MANGROVES AND SEAGRASS BEDS (TROPICAL SEAS)
Background information – by edgardowelelo@yahoo.com
Mangroves and Seagrass beds are important habitats not only protect vulnerable coastlines, but also support an unusual and unique flora and fauna. Dense and usually impenetrable, mangrove swamps are atmospheric places that often fringe shallow tropical seas. They (mangroves) provide an important barrier to the destructive forces of the ocean and they form unique habitats for wildlife. Being less colorful and diverse than CORAL REEFS, they attract less attention, but they are no less important to the diversity of life in the TROPICS. SEAGRASS BEDS, which can stretch for miles along shallow, sandy flats, similarly act as a BUFFER between ocean and land, protecting vulnerable shorelines against erosion. Both MANGROVE SWAMPS and SEAGRASS BEDS not only provide permanent homes to many specialized plants and animals, but also act as nurseries for juveniles that will spend the adult lives out on the REEF or in the open sea, and as havens or hunting grounds for passing visitors.
MANGROVES (Salt – tolerant evergreen trees and shrubs of tropical coastlines)
To visit a mangrove swamp is to step into an unfamiliar world. The occasional shafts of sunlight that penetrate the canopy illuminate a gnarled mass of impenetrable roots below. The air is humid, the water still. Beneath the surface lies a stagnant soup of stinking mud. At first glance, the only signs of animal life are the swarms of mosquitoes ready to plague the visitor. But for those who persevere, mangroves are fascinating places to explore.
Inter – Tidal Forests
Mangroves are TROPICAL TREES that grow in estuaries and on muddy coastlines, where fresh water meets sea water. Here they (mangroves) can form INTER – TIDAL FORESTS. Their most striking difference from typical LAND FORESTS is that they (mangroves) have no ground vegetation (except the developing young mangrove plants themselves) and are made up of relatively few species. There are only 40 mangrove species in the most diverse TROPICAL REGION, THE INDO – PACIFIC, and eight in the ATLANTIC. Tropical timberlands, by contrast, can boast 100 or more species in a single hectare of forest. Mangroves are largely tropical, their distribution being governed by SEA TEMPERATURE: they generally avoid WATER COLDER THAN 200C (680F). Their distribution is therefore not delineated strictly by latitude, but by the presence of cold or warm currents, rather like CORAL REEFS. For example, the cold Humboldt Current running up the west coast of SOUTH AMERICA results in mangroves having a southern limit of only 300S (the PERU/ECUADOR border), whereas on the east coast, water from the tropical Atlantic moves southwards and mangroves are found as low as 330S (the BRAZIL/URUGUAY border). Mangroves are made up of an assortment of unrelated trees and shrubs, which have adapted to the inhospitable habitat between land and sea. They are able to survive in oxygen – starved, waterlogged soil and can also tolerate salty or brackish water. So how have they evolved to cope?
ROOTS AND SALT
Like any other plant, mangrove trees have ROOTS that perform the essential functions of absorption, transport and structural support. The living tissue of roots needs air for oxygen, which is found between soil particles on dry land. But in waterlogged mud there is no air, so the roots grow upwards in various ways, or branch off from the trunk high above ground level. These flying buttresses form a network of shallow roots that provides the necessary support and anchorage. The air passing from the ROOTS above the mud to those lying beneath enters through numerous pores called LENTICELS, then passes along the ROOT through honeycombed, sponge – like tissue. Most of the air circulates by diffusion, but as the tide rises and falls, it seems that grass exchange is enhanced. Under water during high tide, the use of oxygen by the root’s living tissues creates a negative gas pressure. The LENTICELS do not allow water in, so the negative pressure is maintained. But when the tide falls and the LENTICELS are exposed again, they draw in air to equalize the pressure. Mangroves grow in salty places and have evolved three main ways of dealing with excess salt: they restrict how much gets into the ROOTS in the first place, they have adapted to cope with high salt levels in their tissues; and they get rid of excess salt. Some plants use one method, others two and some all three, depending on the saltiness of the place they are in. They are very good at it, and certain mangroves can keep out 97 per cent of salt. Most mangroves rid themselves of excess salt by excreting it on to the surface of their roots, trunks or more commonly on their leaves. Lick a mangrove leaf and you will find it very salty: you can even see the salty crystals lying on the surface. Mangrove trees grow taller where there is more fresh water; but in freshwater habitats they are out – competed by other plants. As a result, they are obliged to eke out a living in the harsh intertidal world.
