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.   

Giant Semi-terrestrial Crabs

Some of the subjects I would like to write about take years to track down. Finding out about them was like solving a mystery where I get one clue at a time. Nocturnal animals in particular fit into this category as finding them and learning about their habits can be very difficult. Sometimes I do not know which animal makes the signs that I am observing so it is very hard to organise the clues I am observing into records. For this reason, special attention needs to be be given to recording animals which are rarely directly observed. I am writing my own software to make the task of piecing such information together easier. It was actually the creature described below that gave me the idea to do so.

The mystery burrow

Not far from home and close to the path to the beach, I found two large burrows that were much larger than usual and I had no idea what creature made them. Each time on my way past, I would watch the holes for signs of life. Just once, I saw a stilt mangrove dropper slowly disappearing down the hole, so now I new what it ate. One day the hole was closed over and remained so for all of the cooler months, reopening only quite late in the year. A second year passed without laying sight on the creature. The hole closed over again. I bought an inspection camera with a 7 m cable and eased it down the hole. Without the ability to steer the camera, it quickly buries into the sandy side of the twisting tunnel and no images were forthcoming. I resisted the urge to just dig the creature up. In late 2015, I caught my first glimpse of the creature, a huge black boxy crab about 15 m from me-too far to see any detail. Gradually, I eased out of the crab's view and ran to grab a camera but the crab was gone when I returned. A few months passed and I was just walking to the beach and cast a glance in the direction of the holes and there it was. This time, I stood frozen, which induced the crab to freeze and I asked someone else to run for the camera. In the weak light of late afternoon and with a superzoom camera at full length, I finally got enough detail to find out what this creature was. It was a giant oceanic land crab (Cardisoma carnifex), which can be 125 mm across the carapace – making it the second largest crab in the mangroves after the mud crab.

Repairing burrow after heavy rain. (Click to enlarge)

Same crab - close up.

There was no record of this species on the Australian mainland in the Atlas of Living Australia, although there is a brief description of the species being present in a book on crabs by by Peter Davie at Queensland Museum so they are known from the mainland. It is certainly not common. Despite searching similar habitats through the area, I found no other similar holes although I did have some near misses with some very large feral pigs whilst exploring some remote backswamps. Apparently these crabs are present in select places around the Cairns/Port Douglas area. I had privilege of claiming the first mainland entry into the Atlas of Living Australia!

The species is one of the dominant life forms on many Pacific and Indian Ocean Islands, where they are an important source of food for local peoples (Lairo seems to be the recognised common name). Yet despite its importance, very little accessible information is available about its ecology. Some additional information is available from the following link. http://www.oocities.org/ericdemuylder/gecarcin.htm

Update February 2016
Two years ago, there was single crab hole, last year there were two holes becoming three by the end of the year.  Now there are twelve holes in the colony.  I am pretty sure that the crabs have only a single hole as sometimes I see the crab above at its nearest neighbour as the same time.  I have not found any other colonies, even in places where the crabs have been reported by others.