Friday, 13 June 2014

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.

polyrhachis sokolova
A mangrove swimming ant - Polyrhachis sokolova 

Polyrhachis sokolova habitat
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.

polyrhachis sokolova looking for trail
Moment of truth - 1 second until the tide arrives - and still no reaction
incoming tide pouring through mangrove roots
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.

polyrhachis sokolova floating on water
Ant floating in a pool left by receding ripples
More information: David Attenborough video clip on mangrove ants.

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