Thursday, 14 April 2016

Hidden Fauna of Floating Mangrove Litter

For almost every type of floating mangrove debris, there is a marine creature that mimics it.  This post covers some of the mangrove litter mimics in the marina at Cairns in tropical Australia.  There are fish that mimic mangrove leaves, strands of seagrass and flakes of bark.  Surprisingly, not much seems to have been written about fish that mimic floating leaf litter. 

Floating mangrove litter can be surprisingly busy with many creatures being present around the same patch of litter.   The best places are where floating litter is spread out in thin lines or small patches.  Bigger patches have fewer creatures.  Perhaps they have been eaten by predators that pick over the larger patches.  Whilst mimics depend on looking like non-food objects, they would also benefit for being spread out thinly over a large area.

Leatherjacket mimicing tree bark (click to zoom in)
Pipefish that mimics floating seagrass leaves (the coarser Zostera leaves)
Mimicry is usually defined as a potential prey species closely resembling another animal species that is unpalatable or dangerous.  The term camouflage is used when an animal blends in with its environment.  However the fish shown in this post are closely resembling the appearance and behavior of discrete floating objects so I would considered the fish as mimics rather than camouflaged species. Many fish actually lie on their sides at the surface when threatened to better imitate floating litter.  This active imitation of the orientation and motion of floating objects goes beyond camouflage.

Unidentified green fish in the head down position beside a mangrove leaf
Same fish pretending to be a leaf when a herring approaches
The surface waters are also home to a number of specialised hunters that do not make use of mangrove litter.  All of these fish have excellent vision and can see predatory birds through the water surface and they relay mainly on speed rather than blending in.  Garfish and Archer fish are abundant. These large fish prey on small fish in the surface waters.

Sevenspot Archerfish checking out a black mangrove seed that looks like a baby squid
Longtoms patrol the open waters
Tiny fish seem to find refuge in a thin layer of water held by upside down leaves against the surface.

Unidentified fish hiding on top of a mangrove leaf
Small grunters are audacious hunters that attack tiny juvenile fish even when they are hiding on top of leaves.  Grunters can zip around at such speed that nothing could escape from them and very little could catch them.  Whereas most creatures of the floating litter move as little as possible, grunters are constantly moving, stopping briefly whilst they carefully observe their surrounds for potential prey.  The largescale grunter which were hunting in the mangrove litter often passed very close to mangrove litter mimics and either did not see them or avoided them on the basis that they were too big or too heavily armoured (scats and pipefish).

A grunter trying get to the garfish on the leaf
Same 50 mm long grunter hiding behind a mangrove flower
Tiny squid are abundant in the floating litter and larger squid often patrol just below.  Squid are predators and aggressively attack fish and even tiny squid would be a threat to juvenile fish.

Baby squid preparing to attack 
A bizarre creature that looked like a small section of tree branch covered by rough bark was flapping its way along like a hatchling sea turtle.  It was just an Estuarine Stonefish, but you don’t expect to see those feeding on the surface.  Scientific advice from a scorpion fish researcher is that stonefish do not have swim bladders so how it was managing to float was unclear.  It was however feeding.  In cases like this, a second observation of a different individual behaving in the same way is needed to establish that the behaviour could be normal behaviour for the species and not just an observation of an individual where something is wrong.    

Estuarine stonefish feeding on the surface
Mangrove herons also take a professional interest in the fauna of floating mangrove litter.  

Mangrove heron on floating walkway

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