official records.  These crabs are sold both within Australia and in North America and Europe.

In Australia, one species of terrestrial hermit crab (Coenobita variabilis) is found across the top end of the Northern Territory and some other species are found on coral cays in the Coral Sea/Great Barrier Reef. Hermit crabs occur in places where they have access to seawater, freshwater and terrestrial vegetation.  In practice this probably restricts them to low energy shorelines or rocky coasts where they can scramble over the rocks to reach pools of seawater without getting washed into the sea.  Most terrestrial crabs drown if they are underwater for a prolonged period and I suspect that these hermit crabs would do so too.  People who keep hermit crabs report a freshwater requirement but I am not sure how they find freshwater in their habit as these places have prolonged dry seasons and the ground is very porous.  The availability of sea shells probably limits where the hermit crabs can live.  Even though good habitat appears to be available in Far North Queensland, for example, this area has very few gastropod shells on the beach but is rich in useless bivalve shells.

The Northern Territory is a land of strange rocky coastlines that are perfect for hermit crabs.  Over aeons minerals leached from the tropical soils are deposited in layers in the ground that area exposed on the coast.  Bauxite, laterite, hard clays, beach rock and occasionally cemented sand create a wild array of coastal geologies.

Bauxite cliff with kaolin and laterite lower layers - a place riddled with hiding places

Hermit crabs appear to mainly inhabit crevices in the rock formations near the water's edge during the day then go foraging in the grasslands and forest patches at night.  For a slow moving animal that makes an unmistakable track, they sure are hard to find at night.  The only time they were easy to observe was when they were climbing up the cliffs and sand dunes in the late afternoon.  As soon as they see you coming, even if you are more than 5 metres away, they often just let go and rolled down all the way to the bottom.

During the day hermit crabs also hide under these cemented sand boulders

Some hermit crabs climb straight up the slip face of the sand dune, which must be really hard

Other hermit crabs seem to follow well-known paths

Hermit crabs tend to live in colonies where they seem to get along peacefully.  Living in colonies helps hermit crabs find new shells as when larger crabs find a new shell the old shell becomes available for smaller crabs and a cascade of shell updating may occur as progressively smaller crabs each upgrade their shells.  There are actually scientific papers written on this phenomenon, which is known as vacancy chaining.  Crabs actually hang around with slightly larger crabs hoping to grab their old shells when they upgrade.  When a number of crabs of different sizes group together in order to upgrade with the big guy does, this is a vacancy chain.

On capture, terrestrial hermit crabs can be quite feisty and will attack fingers, even though they have to come along way out to do so.  If they feel threatened, they pull back into their shells and block the entrance with their nippers and legs, something that marine hermit crabs can't do.  Marine hermit crabs are timid in comparison.

Terrestrial hermit crabs are not shy

Terrestrial hermit crabs can also seal the door with their legs.  

Terrestrial hermit crabs feed on a variety of things but probably eat mainly vegetation.  In captivity, they readily eat the vegetables that people eat.  In the wild, I was only able to observe them eating an old mangrove dropper (Rhizophora stylosa propagule)

Typical seaside grassland habitat

Laterite and beach rock shelves on beaches also provide foraging areas. 

Hermit crabs feeding on mangrove propagule

Hermit Crabs as Pets

When it comes to invertebrates, I think that hermit crabs are one of the best choices there is.  They live up to 10 years, which is longer than almost all insects.  They are also quite active and move around on the surface, unlike many insects which bury themselves in soil as soon as they get a chance.  Insects can also be very robotic with a very limited set of behaviours.  In contrast, hermit crabs seem to explore their enclosures and active hide from us when we get close.  They pull back into their shells and roll down of what ever object they were climbing or try to run under something. They also try to burrow under their water bowl and will climb up anything and will swing from the roof of their enclosure if they get the chance.

