Sunday, 8 June 2014

Crabs and Cyclones

Cyclone Yasi was a category 5 cyclone and possibly the strongest cyclone to make landfall in any settled part of Australia. In Cardwell, there was a storm surge of about 2 m which inundated the first rows of buildings along the coast. What I later found out was that in the uninhabited coastline north of the town, the storm surge was up to 7 m high. This had a dramatic effect on the ecosystems present. Three years later some areas have yet to begin recovery and in later posts, I will show the dead forests. In this post, I want to examine crabs at the mouth of Meunga Creek and compare it with Richters Creek which is well matched in terms of size, climate zone and coastal geomorphology.

The mouth of Meunga Creek is a strange place. I walked in but if you do this, note that the mud can be firm or quite soft and the ground is far less predictable that in healthy swamps. The stand at the mouth of the creek was completely flattened and most of the fallen trees formed a floating raft that was driven inland until it was snagged by the standing trees in the background.

Cyclone damaged mangrove swamp
First 30 m of swamp was completely flattened
Looking toward the mouth, a few low trees survived. Trees that are highly exposed often survive cyclones as they have a lot of experience weathering rough conditions. In both photos there is barely any sign of colonisation by new mangroves – 2 Sonneratia seedlings and about 5 advance Rhizophora seedlings in the entire flattened zone.

Seaward edge of cyclone damaged mangrove swamp
View toward mouth of creek from same location

Did the crabs survive this dramatic change?

The signs do not look good. An algal turf is visible in the top photo and this should not happen. Algae is fodder for a wide variety of creatures. See how flat the ground surface in the above photo is, normally the ground is like a  moonscape with small mounds and craters and is pock marked with crab holes. Surely the crabs would be back after three years? Every year waves of planktonic larvae settle so there is an almost infinite supply of colonists. Maybe this time things were different. Cyclone Yasi was very large and impacted swamps in all directions for more than 50 km. The cyclone may have killed most of the crabs in the mangroves and wiped out the planktonic larvae of 2011. Yet nature abhors a vacuum and if the habitat at Meunga Creek is good, substantial recovery should have occurred by now. There are only a few tens of intertidal crab species and each has populations in the millions so there would have been enough survivors to repopulate the shores. Further up the creek small pockets of habitat also survived the cyclone and provide nearby good habitat today so it is a mystery why are the crabs missing.

mangrove pneumatophores encased in mud
Mud balls on pneumatophores
A peculiar feature of this forest were the mud balls that had formed on the breathing roots of dead Sonneratia mangroves. It suddenly occurred to me why I had never seen mud balls like this before. Let me explain why with a photo.

Uca coarctata female feeding
Female fiddler crab feeding on material on a breathing root
The crabs are not entirely missing. Crabs are digging under fallen logs and causing them to sink into the mud, something that you can see in the photos.  Some of the mud balls on the breathing roots even have small crab holes in them.  In places there were also holes in the ground but even here things were a little strange. But first lets look at a typical patch of ground at Meunga Creek mouth.

Mud surface
The lines are feeding marks from grazing mullet but the texture of the ground is wrong, it is like suede and flies are feeding on it. There are some small crab holes but the area was notable for the absence of adult crabs. A hundred metres away in a slightly different situation, I found colonies of fiddler crabs with absolutely no adults so I am not sure what is happening. Are the crabs colonising then not making it to adulthood?

Normally every millimetre of ground is turned over many times each day by feeding crabs and molluscs. The photos below are taken from a matching area in Richters Creek.  See the feeding balls all over the ground surface.

Creek mouths are normally grazed heavily and feeding balls cover the surface
Mudcreepers graze the sloppy areas
Crabs normally defend their patch but something strange is happening at Meunga Creek. The crabs are almost holding hands. They resemble ants with masses of crabs living in a single hole. Their domain also seems to end only centimetres from their hole finishing at the point where the loosened surface is replaced by the suede-like surface. I am beginning to think that many crabs are farmers rather than gatherers. Their crop would be diatoms.

Twelve crabs live here
Whether these colonies survive and grow is uncertain. Below is a larger complex with fewer signs of life.

Mangrove trees benefit from crab holes which help to drain and aerate the soils. A lack of crabs may be inhibiting the regeneration of this area. Crabs also benefit from mangroves, many eat mangrove leaves or feed on algae growing on mangrove roots. It seems that recovery will be gradual and take a very long time.

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