Monday, 16 February 2015

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

Leaf carrying crab, leaf porter crab
A leaf-carrying or leaf porter crab
Further reading:
Sand Dollars