MANGROVES AND THE PLANTS AND ANIMALS THAT LIVE THERE

MANGROVES

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Mangroves are generally small scrubby trees supported by prop roots. There are many species of mangrove plant. Though mangrove species often look the same or similar, they are often not members of the same family. Many come from different families not even closely related. Different mangrove species are simply plants that came up with the same strategy to survive in a specific environment as plants in the desert have. [Source: Kennedy Warne, National Geographic, February 2007; John P. Wiley, Jr., Smithsonian magazine]

Mangroves are essentially terrestrial plants that have adapted themselves to living in salt water and mud saturated with hydrogen sulfide (the chemical that produces the rotten egg smell) and salt and is rich in organic matter (up to 90 percent) but deficient in oxygen.

Mangrove swamps are difficult to explore. The roots form an impregnable tangle of interlocking roots that make boating through them impossible. Sometimes the roots are covered with a variety of sea creatures and can be as colorful as reefs.

Mangrove swamps are easiest to explore on foot at low tide. But even then making your way through them is no piece of cake They are often covered by barnacles and shells that cut hands and legs. The mud can suck off shoes, stick to the body and swallow people up to their knees. The air is humid, full of mosquitos and the smell of decay and rotten eggs (swamp gas).

Mangrove forests provide vital habitat for endangered species from tigers and crocodiles to rare humming birds the size of a bee. Kennedy Ware wrote in National Geographic, “Forest mangroves form some of the most productive and biologically complex ecosystems on Earth. Birds roost in the canopy, shellfish attach themselves to the roots, and snakes and crocodiles come to hunt. Mangroves provide nurseries for fish; a food sources for monkeys, deer, tree-climbing crabs... and a nectar source for bats and honeybees.”

Mangrove Areas

Nearly 75 percent of the coastlines in the tropics (between 25 degrees north and 25 degrees south) have some kind of mangrove covering. Although most are found within 30 degrees of the Equator some hardy varieties such as those found in New Zealand have adapted themselves to temperate climates.

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Mangrove areas worldwide

Mangroves are most prolific in Southeast Asia, where they are thought to have originated, with the largest total area of mangroves in Indonesia. The Indo-Pacific mangroves are generally richer in species and dense growth than mangroves found elsewhere. In parts of Sumatra mangroves are marching into the sea at a rate of 115 feet a year; in Java advance rates of a 180 feet a year have been recorded. There are 60 species in the Indo-Pacific region compared to only 12 in the New World and three in Florida (the red, the black and the white).

Mangroves in the Asia-Pacific region are harvested for wood for paper. They are also excellent land builders. Their interlocking roots stop sediments from traveling out sea and instead cause them to settle around the mangroves. As mud accumulates on the seaward side of a swamp, mangroves advance and claim it using special seeds that germinate while still hanging from a branch. The seeds sends down green spear-like shoots which may up to 40 centimeters long. Some aboriginals in northern Australia believe their primal ancestor used mangroves to walk across the mudflats to bring trees into existence.

Mangroves, Tides, Freshwater and Saltwater

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Kennedy Ware wrote in National Geographic, mangroves are “brilliant adaptors. Each mangrove has an ultrafiltration system to keep much of the salt out and a complex root system that allows it to to survive in the intertidal zone. Some have snorkel-like roots called pneumatophores that stick of the mud to help them take in air; other use prop roots or buttresses to keep their trunks upright in the soft sediments at tide’s edge.”

Mangroves survive in the salty, brackish water with various kinds of safeguards: membranes that prevent salt from entering the roots, glands on the leaves that secrete salt or move it to leaves that are about to fall off. These adaption help mangrove carve out a niche for themselves where other plants can't grow.

Different mangroves deal with salt water incursions in different ways. Those that move it dying leaves carry the salt water through the stems and deposit it leave salt ready to fall off a die. Those that have glands on their leaves secrete it in concentrations that are 20 times stronger than the sap and stronger than saltwater. Saltwater is damaging to plants and every effort is made to conserve freshwater. The leaves contain mechanisms similar to these found in desert plants to prevent evaporation

Salt marshes and mangrove forest have traditionally served as filters between land and sea. Mangroves have to deal with high tides that swamp the plant and low tides that expose the roots and deal with water that can range from almost completely fresh to completely salty. Currents deposit and remove mud. Some mangroves can live on dry land away from salt water.

