Mangroves #2: wildlife on the mud

Last week I introduced the mangroves, a rich habitat fringing the sea throughout much of the world's tropics and subtropics. Now I want to introduce a few of the animals that live there - and there are very many. So many in fact that I've decided to split this post in two, focusing today on animals feeding down on the mud, and next time looking at those who are mostly above it. Some are tied to the mangroves, while for others the mangroves are important but they also stray beyond them into associated habitats. The proliferation of mangrove boardwalks meandering though the forests, above the mud and high tide, are a great boon to those of us who enjoy watching the life that teems in them. And as I suggested last time, boats are excellent too. Virtually all of these photos were taken from one or other of these vantages.

At low tide, the rich muds of the mangrove forests are alive with crabs, emerging from their burrows to scavenge for detritus that washes in with the tide or falls from the branches above. In turn they are preyed on by many birds, crocodiles and some fish; they are a key part of the mangrove ecosystem. Here are a few, starting closer to home at the Cullendulla Nature Reserve near Batemans Bay, with an excellent boardwalk.

Purple, or Mottled Shore, Crabs Paragrapsus laevis; this one is found from
Tasmania and Melbourne up to north of Brisbane.

Some of the most striking of the mangrove crabs are the fiddlers, in which the males have developed one huge claw, useless for feeding, so he must work twice as hard (or for twice as long) as a female to get enough food. They fight either wandering strangers or encroaching neighbours, and wave the big claw to attract wandering females. If she's impressed - and presumably size matters for both purposes - she follows him into his burrow, which he then seals from the inside. Next day he emerges, but she is sealed in again for a couple of weeks while the eggs develop. On a suitable very high tide she releases the tiny babies to be washed out to sea to develop, before they return to shore to either fiddle or watch the fiddlers.

Orange-clawed Fiddler Crab Uca coarctata, Mission Beach.
The little mud pellets have been spat out by the crab after the goodies within them have been separated and eaten.

Orange-clawed Fiddler Crab males signalling, Centennial Lakes, Cairns

And here are some overseas mangrove fiddlers - all fiddlers are Uca spp., but that's as far as I can go.

Males duelling, Bako NP, Sarawak, Malaysian Borneo.

Santiago, Galápagos.

Flame-backed Fiddler Crab female Uca flammula, Mission Beach Queensland; note how 'normal' her claws are!

Fascinating as the fiddlers are, they are far from the only mangrove crabs.

On the left is a Semaphore Crab Heloecius cordiformis; they too signal with their claws.

Hermit Crab meandering among the pneumatophores, Bako NP, Sarawak.
Like all Hermits (more than a thousand of them, in the superfamily Paguroidea) this one has a soft curved
abdomen, which it protects by reversing into an empty snail shell.

And finally a couple that I can't identify, but who are too handsome to omit on that count!

Mission Beach, north Queensland.

Manglares Churutes NP, Ecuador.

As I mentioned, crabs feed a lot of other animals in the mangroves, every time the tide goes out.

Common Sandpiper Actitis hypoleucos with crab, East Point Mangroves, Darwiin.

Mangrove Skinks Emoia atrocostata don't eat crabs, but they hunt across the muds and sands of mangroves
and nearby habitats, even swimming after prey in tidal pools, across south-east Asia and the Western Pacific,
including Queensland. This one was in Bako National Park, Sarawak.

And much bigger reptiles spend a lot of time in mangroves too, hunting at high tide, and just basking on the mud at other times.

Estuarine Crocodile Crocodylus porosus, Kinabatangan River, Sabah.

Crabs are not the only invertebrates evident in the mangroves of course.

Snails feeding on the detritus in the mud, Pulau Tiga, off the east coast of Sabah.

Also on the mud you are very likely to encounter mudskippers, fascinating fish which spend some of their time out of the water, perhaps to avoid predators. There are 30 or so species in the family Oxudercidae which have adapted remarkably to the amphibious lifestyle. They can move around quite readily on jointed fins, and store an air bubble in their gill chambers, which they seal when ashore. Moreover they can absorb oxygen through their skin and mouth surface, like a frog, but must stay wet. With these adaptations they feed, hold territories and perform courtship rituals on the mud.

Barred Mudskipper Periophthalmus argentilineatus, Darwin.

Mudskippers, Bako NP, Sarawak.

When I watch the mudskippers - and they are most watchable! - I feel as though I'm watching life coming ashore some 375 million years ago, to begin a journey that would transform fish into amphibians, and later into dinosaurs, lizards, crocodiles, birds and mammals, including of course us. What I am really watching though could well be the first stages of a quite separate line of land vertebrates that will develop in the distant future. Or of course they might be quite content with their life between sea and land, in the mangroves.

NEXT POSTING THURSDAY (to finish this series)

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