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.

Soldier crab's weapon

I was wondering why crabs are not present in all the places where I expect to find them.  Fiddler crabs live in the same context in the adjacent creek, why not here?  The only difference between the creeks is that flats below are sandy where as the other flats are have firm mud below the surface.  In some creek mouths, silt is deposited by floods, buried by sand and over a few seasons becomes a firm plasticine-like material.  The consolidated mud being firmer than the sand is probably harder for stingrays to forage in and for some species, this may determine whether survival is possible.  The sandy flat below is cratered with stingray feeding hollows and whilst soldier crabs here seem to be just hanging on, it seems fiddler crabs cannot.

Stingray hollows in soldier crab colony

We have two species of soldier crab in Queensland, the regular one that swarms at low tide (Mictyris longicarpus - below) and a subterranean one that I have never seen above the ground surface (M. livingstonei).  To check which species is present, it is sometimes necessary to dig one up.  It is a case of finding the vertical hole below the sand balls and placing a finger in the hole to find the crab at the bottom.  My finger met with unexpected pain on finding the crab so I had to investigate.  Soldier crab nippers are useless for nipping fingers, so what was the crab doing?  The crab has a spine near where the nipper joins the body and crab was pulling its nippers back hard, spearing my finger against the spine.  It was a raptorial motion like a prey mantis and it was a new trick on me.

Spine soldier crab uses to fight

Same soldier crab escaping

Naturally, that set me wondering what kind of predator might be deterred by such a weapon.  The usual cast of predators would not be deterred at all (kingfishers, ibis, toadfish and predatory crabs), however a newly identified predator may just be feel the spines, the moon snail - Conuba sordidum.  Click here to go to a video of some moon snail and soldier crab encounters.  The scientific paper on the topic is here.

Mud Crab Food

At the mouth of the Richters Creek (Cairns), shells are balanced on the mangrove prop roots.  What is this all about?

Mud-creeper shells in a tree

Many of the shells are severely damaged and this is a clue.  There is only one animal that can follow the spiral, crushing away solid shell - the mud crab.

Mud crab damaged Telescopium shell

Hermit crabs appear to be the main prey although according to the scientific literature mud crabs can prey on the Telescopium and Terebralia mud-creeper snails as well.  After the mud crab has eaten the shell's current inhabitant, an new inhabitant often moves in.  A full sized shell may be reduced in stages down to small remnant as inhabitant after inhabitant is eaten.  It is no wonder hermit crabs have learned to climb trees.

A hermit crab

Saving a Pelican

Pelicans are arriving on the coast in large numbers and are appearing in places that I never seen them before.  When birds are forced back to the coast, it is a sure sign that inland Australia is heading toward drought. Hawks and black swans can also suddenly appear in numbers and such sudden increases are known as irruptions.  At night the pelicans sleep on sand bars in river mouths or on the city esplanade (birds like city lights).

Pelicans at mouth of Meunga Creek, Cardwell

Last week, I was walking the dog on the beach near the mouth of a creek.  In the late afternoon, pelicans fly from local inland waterbodies out to sand bars in the Barron River Delta.  They glide in from a great height, then using the wind shadow of tall trees at the creek mouth they pick up speed before dropping down almost water level where it is easier to fly against the trade wind.  Flights after flight of pelicans pass less than 10 m overhead.  However one pelican spied something in the water and pulled in his wings and went into a dive like a tern.  Just above the water surface, he broke off the dive and headed directly at the kids and fishermen on the far bank.  The kids screamed in fear and the pelican banked and turned away within only a couple of metres of the row of fishermen.  Unfortunately, it collected some of the fishing lines descending from the fishing rods.  I have never seen a pelican do anything this bold before.  The pelican flew a hundred metres out to sea and crashed into the surface.  It was repeatedly half taking off then crashing into the water.  Pelicans sometimes do this when chasing fish so I interpreted the behaviour as extreme hunger.  Luckily for the pelican, someone figured out what happened and asked me to get my boat.

A similar scene to the one the pelican flew through

It was dark by the time I could get a boat prepared with gloves, motors, knife for cutting line etc.  The pelican was a kilometre down the beach and a long way out.  A volunteer assured me he could swim and jumped in the tinny but he looked nervous.  We took a breaking wave over the front which emphasized that light weight car topper tinnys and open sea are a bad combination.  I went out a long way to silhouette the pelican against the fading western sky.  As the pelican could still take off, I came in on him with the 4 hp motor at full speed and he dodged us a couple of times but we got him on about the sixth attempt.  He had a lure embedded in his wing and he was trailing a huge length of fishing line.  On the way back we plugged along slowly to avoid pounding through the waves but it was so dark I could not see the beach break.  Our peripheral vision is slightly better at night and could just make out where we were but at night you can't be certain.  I made it back to the creek mouth avoiding the breaking waves on both sides and my volunteer admitted that he could not really swim.