Mangroves, Oxygen and Global Warming

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Mangrove roots, like those of other plants, need oxygen. Since estuarine mud contains virtually no oxygen and is highly acidic, they have to extract oxygen from the air.

Mangrove roots extract oxygen with above-ground, flange-like pores called lenticels, which are covered with loose waxy cells that allow air in but not water. Some species of mangrove have the lenticels on their prop roots. Others have them on their trunks or have pneumatophores (fingerlike projection that grow up from the organic ooze). A single large tree such as “Sonneratia alba” can produce thousands of rootlike snorels that radiate out in all direction.

Mangroves sit like platforms on the mud. Their roots are imbedded in the mud just deep enough so plants don't wash away. The areal roots also spread out in such a way that act like buttresses.

Scientists have determined carbon inputs and outputs of mangrove ecosystems by measuring photosynthesis, sap flow and other process in the leaves of mangrove plants. They have found that mangroves are excellent carbon sinks, or absorbers of carbon dioxide. Research by Jin Eong On, a retired professor of marine and coastal studied in Penang, Malaysia, believes that mangroves may have the highest net productivity of carbon of any natural ecosystem. (About a 100 kilograms per hectare per day) and that as much as a third of this may be exported in the form of organic compounds to mudflats. On’s research has show that much of the carbon ends up in sediments, locked away for thousands of years and that transforming mangroves into shrimp farms can release this carbon dioxide back into the atmosphere 50 times faster than if the mangrove was left undisturbed.

Achin Steiner, United Nations Under-Secretary General told the Times of London. “We already know that marine ecosystems are multitrillion-dollar assets linked to sectors such as tourism, coastal defense, fisheries and water purification services. Now it is emerging that are natural allies against climate change.”

A United Nations task force on mangroves and the environment recommending: 1) setting up a blue carbon fund to help developing countries to protect mangroves as well as rain forests; 2) place a value on mangroves that takes into consideration their value as carbon sinks; and 3) allow coastal and ocean carbon sinks to be traded in same fashion as those for terrestrial forests. Christian Nellemann, an author a United Nations report on the issue, told the Times of London, “There is an urgency to act now to maintain and enhance these carbon sinks. We are losing these crucial ecosystem much faster than rainforests and at the very time we need them. If current trends continue they [mangrove and coastal ecosystems] may be largely lost within a couple of decades.”

Mangrove Plants and Seedlings

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Puerto Rico
The plants that form mangrove forest are surprisingly diverse, There are 70 species from two dozen families, including palms, hibiscus, holly, plumbago, acanthus, legumes, and myrtle, ranging from prostrate shrubs to 65-meter timber trees.

Fully developed mangroves are very stable. The same can also be said for seedlings. Some species let their seed germinate on their root. The seedlings drop off into the soft mud when they are about two feet high and send out roots at astounding rates to establish themselves.

If the seedlings fall during high tide they can be carried a considerable distance and survive up to a year and feed and grow during that time. Floating seedling hang horizontally in the water and photosynthesize using green cells on their skin. If they float into an estuary they become vertical and implant themselves in mud.

Although the journey is treacherous floating seedling have a better chance of survival than ones that drop near its parents, where competition and crowding are fierce.

Mangroves, Coastal Areas and Humans

Preserving coastal areas and mangroves is vital to people that live in coastal areas, providing them with fish and other seafood and offers protection from storms and tsunamis. Natural coastal environments and mangroves also play a vital role in absorbing carbon dioxide and combating climate change.

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Ganges River Delta, Bangladesh India
Mangroves are useful in many ways. Many commercial important fish and crustaceans spend part or all of their lives in mangroves, which also provide a home of many terrestrial animals. Some 250-acre sections of mangrove produce four tons of shrimp a year.

Mangroves also produce three tons of organic matter per acre a year; protect shorelines from winds, waves and erosion and provide lumber, firewood, charcoal, tannins, medicines, food and alcoholic beverages. Local people do things like harvest wild honey and collect reeds for roof thatching and baskets.

Mangrove forests are vital for protesting farmland from salt water intrusion and buffeting the effects of tropical storms. The great tsunami of 2004 demonstrated how they can save thousands of lives by blunting the force of tsunami waves.

Countries need to set aside protected areas where nature is allowed to run its course without human interference. Managing these habitats is often far less expensive than repairing degraded habitats. In Asia, for example, careful management of mangroves has proved far more effective in protecting coastal areas from storms, surges and waves than man-made coastal defenses. In some places mangrove trees are being planted to create coastal wetlands that will act as a barrier against storms and the effects of sea level rises.

