Imagine taking a
photo of the most featureless and lifeless-looking patch of bare muddy sand in a tropical
estuary and the photo revealing that even this place was seething with life. In fact the place was full of creatures that were just too small to be seen when I was standing up. The video below shows a swarm of springtails (Collembola). It reminds me of the great migratory herds of Africa seen from an aircraft. A single sand fly near the centre of the frame provides an indication of scale.
I filmed the swarm at about four in the afternoon, in the cooler, cloud-shaded conditions when springtails had emerged after the heat of the day and were were swarming. Scientific literature reports that springtails are one of the most common organisms on earth and that densities of 100 000 per square metre are common. As these springtails are on the surface even during the middle of a summer day, I suspect that their yellow colouration is related to protection from UV rays In some of the photos below, these animals are so small, even with high magnification, they are hard to see so please click on the photos to enlarge.
A muddy sand flat between a creek and the beach
Filming set-up, a compact camera with a screw-on close-up lens
The original photo, each yellow or brown fleck is a springtail
Springtail tracks and holes can be seen in fine mud even when the creatures are hiding
Insect predators
were also zipping around on the muddy surface. Large yellow and black striped hover flies fly
at high speed just above the surface and probably catch sand flies.
Rove beetles, which look like tiny earwigs seem to be the most
abundant and roam over the surface and into and out of holes.
Caterpillar-like beetle larvae that were considerably larger than the
rove beetles were also occasionally seen. Tiny bugs, which are as
small as some species of sand fly were also present. In truth it is
difficult to know for certain which of these insects are predators as
most groups have both predators and herbivores.
A tiny fly (robber fly?) that hunts by sight
Rove beetles often make chambers under stuck down mangrove leaves
A sand fly-sized bug, which runs across the surface at speed
In hot dry weather,
a good place to find tiny animals is under a mangrove leave that has
become stuck to the surface of the mud or under loose flakes of
algae. Sometimes a full ecosystem of predators and prey can be found
under a single leaf. On close inspection, there is a rove beetle and hundreds of blue-green springtails in the photo below, all of which were under a leaf.
Rove beetle and blue-green springtails
Some of these
insects are not be restricted to the sheltered waters of a small
tidal estuary. I have also found the rove beetles on open sandy
beaches near the high tide line and on exposed sand flats where the
blue soldier crabs march.
Sand flats at Yule Point
Beetle tracks under the sand, near the work of a sand bubbler crab
A rove beetle beside a sand ball
It is beyond
question that these creatures are present in vast numbers but are
they ecologically important? Juvenile predatory fish such as whiting
seek the shallow waters where these tiny insects live. Sand fly
larvae are also predators and I wonder if springtails are their main
dietary item. Certainly, the tiny fauna would consume algae and help
to prevent purification of surfaces with excessive algal slime.
More research needed.
Big hairy yellow stripe hermit crabs seem to graze algae and not hunt for springtails.
Snapping shrimps,
also known as pistol shrimps make more underwater noise than any
other marine animals. Look up why snapping shrimps make loud popping noises
and you will almost certainly find some dazzling claims such as
“While hunting, pistol shrimps open the hammer part [of their
nippers] and then release it, allowing it to snap into the other
part. This release is extraordinarily fast and results in an
enormously powerful low-pressure cavitation bubble. The bubble that
is created serves the purpose of stunning a prey. How does that work?
The bubble shoots out at a speed of 62 miles an hour, reaching a
temperature of 4700 degrees Celsius for a very brief period of time.
This temperature is actually close to the temperature of the Sun.”
Indeed, it is the holy grail of videographers to capture this ultra
high speed phenomenon as the clips listed below show.
However with my $400
dollar superzoom camera, I have filmed a completely different reason
for snapping. I can find no trace of what the video below shows in
the scientific literature and this may be the first time that this
information has ever been reported.
If mangrove snapping
shrimps use their large nippers to cut up mangrove
leaves so that the leaves can be pulled down into their burrows and consumed, this makes the mangrove snapping shrimp the marine
equivalent of leaf-cutter ants. A radical claim such as this must be supported by a lot of evidence.
I became suspicious
of the standard explanation for snapping shrimp behaviour when it
occurred to me that having a bright orange warning flag in the middle
of an ambush predator’s lethal weapon does not make sense.
Giant snapping shrimp are large and powerful but not very aggressive
Ecological observations did not add up either. There were simply too
many snapping shrimps in some swamps. On some nights, the noise from
snapping shrimps is a dull roar, like giant raindrops falling on a
tin roof. At low tide, these areas are completely cratered with
snapping shrimp holes. As for preying on shrimps, crabs and fish, I
have watched the tide flow into snapping shrimp areas and it carries
with it very few potential prey. Other types of shrimps have eye-shine which is
easily seen in torch light and even the tiniest fish can be seen when
they move.
