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Waterholes

From Tropical Topics newsletter, No. 68 June, July 2001, produced by Stella Martin at the Queensland Environmental Protection Agency. Download the PDF to read the whole issue.

waterlily

Many introduced plants and animals find savanna conditions ideal and are doing considerable damage to the wetlands. Flowering water lilies (Nymphaea spp) make a spectacular sight during the dry season. There are a number of species, with flowers ranging from white to pale blue and pink. Although they appear to float on the water surface, these plants are actually anchored to the bottom, up to 3 metres below. Seeds are contained within spongy berries. As they develop, the stalk forms a coil which pulls the fruit below the water, a strategy which may protect them from being eaten. Aboriginal people eat the fruits and stems raw and bake the tubers, which are considered a good medicine for diarrhoea. They also bake the oily seeds, eating them whole or grinding them to make damper.

Lotus lilies (Nelumbo nucifera), although they similar to water lilies, belong to a different family. Their immaculate leaves can grow from 50cm to nearly one metre in floating on the surface or held above the water. The short lived flowers, which are also held high above, open at first light but drop their petals by midday. They are a spectacular dark pink with dense yellow stamens. Curiously, it seems that the plants can regulate temperature of their flowers, keeping it within the 30–35oC even when surrounding temperatures drop as 10oC. This may serve to attract pollinating insects. are embedded in distinctive conical woody receptacles. These plants are an important source of food for Aboriginal people, who use them in a similar way to water lilies. They are also grown and eaten widely across Asia.

Revered as a symbol of purity in Hinduism Buddhism, the lotus has leaves which never get dirty. The trick to this has recently been discovered by German scientists and adapted to create self cleaning paint for building exteriors and roof seems that the secret is a combination of chemical repulsion and the bumpy, waxy surface of This provides no flat surface for water to cling each drop rolls off. Dirt is more strongly attracted the surface tension of the water than to the is 'captured' by the water and carried away.

Water snowflake (Nymphoides indica) is one of the most commonly seen aquatic plants, easily recognised by the delicately fringed petals of its white flowers. The related wavy marshwort, (Nymphoides crenata) has similar flowers which are yellow.

  pandanus

Pandanus of various species line wetlands, many of them fruiting during the dry season. These plants are important for Aboriginal people. They serve as seasonal indicators — the appearance of the fruit signals the dugong-hunting season in some areas — and are used for food, medicine and materials. Leaves, with thorns removed, are useful for making baskets, mats, ropes, dilly bags and so on while the seed kernels within the fruit can be eaten, baked or raw.

nesting magpie

Magpie goose and goslings Photo: Peter Whitehead

Magpie geese rely on two main habitat types and two main food plants. During the breeding season, from March to June, the adults frequent patchy grass and sedgelands where they, and their young, feast on the seeds of wild rice (Oryza meridionalis). As these areas begin to dry out, the geese move to swamps where bulkuru (Eleocharis dulcis) grows. Also known as water chestnuts, these plants produce starchy tubers in large numbers; one site studied averaged five million bulbs per hectare. From July to September the geese shuffle through the sticky mud, digging up the tubers with their hooked bills and gorging themselves on this high energy food. As the wetlands continue to dry out the geese congregate in ever-increasing numbers on the remaining swamps. The late dry season is a lean period, when food supplies run low, but when the rains come the birds are able to feed on young shoots of grass until the next crop of wild rice seeds is ready. Magpie geese were once widespread across Australia but alterations to wetlands, poisoning and shooting have led to their disappearance in southern, temperate areas and they are now found only in the tropics. However, their wetlands are under threat in the north, with introduced plants such as para grass and olive hymenachne rapidly filling up many magpie geese feeding sites, rendering them useless to the birds. Although the geese do sometimes eat grass, it does not provide the necessary nutrients in the long term. They need seeds and tubers.

  file snake

File snakes are remarkably loose skinned snakes with a flabby appearance. Their common name comes from the pointed scales which give their skin a very rough texture. These snakes live in water, where they are very agile, but they are really quite helpless on land. They can remain submerged for long periods with their nostrils closed by a flap of skin on the roof of the mouth. Nonvenomous, file snakes are an important food for Aboriginal people. There are two species. The little file snake lives mainly in estuaries and mangroves, while the larger Arafura file snake (left) ventures further inland and is most commonly found in freshwater streams and lagoons. These snakes have a habit of wrapping their tails around underwater logs or roots to use them as an anchor while striking at passing fish, the chief item on their menu.

