Australian lungfish

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. It occurs over mud, sand, or gravel bottoms. 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 stout elongated bodies and flattened heads with small eyes. The mouth is small and in a subterminal position. They have powerful long paddle-shaped diphycercal tails. 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. 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. 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 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. 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. 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. The Australian lungfish has very complex courtship behaviour made up of three distinct phases. The first is the searching phase, when the fish will range over a large area, possibly searching for potential spawning sites. A pair of fish will perform circling movements at the surface of the water close to beds of aquatic plants. They breathe air more frequently and more noisily than normal, possibly reflecting a greater physiological requirement for oxygen. Individual fish have been observed to breathe air at regular intervals of about 20 minutes, with air breathing accompanied by a distinct loud burp made in the air. The noisy breathing may be a form of a mating call. The lungfish seem to do their noisy breathing in concert, even responding to each other, but never in close vicinity of where the eggs are laid. Analysis of Granddad's DNA later estimated him to be roughly 109 years old (with an error margin of ±6 years) at the time of his death, and that he was originally captured from the Burnett River in Australia before being transported to the Shedd Aquarium. This discovery also cements the Australian lungfish as the longest-lived subtropical freshwater fish species currently known to science, and one of the 12 longest-living fish species in the world. The Australian lungfish has an unusually large karyotype, very large chromosomes and cells, and a high nuclear DNA content relative to other vertebrates, but less than what is reported for other lungfishes. In spite of this, it displays low genetic diversity between populations from the Mary, Burnett, and Brisbane catchments. This low level of genetic variation could be attributed to population "bottlenecks" associated with periods of range contraction, probably during the Pleistocene, and in recent times during the periods of episodic or prolonged drought that are known to reduce some reaches of these river systems. ==Diet and feeding habits==

The Australian lungfish is primarily nocturnal, and is essentially carnivorous. In captivity, it will feed on frogs, earthworms, pieces of meat, and pelleted food. The Queensland lungfish exhibits the most primitive version of these biomechanical feeding adaptations and behaviors. ==Conservation status==

Although the status of the Australian lungfish is secure, it is a protected species under the Queensland Fish and Oyster Act 1914 and capture in the wild is strictly prohibited. Human activities currently threaten the Australian lungfish, particularly water development. It is potentially at risk in much of its core distribution in the Burnett and Mary Rivers, as 26% of these river systems are presently impounded by weirs and dams. Barriers to movement and altered flow regimens downstream of dams for irrigation purposes could lead to the disruption of existing population structure and cause even more loss of genetic variation. Australian lungfish can be very fast-growing, yet with a delayed first breeding age. For a long-lived species with naturally low mortality rates, successful spawning and juvenile recruitment is not essential every year and may only occur irregularly in medium to long cycles, even in natural environments. The length of these cycles could easily mask the potentially deleterious impacts on recruitment for many years. Additionally, large adults could remain common for decades and give no indication of a declining population in the longer term. ==Recent events==

Proposed 2006 damming projects on both the Mary and Burnett rivers threatened the habitat of the remaining lungfish. The dams would have changed the flow of the rivers, eliminating the slow, shallow areas the fish need for spawning. Scientists worldwide became involved in saving the habitat for these lungfish, citing their evolutionary importance. As of January 2022, the world's oldest living aquarium fish is a 90-year-old named Methuselah. At 4 feet long and 40 pounds, the lungfish resides at the California Academy of Sciences' Steinhart Aquarium in San Francisco. Methuselah inherited the title from Granddad. Granddad, another Australian lungfish, died at the age of 109 at Chicago's Shedd Aquarium in 2017. ==Relationship with humans==

In a 2021 FlyLife article, Karl Brandt proposed the Australian lungfish as the inspiration for Gurangatch, the legendary reptile fish from Gandangara mythology. ==See also==