Hermit crabs can't tell us how they feel so we have to look for indirect signs of whether they are happy to be pets.  So far, I have only had to release one invertebrate because it was so desperate to escape that it was not worth keeping and that animal was a red claw (Cherax quadricarinatus).  Most insects are clearly not worried about being contained provided they have food, water and mates.  If invertebrates are unhappy, they tend to lurch around their enclosures constantly.  I think that hermit crabs are on the verge of intelligence and require some additional care to make their enclosures interesting.  Hermit crabs also require warmth and fresh and saltwater.  They are very easy to feed, eating many human foods.  They also like the company of their kin and lots of empty shells to try out. Give them what they want and they will be happy.  People are crazy for hermit crabs and there are many pages with good advice on keeping them.

Hiding where there is Nowhere to Hide - Life on the Sand Flats

Sand flats can be like deserts however if you look long enough, you will find some very strange creatures.  Many of the beaches in the north have vast sand flats at low tide but many of the creatures that live there are buried in the sand so take some finding.  The shifting sands and lack of hiding places make life very difficult for creatures that shelter in burrows or which need something hard to hide under and this is the primary reason why sand flats have fewer animals.  There are also fewer plants to feed on as macro-algae require something to anchor themselves to.  Seagrasses also avoid very sandy sand flats which are subject to high wave energy, preferring more sheltered areas with muddy sand.

Sand flats at mouth of Barron River, near Cairns

Sand flats at Ellie Point near Cairns with mountains in background

The most abundant creatures by far on the exposed sand flats near the low tide line are sand dollars.  Sand dollars feed by moving just under the sand surface and letting the plankton fall through the short felt-like spines to channels below which sweep the material from the top of the animal around the edge and into the mouth at the middle of the under surface.  In some other parts of the world, sand dollars hide in the sand at low tide and feed on the surface at high tide and I will have to check if our local species does this myself, but my recollection is that they are always covered with at least a fine layer of material.  The sand dollars may also be hoovering up diatoms that live in the sand and which come to the surface to photosynthesis at low tide.  

Sand dollar with a typical hat of sand

On my last trip to the sand flats I observed hermit crabs eating sand dollars, however the hermit crabs live in shallow pools close to the beach.  Perhaps some sand dollars are washed up from their normal habitat into the pools where the crabs live and are scavenged.

A sand dollar being eaten by two hermit crabs which are hidden by their shell houses

Very occasionally, I find file snakes buried in the sand.  File snakes are harmless and are one of the least energetic of all predators.  In fact they have trouble moving on land as they have so little muscle power that their own weight pins them down.  They can bury themselves in wet sand by slowly shuffling their rough skin against the bottom.  To permanently record the details of the last file snake I found, I recorded the sighting on the Atlas of Living Australia.  If you open the preceding link, you can see what the snake looked like before I picked it up and put it on top of the sand to take the photo below.

Little file snake at Cooya Beach, in the process of shuffling back into the sand

Close-up of head and rough skin, which is being rotated around the body to dig into the sand.

Some creatures you see only once and this unidentified eel is an example, so it pays to have camera on you.  The eel was able to retreat backwards into the sand.  From memory, there were no yabby holes or any other gaps, just loose sand, so going into the sand backwards is quite a trick.  The eel was however sucked out of the ground with a yabby pump which is how we got to see it at all.

Unknown species of eel

The final creature which I have seen in Brisbane, Townsville and the Barron River Delta is the leaf-carrying crab.  The crabs last four legs are mounted on its back and point upwards so that the crab can grip a leaf.  Usually these crabs are found in very shallow water such as the leading edge of the incoming tide where they look like mangrove leaves being carried along by the flow, that is until they decide to change direction and then it becomes obvious that something is happening.