Merten's water monitor is at home in the water, feeding mainly on fish, freshwater crabs, frogs and carrion. Growing to about one metre in length it is usually seen basking on rocks, logs and branches overhanging creeks or swamps, but quickly drops into the water when disturbed. It is a good swimmer, its long vertically flattened tail providing propulsion. Nostrils placed high on its snout mean that, like a crocodile, it can remain submerged with just the tip of its nose above the water. Underwater, it uses its long body and tail to herd fish into the shallows where they can be easily snapped up.

Saratoga are found in clear, slow-flowing or still waters around Cape York, the Gulf of Carpentaria and as far west as the Adelaide River, east of Darwin. They are able to tolerate low oxygen levels and are suspected of using their swim bladders as ‘lungs’ for an extra supply. They can grow to almost a metre in length and to over 10 kg in weight.

Saratoga have an interesting and ancient history, belonging to a once numerous group of fish with a fossil history dating back 55 million years. There are now only eight species of saratoga left in the world, two of them in Australia. Since the others are found in Africa, South America and Southeast Asia, saratoga are seen as evidence that these continents were once joined in the supercontinent, Gondwana. While most freshwater fishes evolved from marine ancestors, saratoga and lungfish are among the very few species, still in existence, which evolved in freshwater.

Saratoga have bony mouths and are the only fish known to have bony tongues. It is surprising, therefore, that they are mouth brooders. The female lays between 30 and 130 eggs, each the size of a marble, which she carries in her mouth for five to six weeks until they hatch. (She doesn't feed during this period). Afterwards, the young fry stay close to her head for a similar period, darting back into her mouth if in danger.

long necked turtle

Long-necked turtles, also known as snake-necked turtles, have elongated necks which can be tucked in under the front edge of the shell in a sideways folding motion. Together, the head and neck can be as long as the shell. Sometimes concealing themselves in sediment at the bottom of a waterhole, these turtles ambush prey, using their long necks to strike out, with speed and accuracy, and sucking fish, crustaceans and molluscs into their large mouths.

Normally, inundation would be expected to destroy any turtle or crocodile eggs but some northern long-necked turtle mothers deliberately deposit theirs under the water. Development does not begin until the water evaporates in the dry season, leaving them high and dry (but buried under sediments). Experiments with the eggs have shown that they can develop quite normally even after they have been immersed in water for up to 12 weeks. This strategy, which may have developed in low lying situations where there is little land above water outside the dry season, may help to protect the eggs from terrestrial predators. Researchers, keen to investigate the rumour, finally proved it by catching a pregnant female, inserting an egg-shaped radio transmitter into her ovaries and, a few days later, found the nest which was buried in sediments in 15 cm of water. When the water evaporated, the eggs developed.

Shorter-necked turtles include aquatic plants and fruits, such as pandanus in their diet. Indeed, adults of the northern snapping turtle, which is found in rivers throughout northern tropical Australia and the east coast of Queensland, are almost entirely vegetarian although they prefer meat in captivity.

  northern snapping turtle

The Gulf snapping turtle was known only from a fossil 25,000 years old, found at the Riversleigh fossil site—until a live one was discovered in 1995 in one of the rivers draining into the Gulf of Carpentaria. This 'living fossil', which is one of the largest freshwater turtles in Australia, is very similar to the northern snapping turtle.

Darters can be seen in and around most wetlands. In common with closely related cormorants, but unlike most other waterbirds, they have plumage which absorbs water allowing them to sink below the surface. Often only the bird's long S-shaped neck can be seen, the reason for its other common name, snake bird. Although large webbed feet allow a darter to swim strongly, it does not chase its prey under the water, but instead stalks it or waits until it comes within reach. It then shoots out its long neck, specially equipped with hinged vertebrae for speedy movement, and spears it with its long dagger-like bill. Fish are its main food, but it also eats small turtles, aquatic insects and plants.

darter

Darters have a habit of standing on branches with their wings outstretched. It has always been assumed that this is to dry out their waterlogged plumage. However, studies of cormorants, which in a similar way, suggest that it also aids digestion. Experiments, in which cormorants were given cold fish and some were given warm fish showed that the spread and flapped their wings more. The theory is that the flapping wing produce heat which in turn, warms the fish in the bird’s stomach.

In other parts world female darters are dark, like the males. Australian female darters, however, a pale front with a grey-brown back.

Danger

hidden crocodile

Although wetlands in the dry season, with their tranquil waters, delicate water lilies and dabbling waterbirds are the epitome of peace, this can be deceptive. Wherever there is high productivity, there are large appetites and most animals need to be wary. Both freshwater and estuarine crocodiles are liable to turn up wherever there is water in northern tropical Australia. Young ones feed on insects, crustaceans and frogs, the type of prey changing as they grow. Adult freshwater crocs eat large amounts of fish but food from terrestrial origins, such as invertebrates, birds, and small reptiles and mammals make up as much as 40 per cent of their diet.