ANIMALS OF THE MANGROVE
Hard as it is for mangrove trees to thrive, once established, they do provide a secure home and in some cases food for creatures that live in the canopy, in the water or in the mud beneath. The relationship between the trees and the animals varies, being beneficial to the trees in some cases, but not in others.
Creatures Great and Small
Mangrove forests provide a home for all manner of land – based creatures. Insects live among the branches and in the canopy, ants colonize the trees, spiders weave webs and these smaller creatures attract larger ones. Birds, either resident or migrant, may depend on food housed in the swamp and may build their nests there. Pigs, deer and antelopes graze in the forest, and monkeys, mongooses, racoons, otters and even tigers use mangroves as hunting grounds. All these animals help with nutrient recycling in the forest, and all leave their droppings, which fertilize the swamp. The extensive underwater root system provides an excellent hard substrate to live on in a sea of soft mud. The roots of mangroves are often smothered in encrusting animals, such as BARNACLES, OYSTERS, MUSSELS, ANEMONES, SPONGES, TURNICATES, ANNELID WORMS, HYDROIDS (SEA FIRS) and BRYOZOANS (SEA MATS). Filter – feeding animals, such as BARNACLES, need the support of a root to keep them up, out of the mud so that they can extract food from the water. But if their growth is unchecked by predators, barnacles can cause problems for the mangrove plant by growing over LENTICELS and so reducing gas exchange. Conversely, if an underwater root has a good covering of fouling organisms, such as sponges; it is protected from attack by more destructive root – boring or wood – boring animals. Shipworms, piddocks and Sphaeroma crustaceans bore into the root systems of mangroves and can seriously inhibit growth.
Crabs
There are two types of crab found universally in mangroves and they come from related families: GRAPSID CRABS and FIDDLER CRABS. The grapsid crabs spend high tide under water in their burrows avoiding predators. When the tide recedes, they emerge to forage on the mud and the exposed roots of mangrove trees. Many of them are HERBIVORES and they occur in extraordinary numbers. Densities of 70 crabs per sq m (11sq feet) are common, so they can have a significant effect on the mangroves. Luckily for the trees, many of the crabs feed on old, decayed leaves because the tannins of fresh leaves are indigestible. They will, however, carry newer leaves to their burrows and bury them until they become PALATABLE. Many grapsid crabs feed on the mangrove’s unusually large seedlings. In order to ensure successful reproduction in such a testing environment, mangrove trees have to invest more than other plants in their seeds and fruits. The seed does not leave the parent plant, but stays attached and is actually nourished by it, receiving water, nutrients and even food. The seed germinates and grows into a seedling called a PROPAGULE. During its development, the parent plant regulates the amount of salt the seedling receives, so it can gradually get used to the high concentrations it will experience later on. This high investment in the seedlings gives them a better chance of survival when they drop off into the mud. However, they are also a convenient food source for grapsid crabs. Crabs are as fussy about the seedlings they eat as about the leaves they select. Those with a high sugar content and low tannin and fibre content are the favourites. If seedling – eating crabs are abundant in one part of the mangrove, fewer of their preferred seedlings will succeed in becoming established as trees, so crabs play a part in determining which type of mangrove trees are found where in the swamp.
FIDDLER CRABS
Brightly coloured and best known for the males’ enlarged claw, used in displaying and fighting, fiddler crabs have burrows all along the shore and can reach densities of 60 crabs per sq m (11sq feet). They emerge from their burrows as the tide recedes and immediately start foraging on the mud. Fiddler crabs are deposit feeders. They scoop up sediment from the shore with their claws and sort out particles of food from the grains of sand or mud using specially designed mouthparts, which act like spoons, buckets, sieves and brushes. Different species of crab are adapted to sifting different types of sediment: some species cope better with grains of sand, while others have smaller and finer mouthparts to deal with fine particles of mud. Fiddler crabs do not waste their time. If they’re not getting enough food for the energy they are expending on feeding, they will move on to a more productive part of the shore. Their foraging hours must be as efficient as possible because as time goes by, the mud dries out and becomes too difficult for them to process. When it does, they begin to socialize. Males use their enlarged claw to establish and maintain territories by displaying to other males and also, of course, to attract females. Ritualized displays, which involve claw – waving and bobbing up and down, are common between males and probably reduce the necessity to engage in serious fights. Nevertheless, fights do occur from time to time. Fiddlers cannot roam at random over the mud fighting and looking for food. They require a constant source of water to keep their gills suspended and their bodies supplied with oxygen, so they frequently rush back to their burrows to replenish the water they carry around with them. This is one reason why their burrows are so fiercely guarded. Remarkably, both grapsid and fiddler crabs can cope with very low oxygen levels in the burrows during high tide. They simply build up an oxygen debt, and then, when the tide goes out, they come up into the air so that the water over their gills has a fresh supply of oxygen to absorb. Crabs running around on the mud or hiding in burrows are clearly vulnerable to predation. During high tide, fiddlers’ burrows are closed with a plug of mud, but at low tide, when they constantly dash in and out of their burrows, they are most at risk. Monkeys, kingfishers, herons, snakes, fish and other crabs are just some of the animals on the look – out for tasty fiddlers. One fish in particular, a mudskipper called Periophthalmus, is particularly fond of crabs.
