Saturday, 25 July 2015

Barron River Sunset

At the very same time that I was staring at sand for the tiniest insects that the human eye can see, there was another person on the sand flats photographing the scenery with the best camera I have probably ever seen. The camera was a 36 mega-pixel Nikon D800 full frame DSLR using prime (fixed length) lens. Prepare to see the Barron River sand flats as you have never seen them before. Click to view.

Machans Beach, Barron River Mouth

Machans Beach Sunset

The photos show what we would see if we could look into the sun. I talked to Tony as he waited for the clouds to fire up.  There are a few minutes each day when the clouds are lit from below with the red sunlight of the setting sun.  Trying to be of value, rather an annoying person, I mentioned that the pelicans come in fast and low at about this time of day as they head to their night time retreat.  Tony took up the challenge.  It was about half an hour after the sun had set before the first pelicans showed up in silhouette against a sky orange with volcanic dust.

As the pelicans have to fly into the trade winds, they tend approach the river in the wind shadow of tall trees then suddenly drop down to a metre above the ground to fly under the wind.  Their flight path is almost predictable so we could let them come to us.  A disappointingly low number of pelicans had gone past and the sky was now so dark that even black and white pelicans were hard to seen.  But at that moment Tony spotted some pelicans flying further out and keen to show what a top end professional camera can to took some photos of the distant flock passing in the blue-black of early night.  The results are below and show something quite amazing, the flock of pelicans was being lead by a Black Swan!

Pelicans flying above sea

Pelicans being lead by a black swan

There are a lot more photos of our tropical coast on Tony's web site.
Photo I took of Tony when he was photographing the sunset.

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.

Saturday, 4 July 2015

Natural habitat of the Pet Hermit Crab

Many pet hermit crabs come from the Northern Territory in Australia.  Terrestrial hermit crabs have a planktonic larval stage in the sea, so they are not the sort of animal that is easy to breed in captivity.  As a consequence most pet hermit crabs are wild collected.  At least 30 000 crabs are sold to the aquarium trade every year according to official records.  These crabs are sold both within Australia and in North America and Europe.

In Australia, one species of terrestrial hermit crab (Coenobita variabilis) is found across the top end of the Northern Territory and some other species are found on coral cays in the Coral Sea/Great Barrier Reef. Hermit crabs occur in places where they have access to seawater, freshwater and terrestrial vegetation.  In practice this probably restricts them to low energy shorelines or rocky coasts where they can scramble over the rocks to reach pools of seawater without getting washed into the sea.  Most terrestrial crabs drown if they are underwater for a prolonged period and I suspect that these hermit crabs would do so too.  People who keep hermit crabs report a freshwater requirement but I am not sure how they find freshwater in their habit as these places have prolonged dry seasons and the ground is very porous.  The availability of sea shells probably limits where the hermit crabs can live.  Even though good habitat appears to be available in Far North Queensland, for example, this area has very few gastropod shells on the beach but is rich in useless bivalve shells.

The Northern Territory is a land of strange rocky coastlines that are perfect for hermit crabs.  Over aeons minerals leached from the tropical soils are deposited in layers in the ground that area exposed on the coast.  Bauxite, laterite, hard clays, beach rock and occasionally cemented sand create a wild array of coastal geologies.
Bauxite cliff
Bauxite cliff with kaolin and laterite lower layers - a place riddled with hiding places
Hermit crabs appear to mainly inhabit crevices in the rock formations near the water's edge during the day then go foraging in the grasslands and forest patches at night.  For a slow moving animal that makes an unmistakable track, they sure are hard to find at night.  The only time they were easy to observe was when they were climbing up the cliffs and sand dunes in the late afternoon.  As soon as they see you coming, even if you are more than 5 metres away, they often just let go and rolled down all the way to the bottom.
cemented sand boulders
During the day hermit crabs also hide under these cemented sand boulders
Terrestrial hermit crab on sand dune
Some hermit crabs climb straight up the slip face of the sand dune, which must be really hard
Terrestrial hermit crab tracks
Other hermit crabs seem to follow well-known paths
Hermit crabs tend to live in colonies where they seem to get along peacefully.  Living in colonies helps hermit crabs find new shells as when larger crabs find a new shell the old shell becomes available for smaller crabs and a cascade of shell updating may occur as progressively smaller crabs each upgrade their shells.  There are actually scientific papers written on this phenomenon, which is known as vacancy chaining.  Crabs actually hang around with slightly larger crabs hoping to grab their old shells when they upgrade.  When a number of crabs of different sizes group together in order to upgrade with the big guy does, this is a vacancy chain.

On capture, terrestrial hermit crabs can be quite feisty and will attack fingers, even though they have to come along way out to do so.  If they feel threatened, they pull back into their shells and block the entrance with their nippers and legs, something that marine hermit crabs can't do.  Marine hermit crabs are timid in comparison.
Australian hermit crab
Terrestrial hermit crabs are not shy
Terrestrial hermit crab nipper
Terrestrial hermit crabs can also seal the door with their legs.  
Terrestrial hermit crabs feed on a variety of things but probably eat mainly vegetation.  In captivity, they readily eat the vegetables that people eat.  In the wild, I was only able to observe them eating an old mangrove dropper (Rhizophora stylosa propagule)

Coastal grassland hermit crab habitat
Typical seaside grassland habitat
Coenobita variabilis habitat
Laterite and beach rock shelves on beaches also provide foraging areas. 

Coenobita variabilis feeding
Hermit crabs feeding on mangrove propagule

Hermit Crabs as Pets

When it comes to invertebrates, I think that hermit crabs are one of the best choices there is.  They live up to 10 years, which is longer than almost all insects.  They are also quite active and move around on the surface, unlike many insects which bury themselves in soil as soon as they get a chance.  Insects can also be very robotic with a very limited set of behaviours.  In contrast, hermit crabs seem to explore their enclosures and active hide from us when we get close.  They pull back into their shells and roll down of what ever object they were climbing or try to run under something. They also try to burrow under their water bowl and will climb up anything and will swing from the roof of their enclosure if they get the chance.

Hermit crabs can't tell us how they feel so we have to look for indirect signs of whether they are happy to be pets.  So far, I have only had to release one invertebrate because it was so desperate to escape that it was not worth keeping and that animal was a red claw (Cherax quadricarinatus).  Most insects are clearly not worried about being contained provided they have food, water and mates.  If invertebrates are unhappy, they tend to lurch around their enclosures constantly.  I think that hermit crabs are on the verge of intelligence and require some additional care to make their enclosures interesting.  Hermit crabs also require warmth and fresh and saltwater.  They are very easy to feed, eating many human foods.  They also like the company of their kin and lots of empty shells to try out. Give them what they want and they will be happy.  People are crazy for hermit crabs and there are many pages with good advice on keeping them.