As fish hooks have barbs, they can't just be pulled out.  I had to push the point of the hook out through the skin, crush the barb down with pliers and then gently ease the hook back out.  Pelican skin is very tough and even though I could feel the tip of the hook through the skin, it took as much force as my fingers could take to push the hook through. To calm the pelican, his head is covered with a shirt and he is being held gently but firmly.  The pelican looked fine other than a small wound at the site of the lure so we decided that it would be least stressful on the pelican if we just released it. He took few seconds to get his bearings when the shirt was removed and he flew to the nearest sand bank 50 m away where I hope he spent the night recovering.

Lure in pelican's wing

Pelican being released

The line that the pelican was trailing

Mudskippers get Bitten too

What do sandflies bite when people are not around?  I know they bite herons on the skin around their eyes whilst the poor heron is trying to stand motionless to catch a fish.  The herons certainly feel them.  But do sandflies also bite the smaller ground dwelling animals like crabs and mudskippers.

Mudskipper being bitten by a sandfly

It appears mudskippers are on the menu.  Crabs may also be on the menu but they can only be bitten in joints and I have seen crabs flicking something off, but I am not sure if they were fending off sandflies.  More research.  However sandflies are not the only thing to worry about.  Below a mudskipper has a fish louse on its head.  Fish lice chew flesh.

Granddaddy of Fiddler Crabs

This is probably the largest fiddler crab I have seen in my life.  Like an old bull elephant, it was living alone some distance from a large colony of conspecifics.  I saw the crab from a boat and was able to take a photo without disturbing him.  I may go back and drop a ruler on the ground near his hole, take a photo when he comes out and measure him without hurting him.  My guess that his nipper is around 80 mm long when around 50 is closer to average.

Life in the colony which was about 30 m away is intense and the big guy is probably looking to retire somewhere quiet.

Uca dussumieri and U vomeris

Unexpected Survivors

In the mouth of Richters Creek is a stand of battered mangroves that is directly exposed to the Coral Sea. Waves more than half a metre high persistently slam into the stand whenever the trade winds blow.  For twenty years, I have watched this stand defy the sea and although it is built on soft mud it has barely retreated.  An engineer has solved this mystery, apparently the ability of structure to survive destructive waves is related to the forth power of the empty space the structure contains.  Emptiness is the key to the strength.  Modern seawalls are built in this principle but a mangrove swamp on soft mud riddled with crab holes provide its best demonstration.  The weakness of a solid mass is also demonstrated by this curious stand of mangroves.

This season, the stand was also tortured by two cyclones.   Whilst the mangroves stand their ground, the beach and foredune that had built up within the stand was washed away.  In the background of the photo below is sand cliff two metres high.  Roots that were buried and are now exposed but remain brown up to the former sand level where the bark turns grey.  Beach creepers that grew on the sand are draped over the roots.

There is even a Pongamia tree, which is a rainforest tree rather than a mangrove, that grew when the mangroves were previously filled with a sandy foredune.  By the time the previous foredune had washed away, the tree had developed long roots that ran back into the hind dune.  When I first saw this tree, the foredune was gone and the tree was suspended by only by the tangled stilt roots of the mangroves and it was perfectly healthy.  A mature rainforest tree growing suspended in empty space by mangroves!  Conduit-like roots that ran across tops of the stilt roots for as much as 30 m and kept the tree alive.  With the attack of the cyclones, the mangroves that supported this tree have fallen and the tree now lies exposed on the mud at the front of the stand, a single branch still alive.

The Pongamia tree is not the only unexpected survivor in this stand.  On the very exposed end of the stand was a tree that bears witness to an animal that I have never seen in the mangroves, the Striped Possum.  Striped possums eat beetle larvae that feed on the timber or dead and dying trees, leaving deep pits where they have chewed into the trunk to catch their prey.  In this case, the trees at the front of the stand which are succumbing to the relentless punishment of the sea.

Survivors such as these are interesting but they are also important.  Following the 2004 tsunami, the importance of mangroves as a sea defense is widely recognised.  As sea-level rises, we will need trees that can survive marine incursions as many developed areas and natural areas are not defensible.  And important native animals such as stripped possums - Australia's most intelligent marsupial - can show up in the most unexpected places.