The Australian lungfish (Neoceratodus forsteri), also known as the Queensland lungfish, Burnett salmon and barramunda, is the only surviving member of the family Neoceratodontidae, and endemic to Australia. It is one of only six extant lungfish species in the world.
Fossil records of this group date back 380 million years, around the time when the higher vertebrate classes were beginning to evolve. Fossils of lungfish almost identical to this species have been uncovered in northern New South Wales, indicating that Neoceratodus has remained virtually unchanged for well over 100 million years, making it a living fossil and one of the oldest living vertebrate genera on the planet.
It is one of six extant representatives of the ancient air-breathing Dipnoi (lungfishes) that flourished during the Devonian period (about 413–365 million years ago) and is the outgroup to all other members of this lineage. The five other freshwater lungfish species, four in Africa and one in South America, are very different morphologically from N. forsteri. The Queensland lungfish can live for several days out of the water, if it is kept moist, but will not survive total water depletion, unlike its African counterparts.
The small settlement of Ceratodus in the Wide Bay–Burnett region of Queensland derives its name from that of the Australian lungfish. The species was named, by Gerard Krefft, in honour of the squatter and politician William Forster.
Distribution and habitat The Australian lungfish is native only to the Mary and Burnett River systems in south-eastern Queensland. It has been successfully distributed to other, more southerly rivers, including the Brisbane, Albert, Stanley, and Coomera Rivers, and the Enoggera Reservoir in the past century. The Australian lungfish has also been introduced to the Pine, Caboolture, and Condamine Rivers, but current survival and breeding success are unknown. Formerly widespread, at one time at least seven species of lungfish were in Australia.
This species lives in slow-flowing rivers and still water (including reservoirs) that have some aquatic vegetation on banks.Martin F. Gomon & Dianne J. Bray, 2011, Queensland Lungfish, Neoceratodus forsteri, in Fishes of Australia, accessed 07 Oct 2014, https://fishesofaustralia.net.au/home/species/1988 It occurs over mud, sand, or gravel bottoms. Australian lungfish are commonly found in deep pools of depth and live in small groups under submerged logs, in dense banks of aquatic macrophytes, or in underwater caves formed by soil being washed away under tree roots on river banks. The lungfish is tolerant of cold, but prefers waters with temperatures in the range .
The Australian lungfish cannot survive complete desiccation of its habitat, but it can live out of water for several days if the surface of its skin is constantly moist. Unlike the African species, Protopterus, it does not survive dry seasons by secreting a mucous cocoon and burying itself in the mud.
The Australian lungfish is essentially a sedentary species, spending its life within a restricted area. Its home range rarely extends beyond a single pool or, occasionally, two adjacent pools. It does not follow a set migratory path, but may actively seek out suitable spawning habitats between July and December.
Description Australian lungfish are olive-green to dull brown on the back, sides, tail, and fins, and pale yellow to orange on the underside. They have been described as having a reddish colouring on their sides which gets much brighter in the males during the breeding season. This colouration is the only obvious distinguishing sexual characteristic of the lungfish. They have stout elongated bodies and flattened heads with small eyes. The eyes have five types of photoreceptors; four types of cones and a rod. Some of these photoreceptors can reach 20–24 μm in diameter, possibly the largest in the animal kingdom.The Biology of LungfishesEvolution's Witness: How Eyes Evolved The mouth is small and in a subterminal position. The lungfish can grow to a length of about , and a weight of . It is commonly found to be about and on average. Both sexes follow similar growth patterns, although the females grow to a slightly larger size. They are covered in slime when taken from the water.
The skeleton of the lungfish is partly bone and partly cartilage. The vertebrae are pure cartilage, while the ribs are hollow tubes filled with a cartilaginous substance. The body of the lungfish is covered with large, bony scales. Ten rows occur on each side, grading to small scales on the fins. The scales are each embedded in their own pockets, and overlap extensively, such that vulnerable areas of the body are covered by a thickness of at least four scales. Two unusually large and thick interlocking scales cover the back of the head where the bony skull is thin. Their cranial muscles (around the skull and jaw) follow similar patterns observed in other vertebrates, whereby the muscles tend to first develop from anterior to posterior, and from their region of origin toward insertion. They have powerful long paddle-shaped diphycercal tails. Unlike all other extant lungfish species, the pectoral fins and pelvic fins are large, fleshy, and flipper-like. The dorsal fin commences in the middle of the back and is confluent with the caudal and anal fins.
The dentition of the lungfish is unusual: two incisors, restricted to the upper jaw, are flat, slightly bent, and denticulated on the hind margin. These are followed by dental plates on the upper and lower jaws.