A leaf-carrying or leaf porter crab

Further reading:
Sand Dollars

Creatures that Live in Logs in Mangrove Swamps

There is a small fish that lives inside worm holes in mangrove trees.  I first saw this fish 25 years ago on a university field trip and I remember the fish being called a cobra fish but googling that term finds nothing and I cannot find its name.  Finding names for mangrove log creatures seems to be harder than normal as not much is published.  The first step is to get a good photo of the creature, not easy when you have to open up muddy logs to find them and then try to get clean enough to use a camera.  What I discovered is that the fish are certainly not alone.  The biodiversity in rotting mangrove logs is as high as the biodiversity of a rock pool.

The mangrove forest where most of the photos were taken - Redden Island

Shipworm holes can be large enough to put your finger in and many are calcified

A small shipworm showing the shells that bore into the wood

The key species in the rotting log ecosystem is the ship worm. These worms, which are actually bivalve molluscs grind away wood using their shells.  Logs soon end up riddled with hole that provide habitat for a wide range of creatures.  Logs also have other habitats. Common mangrove crabs make tunnels under logs, not that they need logs, just that they prefer to tunnel under logs than burrow out in the open.  Another range of creatures live in the space that opens up under the bark when the bark becomes lose.  As logs can be different sizes, in different states of decay and in a different position relative to tide heights, each logs is different and no log supports all species.  In all at least 20 species of macro-invertebrate depend on mangrove logs in north Queensland. 

The photos that follow show some of the creatures that I found under or within logs.  Every tenth log has something interesting.

These dartfish were under the bark of a tree that was completely exposed and had been so for possibly hours or days.  I assume the fish were in this position before I lifted the bark

The fish which are about 20 mm long can also be found inside the trees in shipworm holes.  This fish came out of a hole when I chipped into a shipworm infested log.

Non-descript crabs are the most common inhabitant

Giant flatworms up to 60 mm long glide through cracks

A giant flatworm coming out of a shipworm hole.  See the eye spots.

My new species for yesterday - looks like a species of stone crab

This snapping shrimp was as long as my hand and was found under a log

A colony of shrimps was present in the tunneled log shown above

I have very little idea what these 80 mm long creatures are

Grassy Salt Pans of Cairns

Almost exactly 100 miles/160 km down the coast from the succulent covered salt pan near Cooktown is one of the few remaining salt pans near Cairns. Despite being so close that a lazy crocodile could drift from one to another in less than a week, the botany of these two salt pans is very different. At Cooktown, it was all succulents, near Cairns it is usually nearly all grass. In Google Earth the photo pattern of each vegetation type is remarkably similar. Surprises like this remind me that neither I nor anyone else I know of can fully understand why things grow where the do. Until we can understand why what grows where, environmental management is just educated guess work.  I think students and rangers could do mini science projects to find out why some places that should be similar are entirely different.  Professional scientists don't answer these questions as they need to maintain their fame and are not interested in easy research.

Saltwater couch grassland/salt marsh at Yorkeys Knob

Most of the original salt pans and samphire flats around Cairns have been lots to development so I have to be careful about deciding that Cooktowns salt pans are different to those found near Cairns. Many salt pans within Cairns City were reclaimed. Cairns Airport is on what once was a salt pan and the major agricultural folly of East Trinity also included saltpans. Fortunatelly, we have a couple of good examples left and I think the remaining examples are similar to the ones lost. It is also unlikely that any halophytic (salt tolerant) species would be lost from the entire region. Salt pan plants are great colonists and show up even in salty ruts around the edges of sugarcane fields. For these reasons, I suspect that the succulent flats like those near Cooktown never occurred on salt pans near Cairns.