Threats to Mangroves and Coastal Areas

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Before shrimp farming, Honduras 1987
Coastal habitats have been lost to development, shrimp farms, fish farms and land reclamation. Run off, agriculture, overfishing, dumping of garbage, surface mining and construction all degrade the coastal environment. Untreated raw sewage, industrial chemical and other pollutants are released directly into the sea. Even in the United States this is still occurring on a grand scale.

Houses and hotels are built too close to the water. Dredging and filling have destroyed near shore habitats. Oil spills equal in size to the Exxon Valdez spill occur on average around the world every eight months. In some places the accumulation of pollutants is so bad that dead zones — areas where there is so much algae that all the oxygen is sucked out of the water making it impossible for most life forms to exist — have sprung up. There are nearly 150 of these worldwide. Even when progress is made improving water quality the improvements can not keep up with the waste produced by the increasing number of people that are migrating to coastal areas all the time.

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After shrimp farming, Honduras 1999
Mangroves are regarded by some as the world’s most endangered habitat, with more than third being lost to development between 1990 and 2000. Mangroves and coastal habitats are being lost at a rate of seven percent a year, 15 times faster than rain forests. Parts of Asia have lost 90 percent of their mangrove forests., robbing fish of a place to spawn and people protection from storms.

Mangrove forests are being destroyed to make way for fish and shrimp farms, coastal development, salt pans, port facilities, farms, golf courses and roads. The mangroves themselves are chopped up to provide chips for the production of rayon or processed into charcoal in rudimentary ovens. They die as a result of pollution, oil spills, sediments overlaid and disruption to their sensitive water and salinity balance.

Perhaps the biggest threat comes from shrimp farms, which are easy to set up in mangrove areas and provide much need jobs in poor countries, When there a choice between leaving a mangrove undeveloped for the sake of the fish and crabs and carbon-consuming tree or developing the site for money and jobs you know who is going to win out. To make matters worse, shrimp farmers typically abandon their ponds after a few years to avoid disease outbreaks and declining productivity and move to new sites, leaving behind degraded areas and plowing up new ones.

Mangrove and Coastal Zone Life

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Ferns, vines, orchids, lilies, terns, herons, plovers, kingfishers, egrets, ibises, cormorants, snakes, lizards, spiders, insects, snails and mangrove crabs thrive on land or upper parts of the mangrove plants. Barnacles, oysters, mussels, sponges, worms, snails and small fish live around the roots.

Mangroves water contain crabs, jellyfish and juvenile snappers, jacks, red drums, sea trout, tarpon, sea bass, snook, sea bass. The only sharks and barracudas are babies.

Lemon sharks give birth to live young and breed in shallows and young sharks spend their first year around mangrove swamps, feeding on small fish and crustaceans and staying shallow waters were there are less vulnerable to attacks from larger fish, especially other sharks. In the Bahamas there are large numbers of youngsters living in mangrove swamps which offer them a plentiful supply of food and few dangers than in the open sea and around reefs.

Mangroves begin the food chain by transforming sunlight into energy and food that support microorganisms that in turn support larger and larger animals. Leaves that fall in the water are broken up crabs and snails and in turn provide nutrients for other life forms.

Pieces of leave are attacked by bacteria, fungi and yeasts that break down the leaves into particles that can be consumed by protozoa and microscopic animals. They are fed on by small fish, worms, crustaceans and other invertebrates. They in turn are fed on by crabs and bigger fish, which are sometimes gobbled up by herons and eagles.

Some mangrove snails avoid being submerged by crawling up and down mangrove roots. They have an acute sense of timing and anticipate tide changes by moving up and down the roots just ahead the rising and falling water. When they tides are at their highest each months they stay at the highest perch and don’t drop down at low tide.

Mudskippers

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Gambian mudskippers
Mudskippers are small fish found in mudflats that spend a great deal of time out of water. There are the only fish that feed, court and defend their territories on land. Residing in Old World mangrove swamps and muddy estuaries from West Africa to Papua New Guinea and Australia, they spend about half their time on land and can live up to week without water. The largest species reach lengths of about 20 centimeters.

Mudskippers are somewhat similar to the first creatures that moved from the seas to land and evolved into amphibians, reptiles, dinosaurs, mammals and other terrestrial animals. Most species feed on plankton and algae. Some feed on worms, crustaceans and insects and other food and small animals they can extract from the mud.

