Tuesday, 26 January 2016

Fauna on Floating Mangrove Debris

A few days ago, I was cruising along a rock coastline near Cairns when I realised that many of the floating objects had a passenger. Photographing these passengers is ridiculously hard as they hide on the other side of the object when you approach and everything is vigorously bobbing up and down. In smooth weather, one also has to consider that Irukandji jellyfish are likely to be present. These small jellyfish which are as clear as ice are occasionally lethal. Irukandji and their relatives hunt for copepods at the water surface in glassy weather. Irukandji jellyfish kill by inducing so much pain that people can and do die. That would not be a nice experience on a remote coastline and I fear these creatures more than sharks and crocodiles.  Tourists should go on organised tours to minimise the risk of jellyfish sting.

Rocky coastline near Cairns
I did manage to photograph a couple of creatures that one would not expect to be floating around the sea on bits of mangrove. The first creature was a sea hare and the second a crab.

Sea hare on a stilt mangrove propagule
Crab on a floating branch about 1 km from land
I really do not know why animals ride on floating objects. It would seem that the risk of being taken out to sea or getting washed up on a beach would be so high that it would almost suicidal. It obviously takes these animals some deliberate effort to find a floating object. For example, I have never found a sea hare any near a mangrove swamp, they are an animal of the sub-tidal zone. Likewise the crab has chosen to ride the branch. It is unlikely that the crab just happened to be on the branch when it started floating.  Most of the animals that associate with floating mangrove material also have planktonic larval stages so they do not need mangrove help to disperse.

One day I would like to do a proper job of photographing mangrove hitchhikers because there are so many of them. Many actually mimic common floating mangrove debris including orange leaves, specks of bark and even thin green eel grass leaves. The only thing that is not mimicked to my knowledge are mangrove flowers.   

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