The natural park lands are quite pleasant provided you wear long clothes and don't dally around after sunset

Cooktown is in the Cape York Peninsula Bioregion and Cairns is in the Wet Tropics Bioregion. Bioregions are like countries for flora and fauna so this suggests that the salt pan vegetation could be different. Queensland has 13 bioregions. The bioregion boundaries were originally drawn by smart people with marker pens on paper maps. They were not something that came from a sophisticated analysis. Yet it is strange that even intertidal communities seem to have a sudden transition at the bioregion boundary. It is strange because most mangrove plants and animal species (fish, crabs, molluscs) occur from here to Africa and on almost every coastline in between. Given that most species are so widespread, why to comparable salt pans that are so close by natures standards have such different vegetation? I can only guess. Climate and geology and tidal inundation cycles are all quite similar. The founder effect may be at play. The founder effect is natures version of losers/keepers. The founder effect is when the first plant species to find a new island or in this case salt pan multiplies and occupies all the space making it very hard for new species to establish.  In very flat places, I have noticed that even a few centimetres in altitude makes a big difference to how long an area is flooded in inland areas or how many tides per month are experienced in coastal areas and I would start my investigations here. Succulents (Halosarcia sp.) do grow on slightly lower ground around the margins of the grassy swards in Cairns but are low and straggly.

Saltwater couch (Sporobolus virginicus) is the grass that covers the salt pan. We also have a second species of saltwater couch (Paspalum vaginatum) which often grows with the first species and looks very similar. The leaves of the first species are spirally arranged on the stems and the on the second only grow on opposite sides (called distichous).

Sporobolus virginicus has little tufts of leaves from underground runners.

Crabs and mud lobsters thrive in the saltwater couch and the ground is so full of hummock and hollows that I stumble through the grass rather than walking through it.  I wonder how the productivity of this marine grassland compares with mangrove ecosystems.  

Mud lobster mound peppered with crab holes

Shed exoskeletons were hanging in the grass

Ant plants also like the margins of salt pans and I suspect this is related to mists but I don't really know.  

Myrmecodia beccarii - the spiny ant plant

Crocs, People and Creek Mouths

Creek mouths are magnets for people on warm sunny day. The crab habitat gets trampled and I wonder how the crabs manage to survive.

On a sunny day, everyone is chasing fish at the creek mouth

If you are not interested in fishing, you can swim in the mud with your brothers and sisters

Sentinel crabs that share habitat with the mud-swimmers

In the search for more pristine habitat I went to another creek mouth about 50 km from Cairns to look for the  locally elusive Uca polita fiddler crab but had to rethink my plans.  There was a crocodile that was larger than my boat and faster too so I had to put some distance between it and myself.  Around the bend and about 100 m away were the weekend crab tramplers.  It is not uncommon for crocodiles to be within visual distance of people and the people to be totally unaware.  I sometimes took binoculars to a creek mouth, lay down on the sand so that I could see under the overhanging branches and looked for crocs and I would see them and nobody else was aware of their presence.  When boats approached, they would quietly slide into the water unseen and come back out when the boat moved on.  Crocs are not put off by noise and human activity.  One of the best places to see crocodiles was on the Barron River where the Cairns Airport landing lights span the river.  Screaming jets tensing up before landing would pass only tens of metres above the crocodiles.  They even nest in unlikely habitats like than.  

3.5 m croc and girl friend at creek mouth

Crocs run and jump off banks and can end up in your speed boat if you cruise at speed close to the bank - they have hit the side of my boat on a few occasions.  Years ago, when I had been padding around in the sea on a surf ski for hours, I paddled into the creek mouth shown above and up the edge of the channel to stay out of the wind.  A croc came running out of the mangroves and stopped with one foot hanging over the lip of the bank just in front of me.  On a 17 foot long surf ski, turning fast is not possible so I had to maintain my composure paddle within 2 m of a 2.5 m croc perched on the bank.  Sometimes you can even run into crocs in coastal waters.  With me it has usually been near misses caused by me surprising crocodiles.  Crocs generally avoid us but always stay on guard.  I recommend that you make an effort to see the crocodiles that share our waterways as it helps to know your potential enemy.