Mudskippers can breath on both land and in the water. Like all fish they have gills. But what makes them unique are the little chambers they have outside their gills which entrap water and enables them to breath on land, sort of like a scuba tank in reverse. To breath in this way they need to regularly fill their mouths with water. They can also absorb oxygen through their skin like a frog does but to do this they need to keep their skin wet and often roll around in the mud to achieve that end.

Fiddler Crabs

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Fiddler crabs live in holes and pick up food with their pincers that deliver it to a set of hair-fringed blades that scissor back and forth in front of their mouths. One set of hairs sorts out grains of sand and mud. Another set moves potentially edible material to the mouth. Inedible material collects at the bottom of the mouth and is coalesced into a pellet that is removed with the pincer. [Source: Douglas Fox Natural History, April 2004]

Fiddler crabs are seen by the hundreds in mud flats. They make slurping noises as they take in mud, extract organic material and eject little balls. They rarely venture more than a meter or two from their burrow. Somehow in their brains they count their steps and use triangulation to figure out where they are in case they have to make a run for it to the relative safety of their burrows.

Fiddle crab life revolves around its burrow. Douglas Fox wrote in Natural History: “A crab’s most precious resource is its burrow. That’s where the animal hunkers down at hide tide, hides from birds, mates. And other crabs that leave the safety of their own burrows in search of a larger or better-positioned burrow are often the biggest threat. When a crab ventures even a few crab steps from its burrow to slurp some mud, other crabs are constantly trying to steal its burrow, forcing it to dart back time and time again to defend its home.”

In the world of the fiddler crab most everything on land level are other crabs and things that come from the sky are predators. If a dummy is placed next to a crab the crab treats it as another crab and either ignores it or tries to fight with it or mate with it. If you wave a dummy over their heads from the sky they immediately run for cover to their burrows.

Fiddler Crab Breeding

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Fiddler crab burrows
Female fiddler crabs have two pincers that are the same size. Males have one pincer like the female’s. The other is very large and conspicuously colored pink, red, blue, purple or white. The large claw looks fearsome but actually they are virtually useless in catching prey and defending the crab from predators, Its primary purpose is to attract mates.

The large claw is waved (the fiddling) by the male, often accompanied by a little dance, to signal females that he is ready to mate. Responsive females follow the male to his hole. The dance and style of fiddling varies from species to species. Some stand as high as they can and wave their claws back and forth. Other hold their claws still and jump up and down.

A group of a dozen or so male fiddler crabs may surround a female and wave their large claws, seemingly in unison. The female then selects one of the males and goes down his hole to mate. Why did she chose one and not the others when they all seem to be acting the same. Studies have shown that the victor often begins his fiddling a fraction of a second earlier than the others.

Fiddler Crab Vision

Fiddler crabs have compound eyes located on stalks that emerge from the head. Each eye is composed of 10,000 ommatidia, the individual eyes that make up compund eyes. Most are on the stalks rather than at the end of the stalk. Even where there is a clear division of shapes fiddle crabs can only make out objects only about two percent as well as humans can.

Describing fiddler crab vision Douglas Fox wrote in natural History magazine wrote: “A fiddler crabs eyes are mounted on stalks that point straight up and they command a panoramic 360-degree view. The mudflat comprises the entire outer edge of the visual field, and the arching sky dominates the middle...Unlike human vision the crabs vison is sharpest around the edges. That’s a reasonable emphasis. After all, the outer edge is where other members of the species are scuttling about: both rival animals looking to steal one’s precious burrow and females in the market for a mate. But in the great round center of the crabs visual field there is nothing but sky — and the occasional bird swopping in for a crabmeat cocktail.”

Scientists at Australia National University in Canberra studying fiddler crabs have developed a “crab camera” that mimic the vision of a fiddler crabs, giving a sky-centered “donut view” of the world.

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Fiddler crab anatomy

Image Sources: Wikimedia Commons, National Oceanic and Atmospheric Administration (NOAA)

Text Sources: New York Times, Washington Post, Los Angeles Times, Times of London, Yomiuri Shimbun, The Guardian, National Geographic, The New Yorker, Time, Newsweek, Reuters, AP, Lonely Planet Guides, Compton’s Encyclopedia and various books and other publications.

Last updated January 2012


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