MUDSKIPPERS
The semi – aquatic mudskipper is an odd – looking fish found in Indo – Pacific mangroves. Some spend the high tide in burrows under water, while others retreat up the shore and even up the mangrove trees to avoid drowning as the tide comes in. As their name implies, they are mostly seen skipping across the mud when disturbed, using their long supple tails as a spring. Their specially adapted pectoral fins vary between species, some using them like crutches, others like suckers to climb trees. Periophthalmus lives on mud among the trees, is carnivorous and highly amphibious. Other mudskippers, such as Boleophthalmus, a type of goby, are deposit feeders. They live lower down the shore away from the trees and sweep their heads across the mud scooping up mouthfuls. Like fiddler crabs, they sort food from grains of mud or sand in their mouth, then get rid of the water and sediment they do not want. Boleophthalmus does not avoid the rising tide, but spends high tide in a burrow under water and has gills more like that of other gobies. Like crabs, mudskippers are often seen returning to their burrows, to prevent their skin drying out under the tropical sun. As gills are not very effective in the open air, where many mudskippers spend a great deal of time, it makes sense for them to get oxygen in another way. Mudskippers have skin that is full of blood vessels, particularly at the front of the head. Periophthalmus, which spends more time exposed to air than Boleophthalmus, depends more on its skin for its oxygen supply.
OTHER MANGROVE ANIMALS
Many marine animals, including catfish, mullet, cardinalfishes and numerous juveniles of reef dwelling or pelagic fish, can invade mangroves only when the tide is high. When the water is out, these creatures can be found concentrated in the creeks and pools that lace the swamp, but, of course, they attract predators. These include the estuarine crocodile, whose distribution ranges from India through Southeast Asia to Fiji and Australia. Where left undisturbed and not hunted, these crocodiles can grow up to 7m (23 feet) or so in length. However, the saltiness of mangrove water does pose a problem for them. Their largely impermeable skin helps to keep the salt out, and they are careful to drink only fresh water when they can. But inevitably, when they eat, they take in salt water with their food. Along with sea snakes and other reptiles, crocodiles have evolved ways of controlling the salt content of their bodies. In their case, excess salt is excreted through salt glands on their tongue.
HIDDEN SNAILS
The most abundant snails in mangroves are PERIWINKLES. They live above the water line, different species being adapted to live in different parts of the tree and in different zones of the forest. Some live on the roots, trunk and branches, grazing the blanket of algae growing there. Others live in the canopy, and prefer to live near land where there is more fresh water. Their shells are coloured and constructed according to where they live. Snails living low on the tree’s roots and branches have darker shells, so they are better camouflaged, thicker shells to protect them from predation by crabs, and the entrance to their shell is rounded to fit the curve of the roots. Those living in the canopy can afford thinner shells with a flat opening and they are typically lighter in colour. Snails eat most of what is available in the mangrove forest. Some get food from the seabed or the mud by scraping the surface of DETRITUS. Herbivorous snails eat dead and fallen leaves or graze on algae. Predatory snails, such as Thais, are found in mangroves throughout the world feeding on BARNACLES.
SEAGRASS BEDS
SEAGRASS BEDS occur on shallow, sandy plains that often neighbour CORAL REEFS and MANGROVE FORESTS. These huge meadows, which carpet the SEA FLOOR, house many creatures, most of which are either buried in the sand, or are well camouflaged against the green of the SEAGRASS.