Stilt mangrove swamp near the seaward fringe
The entire floor of this area is pitted with snapping shrimp holes
The mangrove swamps
that fringe the open mudflats of Trinity Bay are flooded by seawater
that crosses at least one hundred metres bare sand and mud flats. Not
that many small shrimps and juvenile fish want to cross such a space - so they hide in creeks. Many
mangrove areas are more than 500 m from the seaward fringe of the
mangroves and are flooded by seawater for less than 6% of the time,
yet snapping shrimps are still abundant. The idea that snapping
shrimps are mainly predators was starting to fall apart.
Snapping shrimp holes in stunted Ceriops backswamp near a salt pan
So far, I have
discovered that snapping shrimps rarely leave their holes or even
come to the entrances of their burrows. At low tide, there are
occasional snaps. Snapping is most intense in the evening and when
the outgoing tide has exposed the mangrove floor. Waters continue to
drain from the forest floor for minutes to hours when the tide
recedes and leaves are sometimes transported in the thin film of
receding water. Snapping shrimps can be observed grabbing these drifting leaves and pulling them into their burrows. Very occasionally, it is
possible observe a leaf being cut up on the surface.
Snapping shrimp capturing a mangrove leaf
Observing the snapping shrimps is difficult as during the day, the sky reflects on the waters surface, making visibility through the surface quite low. At night, a powerful light is needed for photography and this usually suppresses snapping shrimp activity. Once I turned my
light out after scanning an area for several minutes and was greeted
with roar of snapping shrimp pops. Snapping shrimp may also
occasionally respond to snapping noises by making a pop themselves.
Popping up my camera flash makes enough noise to trigger the
occasional response.
The one time that
snapping shrimps are quiet is at high tide. Bream and other snapping
shrimp predators can be seen swimming through the milky waters and it
is too dangerous for shrimps to be active. On the incoming tide, the
shallow film of moving water that snapping shrimps need to catch
floating leaves becomes too deep and too swift only sixty seconds
after the incoming tide has arrived and they fall silent seconds
later.
In the soft muds of
the seaward edge of an accreting stilt mangrove swamp, snapping
shrimp appear to outnumber crabs. Only in the infrequently flooded
and very saline Ceriops swamps do numbers of snapping shrimp fall.
The video in this post was filmed in a Ceriops swamp. Sometimes a few snapping shrimps even
persist around the edges of salt pans. Middle areas which
have a mix of mangrove species seem to have equal numbers of snapping
shrimp and crab burrows. This unreported and extraordinary abundance
would make snapping shrimps one of the most important components of
the mangrove ecosystem.
Two tiny crabs can be seen but the glossy mud surface indicates that crab grazing is insignificant
My work in
identifying the species of mangrove snapping shrimp is ongoing. They
belong to the genus Alpheus. Alpheus strenuus and A. cf. lobidens are the leading
contenders and are found from Australia to at least South East Asia.
American mangroves have similar species.
Mangrove snapping
shrimp live in burrows that resemble a the branches of a stag horn
coral. Several sloping burrows that radiate out from a central
chamber. The central chamber can be as much as 0.5 m below the
surface and possibly even deeper. Tracing tunnel systems with ones
fingers in semi-liquid mud has its limitations. The burrows also
sometimes intersect the burrows of animals that I would not like to
grab such as mantis shrimp. I have traced several burrows in newly
colonised mangrove swamps where the snapping shrimp burrows are first
generation and have not become part of an interconnected network of
burrows. Most burrow systems have approximately a dozen entrances.
Craters may form where part of the branched burrow system has a roof
collapse. Originally, I thought the craters were by design and were
to increase the surface area water within the burrow mouth to improve
oxygenation.
Collapsing tunnels in sandy areas reveal the tunnel structure created by a single shrimp
Usually one or two small snapping shrimp is recovered
together with a goby. It appears that even mangrove snapping shrimp
have gobies as friends.
Goby and shrimp are rarely observed, the shrimp was touching the goby before the photo.
This goby was restricted to this hole for at least consecutive five days due to very low high tides.
When the tide goes
out the goby become a prisoner within the snapping shrimps tunnel
complex. In Cairns, which has two tides per day and tides are
moderately large, six days can pass between tidal flushing of the
seaward zone. That is a long time to be confined to a muddy hole the
size of a tea cup. At night, a goby can be seen in one hole in about
50. Only once have I seen a snapping shrimp antenna running over the
back a goby and I wonder if the relationship between shrimp and goby
is as tight as the relationship is for reef living species.
Goby and shrimp obtained from tunnels in a mangrove creek bank.
There is much more
to this discovery that I can report and many of the details remain
shrouded in mystery. There is much more work to be done
