Australian lungfish  (Neoceratodus forsteri)

Description

The Australian lungfish (Neoceratodus forsteri), also known as the Queensland lungfish, Burnett salmon and barramunda, is the only surviving member of the family Neoceratodontidae. It is one of only six extant lungfish species in the world. Endemic to Australia, the Neoceratodontidae are an ancient family belonging to the class Sarcopterygii, or lobe-finned fishes. 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. 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. It occurs over mud, sand, or gravel bottoms. Australian lungfish are commonly found in deep pools of 3–10 m (9.8–32.8 ft) 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 15–25 °C (59–77 °F). 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 distinguishing sexual characteristic of the lungfish. They have stout elongated bodies and flattened heads with small eyes. The mouth is small and in a subterminal position. The lungfish can grow to a length of about 150 cm (4.9 ft), and a weight of 43 kg (95 lb). It is commonly found to be about 100 cm (3.3 ft) and 20 kg (44 lb) 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. The pectoral fins are large, fleshy, and flipper-like. The pelvic fins are also fleshy and flipper-like and situated well back on the body. 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. 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 25 mm (0.98 in) long.