FLOWERING PLANTS
Most plants in the OCEAN have no root system and do not produce flowers: they are known as SEAWEEDS or ALGAE. Those that do produce flowers are called SEAGRASSES. There are only 50 PLANT SPECIES that have managed to adapt to a salty, underwater life yet still retain the ability to reproduce using FLOWERS. In some seagrasses, threads of pollen are released by male plants and carried by water currents to a female plant, which is then fertilized. Other seagrasses have FLOWERS that contain both male and female organs, a common occurrence in land plants.
Seagrasses also have an extensive underground network of roots. These are important in anchoring the plant to the ground, as well as for absorbing nutrients from the sediment. But oxygen for the plant is obtained from water absorbed through the leaves. Seagrass leaves are typically flat and brade-like with a high surface to volume ratio. This maximizes the diffusion of gases and nutrients into the leaf from the water, and also exposes the largest possible surface area of leaf to the sun for photosynthesis. As seagrasses have no STOMATA (small openings) in the leaf as land plants do, their leaves have a thinner, protective outer layer of cutin, as well as a sponge – like interior to help speed up gas exchange. Despite this, in the afternoon, when photosynthesis and therefore oxygen production is at its peak, the leaves of Thalassia testudinum swell up 250 per cent and become oval in shape because they cannot get rid of the oxygen quickly enough.
SEAGRASS INHABITANTS
SEAGRASSES often form extensive beds or MEADONS, which can be very important to shallow tropical seas of the CARIBBEAN and the INDO – PACIFIC for a number of reasons. One of the key roles of seagrass beds is in preventing coastal erosion. The root systems stabilize and hold SEDIMENTS on the SEA FLOOR even during hurricanes and storms, and the swaying action of the leaves slows the rate at which water moves over the seabed, thus buffering the effect of wave action and water currents. Seagrass meadows also grow quickly, some plants extending by as much as 5-10 mm (0.2-0.4 in) per shoot per day, so they are a valuable source of food for herbivores, such as SEA URCHINS, FISHES, TURTLES and DUGONGS (NGUVA). Sea grasses also provide a safe SHELTER for many small creatures living in the SEABED or between the blades of grass. Some live there all their lives, while others are transient and spend time there only to feed, or as a temporary home while they are young. Among the latter are some fishes, lobsters and shrimp, which spend their early days in seagrass beds and their adult life in CORAL REEFS. Those that come to feed there are often found hiding on nearby REEFS during the day. The sea urchin, Diadema, is a good example of this. These urchins cannot afford to travel far from their daytime hiding places because of the risk of predation when the sun rises, so they graze a seagrass bed close to their home reef. This can result in a halo effect on the seagrass bed, the plants close to the REEF being heavily cropped while those beyond grow taller. The floor of a seagrass meadow is home to many different types of INVERTEBRATES. Worms burrow into the sediment, while starfish and conch live on the surface feeding on DETRITUS and dead blades of seagrass. Sea urchins that do stay in the seagrass beds all day and night usually camouflage themselves with bits of grass and broken shell, or bury themselves in the sediment. Thanks to the stabilizing effect of the plants on the sediment, anemones can dig themselves into the sea floor and be sure they won’t become exposed. Their stinging tentacles remain up in the water column to catch food passing by. SEAHORSES can sometimes be found swaying in the current, their tails wrapped around the blade of a plant, but they are usually extremely well camouflaged and difficult to find.
SEA COWS (NG’OMBE BAHARI)
Perhaps the most exciting animals to be found feeding in seagrass beds are SEA COWS – the dugong (Nguva) of the Indo – Pacific and the manatee of the CARIBBEAN and WEST AFRICA. Manatees, however, spend at least part of their lives in FRESH WATER, so dugongs are the only herbivorous, truly marine mammal. The most obvious difference between these two types of sea cow is the shape of their tails. Dugongs have notched, fish like tails, whereas manatees have round, paddle – like tails with no notch. The other key difference, though not so visible, is their teeth. Dugongs have tusk – like incisors, while manatees have only molars and premolars, which move forward over time and are continually replaced by more teeth from the back. Being slow swimmers, manatees and dugongs are found in the sheltered waters of LAGOONS and BAYS, rather than far out at sea. They have a slow metabolic rate, although they are reasonably well insulated, they cannot generate enough heat to tolerate cool water. Thus they are found in the WARM WATER of the TROPICS and in some SUBTROPICAL AREAS. Dugongs and manatees have dense, massive bones, which help to keep them submerged. Their lungs lie along their back and act like floats, keeping them horizontal in the water. A pair of nostrils, sealed with a flap of skin when under water, is situated near the top of the head so that they can take a breath of air with minimal effort. Their lobed lips, covered with sensory bristles, are prehensile (grasping) and pass seagrass to the mouth. Dugongs feed voraciously and can eat as much as 40 kg (88 Ib) of seagrass a day. But whatever the species, dugongs eat by grubbing up the whole plant, which leaves distinctive troughs in a meadow.
