Miniature Insects of Tidal Sand Flats

Sometimes if you just look hard enough you may see something no one else has seen.  Unable to find the time to go somewhere new, I decided to sit on the edge of a sandbar and watch at the seawater gradually draining from the sand and I kept up my concentration until I saw something that I had never seen before.  Tiny flecks of life appeared to be moving around on, under and above the water surface above black patches of mangrove debris.  Moving above the surface implies insects and insects the size of dust particles can fly through water as well as air! However it was hard to believe that such infinitesimally small creatures could survive in an environment that lasts only a few hours before the tide comes in violently reworks the very ground.  I can remember ten men trying to open a metre wide channel from a nearby blocked creek to the sea.  Such is the rate at which the sand is moved around that digging like mad they could not open the channel faster than the sea could fill it back in. Yet it is in this ocean of highly mobile sand that the flecks of life are present.

The sand flats can look like this one day (click to view)

I described these insects to the entomologists at the local university to a muted response.  It was uncomfortable to call what I had seen a community or ecology as how can a community exist when the very terrain is destroyed and recreated every few hours.  Worse still, finding the flecks of life a second time eluded me.  Even scraping up the mangrove debris and going through it under a microscope failed to yield anything other than a few larval crabs.  For nearly two years I have been looking. If I put a bright LED torch on the sand at night, flecks could be seen jumping through the beam.  However the nights are seldom still and it was difficult to be sure that the flecks were not flakes of matter propelled by the breeze.

Hours later the sand flats can be a very different place, in this case turned over by stingrays

A few days ago, I tried the old trick of just looking until I saw something, this time in a different place and I rediscovered this lost community.  Perhaps this community is only present in the winter (June-July) or perhaps it is only present in certain areas.  I am yet to work this out.  It was late in the day and the sun was so low that the shallows had become dark yet anything on the water surface was still illuminated and suddenly the true number of these little creatures was revealed. To see the creatures, you must approach the waters edge with great care as the weight of a foot fall even a half a metre away gently shifts the sand, causing the sand to slump just slightly and warning the little creatures to depart the area.  Video confirms that they are moving around the edges of the water, as I cannot easily see them with the naked eye.  I can only see them on the surface of the water and here they are caught by the wind and streak across the surface so quickly that in a few video frames they are gone.  Falling into the water does not seem to bother them and may in fact be part of their normal habitat.

A rill between sand ripples at the mouth of the Barron River

Springtails floating on the water seen in silhouette

I filled a small bottle with surface water and took it home for a look under the microscope.  Even under a microscope at 40 x the creatures are small.  They proved to be springtails, small wingless insects that can fly by flicking themselves into the air with a spring-like rod that folds under their body.  To the naked eye they look white or brown (could be a few species) but the brown ones are actually bright yellow.  The captive creatures were placed on a drop of water on a black surface and I photographed then with my compact camera held over the eye piece of the microscope.  The first photo shows the springtail beside a fine blonde human hair.  The other photos show the springtails rolling around on the surface of a drop of water.  Under the abdomen in the last photo is the spring that gives springtails there name.

Springtail beside a human hair

Are the springtails just a curiosity or are they important?  Their sheer numbers probably make them important and potentially they are food to the many juvenile fish that live in the pools.  Aside from that, I wonder if their yellow colour is a sunscreen.  I also wonder if they are part of a detrital community based on bacteria that live on buried organic matter as they seem to be present mainly in shallow areas where detritis is present in the sand.

Do Mangrove Ants know about Tides?

I often wonder why crustaceans dominate the mangroves and not insects.  In comparison to crustaceans insects seem so advanced.  Just to name a few advantages of being an insect:

• Being able to fly;
• Living in colonies with division of labour;
• Having long antennae that allow an animal to feel its way along in the dark;
• Different breeding strategies e.g. making nests, bearing live young; and
• Lack of planktonic larval phase which probably facilitates rapid speciation (i.e. local populations can be reproductively isolated.

So why aren’t insects dominant?  The answer to that question would need a book but one chapter in the book would be the ability of animals to match their activity with the tides.

When I was young, I created a small mangrove swamp in a fish tank and kept fiddler crabs.  Even though the fiddler crabs were separated from the sea, they still knew about the tides times and would make their little doors which   they use to seal themselves in their burrows at high tide.   In the wild it is easy to observe fiddler crabs closing their burrows before the advancing tide.  Whether they just use their internal clock or whether they pick up environmental cues as well, I do not know but tides are complex so I suspect that the crabs use both.  East coast Queensland has two high tides a day but the Gulf of Carpentaria has only one high tide a day.  At the tip of Queensland, where the two seas meet it can be high tide in one and low tide in the other which results in the famous horizontal waterfall.  Thursday Island which is three kilometres long can have high tide at one end and low tide at the other.  It would interesting to see if fiddler crabs from one end of the island measure tides differently to those at the other  

If fiddler crabs are masters of telling the tide, what do ants know?  I found an ant trail of the mangrove swimming ant (Polyrhachis sokolova) and decided to watch how the ants reacted to the incoming tide.  The trail was about 20 m long and ended at a tree on the seaward fringe.

A mangrove swimming ant - Polyrhachis sokolova 

Ant ascending tree at end of trail

Despite the approaching tide the ants continued to march the trail.  Some early sets of ripples flooded the area around the tree and I thought that the ants would get the hint, but when the water drained away, they just resumed marching the trail.  However as the trail had been washed away they were wandering around trying to get some bearings.  Still there was no panic.

Moment of truth - 1 second until the tide arrives - and still no reaction

Too late!

The ants are good at floating on the surface film and are good at swimming, however they would be no match for incoming tide, especially on a day with a strong trade wind, which add wind-driven surface currents to the flow.  There must be more to the story as I doubt that the ants could survive loosing many of their number with each incoming tide.

Ant floating in a pool left by receding ripples

More information: David Attenborough video clip on mangrove ants.