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

Swamp-oak Forest - Endangered but Hard to Love

Swamp-oak forest which occurs around Brisbane is an endangered vegetation type.  Despite being endangered, it was one of the most wildlife unfriendly vegetation types I had ever seen.  Even though swamp-oak forest straddles the highest high tide line, it has a fraction of the wildlife present in adjacent ecosystems.  Mangroves are full of life and the paperbark swamps and eucalypt forests have myriads of honeyeaters and even koalas.  Swamp-oak forest in contrast is so poor that a vast area of swamp-oak was planted around Brisbane Airport as it is so unattractive to birds, it would reduce bird strike at the airport.  Jet engines usually have to be rebuilt after they ingest birds, so bird strike can serious impact airline profitability.  Ansett Airlines threatened to quit Townsville due to this issue and I had to investigate the contribution that vegetation making in attracting birds to the airport.

Ibis eats McDonalds for breakfast in Brisbane CBD

In fact is was reported that the white ibis, which is the sworn enemy of the jet airplane, only became a nuisance after some were hand raised at a Gold Coast wildlife sanctuary and released into the wild.  These ibis were not afraid of people and became urban birds that raided rubbish tins and breed up into an unnatural population.  Unfortunately for Coolangatta Airport, the birds nested in a wetland not far away.  The bird strike issue was so dire, that people with long poles were hired to go into nearby forests and poke ibis nests to bits to force the birds to breed elsewhere.  Brisbane being not far from Coolangatta  for an ibis was reason enough for Brisbane Airport to plant swamp-oak on every patch of unused land to discourage the dreaded ibis and other better behaving wetland birds.  In my youth on the south side of Brisbane (~1980's), ibis were uncommon and were usually feeding on the banks of mangrove creeks and were never raiding rubbish.

An open forest of swamp-oak (Casuarina glauca)

Now that I have established that swamp-oak forest is a wildlife desert, I will ask you to reverse that position. It is not that the swamp-oak forest has improved, rather the surrounding land has become worse.  Bushland remnants in Brisbane are generally occupied by noisy miners, which are a vicious species of honeyeater that lives in large colonies.  Noisy miners harass small birds in to leaving or dying.  Magpies and butcherbirds help them in this nasty business. Brisbane bushland has also been overrun by invasive garden plants and barely resembles the native forest that once existed.  Now that the good environment has been overrun by baddies, the poor environments which are too poor for weeds and noisy miners have become important sanctuaries for the smaller birds.  Imagine what this means for the pre-development environmental assessment process.  Previously the poorer habitats would have been the automatic choice for development and arguments would be made for preserving the areas with the best remnant trees.  I will certainly be careful before making that sort of argument in the future.

A mistletoe bird eating mistletoe berries

This mistletoe is common in swamp-oak forest and has leaves that look like swamp-oak needles

Mistletoes with broad leaves are also found on swamp-oaks negating
the idea that mistletoe foliage resembles the foliage of its host tree

Mistletoes are a heavy burden for a tree

Besides being a place where you can still hear the voices of little birds, swamp-oak forest and saltwater couch grasslands are places of wide open space.  Only 8 km from the centre of the city (Storey Bridge) are the wetlands of Bulimba Creek, known as the Hemmant Recreation Reserve.   This area is about 1 km wide, 2 km long and between bridges, Bulimba Creek flows for 7.5 km through the wetlands.  Paradise it is not, but in a crowded city you take what you can get.  There is a canoe trail with racks and taps for washing canoes.  It is also possible to walk around but there are no formed trails.  If you need blue sky and open space, this could be your place.

Saltwater couch grassland bordered by mangroves and swamp-oak forest

Bulimba Creek

Saltwater couch grasslands are the home of the saltwater mozzie, so go prepared or go in winter when they are dormant.  And don't wear you good shoes. On very high tides, the saltwater couch grasslands are submerged.

Samphire plants grow in patches in the grass

Mangrove seedlings floating in a saline pool

Further Reading:

• Status of Swamp-oak Forest in NSW
• Regional Ecosystem of Swamp-oak Forest in  Queensland

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.