SEX AMONG SEA COWS
Dugongs are thought to use the “lek” system whereby males establish and defend COURTSHIP TERRITORIES in traditional areas where females come only to mate. In Australia, when male dugongs encounter one another; they swim in parallel or root around on the seabed facing each other. Only as a last resort will they engage in a serious fight. When the females eventually arrive, sometimes as long as three months later; the males advertise their presence and prowess by thrusting themselves as high out of the water as they can and creating a tremendous splash. A female chooses a partner and mates, and a calf is born approximately one year later. Manatees have a rather different mating system. The females, who are sexually mature at three years as opposed to the dugong’s ten, come into oestrus and are pursued by a herd of males. Up to 20 males can be jostling for access to one female and she may mate with a number of them.
UNUSUAL FISH – SEAHORSES
Seahorses are unusual fish; they move, swim, feed and reproduce unlike any other. Their distinctive snout has a powerful suck and is used to capture tiny crustaceans, fish larvae and Plankton. Like chameleons, they have independently moving eyes, which help in spotting their minute prey. A coronet adorns their head, and each is as individual as a human fingerprint. Although seahorses are FISH, they have no scales: instead they have skin stretched tight over a frame of interlocking bony plates, which are hinged so that they can bend. They propel themselves through the water by using only their dorsal fin, while their pectoral fins are used for steering and balance.
SEAHORSES AT HOME
There are 35 species of seahorses worldwide, ranging in size from the tiny pygmy seahorse, which is about 2 cm (0.75 in) long, to the eastern Pacific seahorse, which is 36 cm (14 in) long. They usually live in shallow coastal seas, such as SEAGRASS BEDS, where they can attach themselves to individual blades of grass and mangrove roots. They can change colour within a few seconds to match their background perfectly, grow tendrils which blend in with their surroundings, and allow encrusting organisms and algae to cover their skin, making them incredibly difficult to find.
REPRODUCTION
Seahorses are monogamous; maintaining the bond with their partner by a ritual dance performed every morning just after dawn. The female finds the male in his territory and they greet each other with a brightening in colour. They attach themselves to the same seagrass shoot, using it as a central pivot while they circle around it. Then, swimming closely together with their tails entwined, they move to another place for more circling. At the beginning of the breeding season it takes some time for the females’ eggs to ripen. Only after three days or so of courting, when both change colour dramatically and conspicuously, are the couple ready to mate. On that morning the male indicates his readiness by jack – knifing energetically, bringing his tail up to his body. The female responds by stretching upwards, pointing her snout at the water surface, while keeping the tip of her tail on the seabed. The pair then slowly rise together; belly to belly, the female transferring her string of sticky eggs to the males’ pouch.
PREGNANT MALES
The eggs are fertilized just after they enter the males’ pouch and soon become embedded in its wall. The pouch functions very like a mammalian womb: each embryo is supplied with oxygen, housed and nurtured. Male seahorses even release the same hormone (prolactin) as female humans, which helps to nourish the embryos. The fluid in which the embryos are suspended changes during the pregnancy so that it becomes increasingly like sea water. This prepares the young for the shock of being evicted from the pouch. Pregnancy lasts about two weeks in tropical seahorses, and during this period the female continues to arrive every morning for the greeting ritual. Apart from maintaining the bond between the pair; the ritual reveals when the male has given birth and is ready for the female’s next batch of eggs to be transferred. When the young are ready to be born, the male performs his jack – knifing contortions again, expelling the miniature seahorses from his pouch. This can take hours or even days, but from this moment on the young are totally independent. They rise to the surface, take a gulp of air to fill their swim bladders, then sink again, holding on to anything they can find with their prehensile tails. The female mates with him again that same day or the next, so males are pregnant continuously throughout the breeding season. `
