Tuesday, 14 July 2015

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.

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