Mangroves 3: Mangrove as Habitat

MANGROVES 3: Mangrove as Habitat

Mangroves at high tide, Channel Island (Michael Michie)

Mangrove as a habitat during high tide

The intertidal zone is an interesting habitat whether it is on beaches, rocky shores or in mangroves. The animals and plants that live there have the problem of coping with an ever-changing environment, and the changes are from one extreme to the other. At one stage the habitat is flooded and marine animals such as fishes can move about freely. Six hours later it is dry and exposed to the sun.

During periods of inundation the mangroves are home to a large range of sea life, particularly the early life stages of fish and crustaceans. Many of these early stages of marine life help to make up the plankton which floats in the water and provides food for small fish like Hardyheads () and Anchovies (). The plankton is also taken in by filter feeders such as oysters and barnacles attached to the mangrove roots. A carnivorous shellfish, the Thai or Mulberry shell (Thais spp.), feeds on the oysters and other molluscs of the mangrove. The Thai also preys on the lined nerite which is a black snail-like mollusc that scrapes algae off tree roots and rocks

Strong direct sunlight and warm temperatures help the mangroves to produce an abundance of foliage, and their productivity is almost in the same class as that of rainforests. The leaves and other debris fall into the estuary and are the main source of energy at the beginning of mangrove food chains. The decaying leaves of the mangroves break up into very fine particles which along with other organic matter (waste material and dead animals of all sizes) make up the detritus, a fine silt on the sea bed. Detritus provides food for young prawns and other small crustaceans such as isopods. Some polychaete worms live in tubes in the mud and spread out tentacles like spaghetti tubes to catch small particles of food caught up in the detritus. Diamond Mullet (Liza alata) feed on algae which may be attached to rocks or contained in the detritus. Cuttlefish feed in the mangroves at high tide and larger fish - Mangrove Jacks (Lutjanus argentimaculatus), Threadfin Salmon (), Barramundi (Lates calcarifer) and Stingrays () - seek out the small fish, crustaceans and worms in the rich waters.

The Mud Crab (Scylla serrata) is active during high tides. When the tide goes out it burrows into the mud with only its eyes protruding. Other crustaceans include the Mud Lobster (Thalassina anomala) which lives in burrows, and the Pistol Shrimp, so named because of the noise it makes when it snaps its front claw.

Mangrove as a habitat during low tide

Mangroves at low tide, Channel Island (Michael Michie)

When the tide is out, the mangroves become a hunting ground for a different set of animals. Some, like the Egrets (Ardea sacra, A.alba, A.intermedia and A.garzetta) stalking the water's edge on the mud flats are visitors. Other animals, permanent residents, emerge from their shelters where they hid at high tide. The most noticeable are the Fiddler Crabs (Uca spp.) that live in burrows in the mud. The male Fiddler Crab has one brightly coloured claw enormously enlarged which the crab waves in courtship displays. Fiddler Crabs extract algae and other organic matter from the detritus. Less conspicuous are the dull Sentinel Crabs which may occur in large numbers on the mud flats beyond the mangroves. The larger Mangrove Crab (Sesamia spp.) lives at the landward side of the mangrove forest and is responsible for the crab burrows so characteristic of this habitat. They are about 40 mm in width (i.e. across the carapace or shell) and are scavengers and predators.

Mudskippers are commonly seen around the mangroves at the time of low tide. These unusual fish vary in size from 50 mm. to 250 mm. The smallest Mudskippers (Periphthalmus spp.) eat insects, spiders, small crabs, amphipods (beach-hoppers) and marine worms. They measure about 50-100 mm in length and live in short burrows which they dig with their teeth.

Slightly larger species of Mudskippers (Boleophthalmus spp.) are found where the mud is semi-liquid over a firm base. They eat by straining through the soupy mud, moving their heads from side to side as they filter out fine algae, tiny worms, diatoms and small crustaceans. These mudskippers have the largest and most colourful dorsal fins and have an extensive underground system of tunnels.

The Giant Mudskipper (Periphthalmus spp.) is up to 250 mm in length and lives near the top of the steep mangrove banks, catching crabs and other mudskippers as food. It is a wary animal and not easily approached.

Reptiles are represented amongst the permanent residents of the mangrove habitat. The Bockadam (Cerberus rynchops) is a water loving snake and does not like to crawl onto open spaces of mud, preferring to swim across shallow pools close to the bottom when disturbed. It is about 120 cm long and may be coloured light grey, reddish brown or olive with irregular cross bars. The Bockadam eats mudskippers, crabs and fishes that become stranded at low tide. It has poisoned upper rear fangs but is not thought to be dangerous to humans. Another reptile is the Mangrove Monitor (Varanus indicus). It has a black colouration with yellow markings on its back and side. The throat and belly are a bright yellow. The mangrove monitor hunts fish, crabs, eels, small mammals, insects, reptiles and birds. It is equally at home in the water or in the dense foliage of the mangroves.

The Little File Snake (Acrochordus granulatus) and the White-bellied Mangrove Snake (Fordonia leucobalia) are two other common reptiles of the tropical mangroves. Another reptile, the Saltwater Crocodile (Crocodylus porosus) is the largest predator in the mangroves and is the final link in many tropical estuary food chains.

Mangrove islands in the estuaries of the tropical north are favourite places for large colonies of Black Flying-foxes (Pteropus alecto). These animals like roosting in high trees with dense foliage, and the mangrove is an ideal place of refuge. At dusk thousands of flying-foxes stream out of the camps, and individuals may travel to as far as 50 km in search of food. The flying-foxes have another link with the mangroves, for they are known to assist in the pollination of the species Sonneratia alba.

Three species of kingfisher are often seen in the mangrove forests. The Forest Kingfisher (Todiramphus (Halcyon) macleayii) has a rich blue colour for its wings and back and white underparts. The Azure Kingfisher (Alcedo azurea) is smaller and has tan to rufous underparts. The smallest is the Little Kingfisher (Alcedo pusilla) which has a white spot before its eye and on the side of its neck. It has a white throat and belly.

Other birds of the mangrove habitat include the Red-Headed Honeyeater (Myzomela erythrosephala) and the Mangrove Golden Whistler (Pachycephala melanura). The migrant waders are represented by the Eastern Curlew (Numenius madagascariensis), the Terek Sandpiper (Tringa terek), Common Sandpiper (Tringa typoleucos) and the Broad-billed Sandpiper (Limicola falcinellus), all well adapted to the wet muddy conditions of the mangroves.

The Eastern Reef Egret (Ardea sacra) and the Intermediate Egret (Ardea intermedia) both find the mangrove habitat suited to their needs and they are conspicuous members of the bird populations. Not so conspicuous are the Black Bittern (Ixobrychus flavicollis) and the Striated, or Mangrove, Heron (Ardeola striata), both of them experts at freezing in a head-up position to escape notice. These birds have unobtrusive coloration and are camouflaged with patterns of broken stripes and streaks. 

1. Mangrove as forest 2. Mangrove as plant   4. Mangrove as resource 5. Use by Northern Aboriginal Clans 6. Activities

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