Crabs and Cyclones

Cyclone Yasi was a category 5 cyclone and possibly the strongest cyclone to make landfall in any settled part of Australia. In Cardwell, there was a storm surge of about 2 m which inundated the first rows of buildings along the coast. What I later found out was that in the uninhabited coastline north of the town, the storm surge was up to 7 m high. This had a dramatic effect on the ecosystems present. Three years later some areas have yet to begin recovery and in later posts, I will show the dead forests. In this post, I want to examine crabs at the mouth of Meunga Creek and compare it with Richters Creek which is well matched in terms of size, climate zone and coastal geomorphology.

The mouth of Meunga Creek is a strange place. I walked in but if you do this, note that the mud can be firm or quite soft and the ground is far less predictable that in healthy swamps. The stand at the mouth of the creek was completely flattened and most of the fallen trees formed a floating raft that was driven inland until it was snagged by the standing trees in the background.

First 30 m of swamp was completely flattened

Looking toward the mouth, a few low trees survived. Trees that are highly exposed often survive cyclones as they have a lot of experience weathering rough conditions. In both photos there is barely any sign of colonisation by new mangroves – 2 Sonneratia seedlings and about 5 advance Rhizophora seedlings in the entire flattened zone.

View toward mouth of creek from same location

Did the crabs survive this dramatic change?

The signs do not look good. An algal turf is visible in the top photo and this should not happen. Algae is fodder for a wide variety of creatures. See how flat the ground surface in the above photo is, normally the ground is like a  moonscape with small mounds and craters and is pock marked with crab holes. Surely the crabs would be back after three years? Every year waves of planktonic larvae settle so there is an almost infinite supply of colonists. Maybe this time things were different. Cyclone Yasi was very large and impacted swamps in all directions for more than 50 km. The cyclone may have killed most of the crabs in the mangroves and wiped out the planktonic larvae of 2011. Yet nature abhors a vacuum and if the habitat at Meunga Creek is good, substantial recovery should have occurred by now. There are only a few tens of intertidal crab species and each has populations in the millions so there would have been enough survivors to repopulate the shores. Further up the creek small pockets of habitat also survived the cyclone and provide nearby good habitat today so it is a mystery why are the crabs missing.

Mud balls on pneumatophores

A peculiar feature of this forest were the mud balls that had formed on the breathing roots of dead Sonneratia mangroves. It suddenly occurred to me why I had never seen mud balls like this before. Let me explain why with a photo.

Female fiddler crab feeding on material on a breathing root

The crabs are not entirely missing. Crabs are digging under fallen logs and causing them to sink into the mud, something that you can see in the photos.  Some of the mud balls on the breathing roots even have small crab holes in them.  In places there were also holes in the ground but even here things were a little strange. But first lets look at a typical patch of ground at Meunga Creek mouth.

Mud surface

The lines are feeding marks from grazing mullet but the texture of the ground is wrong, it is like suede and flies are feeding on it. There are some small crab holes but the area was notable for the absence of adult crabs. A hundred metres away in a slightly different situation, I found colonies of fiddler crabs with absolutely no adults so I am not sure what is happening. Are the crabs colonising then not making it to adulthood?

Normally every millimetre of ground is turned over many times each day by feeding crabs and molluscs. The photos below are taken from a matching area in Richters Creek.  See the feeding balls all over the ground surface.

Creek mouths are normally grazed heavily and feeding balls cover the surface

Mudcreepers graze the sloppy areas

Crabs normally defend their patch but something strange is happening at Meunga Creek. The crabs are almost holding hands. They resemble ants with masses of crabs living in a single hole. Their domain also seems to end only centimetres from their hole finishing at the point where the loosened surface is replaced by the suede-like surface. I am beginning to think that many crabs are farmers rather than gatherers. Their crop would be diatoms.

Twelve crabs live here

Whether these colonies survive and grow is uncertain. Below is a larger complex with fewer signs of life.

Mangrove trees benefit from crab holes which help to drain and aerate the soils. A lack of crabs may be inhibiting the regeneration of this area. Crabs also benefit from mangroves, many eat mangrove leaves or feed on algae growing on mangrove roots. It seems that recovery will be gradual and take a very long time.