Juveniles have different body proportions from mature adults. The head is rounder, the fins are smaller, and the trunk is more slender. Also, the brain is relatively larger and fills more of the cranial cavity in juveniles compared to adults. The mouth is initially terminal, but shifts back as the fish grows. The dorsal fin typically reaches to the back of the head in young juveniles, and gradually moves caudally until it only extends to the mid-dorsal region in adults. They show a gradual change in body form as they develop, but no metamorphosis is externally detectable and no obvious point occurs at which they can be termed adult. As a juvenile, the lungfish is distinctly mottled with a base colour of gold or olive-brown. Patches of intense dark pigment will persist long after the mottling has disappeared. Young lungfish are capable of rapid colour change in response to light, but this ability is gradually lost as the pigment becomes denser.
The lungfish is reputed to be sluggish and inactive, but it is capable of rapid escape movements using its strong tail. It is usually quiet and unresponsive by day, becoming more active in the late afternoon and evening.
Breathing
A distinctive characteristic of the Australian lungfish is the presence of a single dorsal lung, used to supplement the oxygen supply through the gills. During times of excessive activity, drought, or high temperatures (when water becomes deoxygenated), or when prevailing conditions inhibit normal functioning of the gills, the lungfish can rise to the surface and swallow air into its lung. More frequent air breathing is correlated with periods of greater activity at night when it uses the lung as a supplementary organ of respiration.
Unlike the South American and African lungfishes, the Australian species has gills on all the first four gill arches, while the fifth arch bears a hemibranch. It is also the only facultative air breather lungfish species, only breathing air when oxygen in the water is not sufficient to meet their needs. The lung is a single long sac situated above and extending the length of the body cavity, and is formed by a ventral outgrowth of the gut. Internally, the lung is divided into two distinct lobes that interconnect along its length, compartmentalized by the infolding of the walls. Each compartment is further divided to form a spongy alveolar region. Blood capillaries run through this region close enough to the air space in the lung to enable gas exchange. Lungfish breathe in using a buccal force-pump similar to that of amphibians. The contraction of smooth muscles in the walls of the lung results in exhalation.
The sound of the lungfish exhaling air at the surface prior to inhaling a fresh breath has been compared to that made by a small bellows. Young lungfish come to the surface to breathe air when they are about long.
Reproduction and development The Australian lungfish spawns and completes its entire lifecycle in freshwater systems. The age of first breeding is estimated to be 17 years for males and 22 years for females. Males typically become mature at and females at . After an elaborate courtship, the lungfish spawn in pairs, depositing large adhesive eggs amongst aquatic plants. They spawn from August until November, before the spring rains, in flowing streams that are at least a metre deep.
Eggs are most abundant during September and October. The stimulus for spawning is believed to be day length. The lungfish is known to spawn both during the day and at night. The lungfish is selective in its choice of spawning sites. Eggs have been recorded on aquatic plants rooted in gravel and sand, slow- and fast-moving waters, in shade and in full sun, but never on aquatic plants covered with slimy algae, in stagnant water, or where loose debris was on the water's surface.
Contrary to its South American and African relatives, the Australian lungfish does not make a nest or guard or care for its eggs. When spawning does take place, the pair of fish will lie on their sides or become entwined. They usually deposit their eggs singly, occasionally in pairs, but very rarely in clusters. The male lungfish fertilizes each egg as it emerges, and the eggs are deposited in dense aquatic vegetation. The newly laid egg is hemispherical, delicate, heavily yolked, and enclosed in a single vitelline and triple jelly envelope. The egg about in diameter; with the jelly envelope, it has a total diameter of about . The egg is sticky for a short while until silt and small aquatic organisms have covered it, but long enough for it to become attached to submerged vegetation. It is negatively buoyant, and if it falls to the lake or river bed, it is unlikely to survive to hatching.
The female has a large ovary and the potential to lay many eggs, but in the wild only produces a few hundreds of eggs, at most, during her lifetime. In captivity, 200 to 600 eggs have been laid in a single event. The lungfish does not necessarily spawn every year. A good spawning season occurs usually once every five years, regardless of environmental conditions.
The eggs and young are similar to those of frogs, but the offspring differ from both frogs and other lungfishes by the absence of external gills during early development. Within the egg, head structures and pigmentation start to appear by day 17. They hatch after three to four weeks, and resemble tadpoles. The young fish are slow-growing, reportedly reaching after 110 days, and about after 8 months. During the first week, it lies on its side, hiding in the weeds, and moving only when stimulated by touch. It will swim spontaneously, and often retreat back into the gelatinous envelope when disturbed. Newly hatched larvae develop a ciliary current over their skin and gill surfaces. This is believed to either provide respiratory exchange across the skin and gills without necessitating any movements of the jaw or brachial apparatus, or to keep the skin of the unprotected larvae free of debris, parasites, and predatory protozoans. Larvae are reported not to feed for two to three weeks while the yolk is still present. By the time the yolk is fully used, a spiral valve has developed in the intestine and the fish starts to feed. The young can grow about per month under optimal conditions.