Australian lungfish

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 738–790 mm and females at 814–854 mm. 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.

Opposite of 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 3 mm in diameter; with the jelly envelope, it has a total diameter of about 1 cm (0.39 in). 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 27 mm (1.1 in) after 110 days, and about 60 mm (2.4 in) 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 2 inches per month under optimal conditions.

The Australian lungfish has very complex courtship behavior 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.

The next phase involves behavior, similar to “follow-the-leader”, during which one fish, the male, shows interest in the female and nudges her with his snout. Up to eight individuals may be involved in follow-the-leader behavior. The male lungfish may occasionally take a piece of aquatic plant into its mouth and wave it around. In the third phase, the fish dive together through aquatic vegetation, the male following the female and presumably shedding milt over the eggs.

Adults have a high survival rate and are long-lived (at least 20–25 years). An Australian lungfish named "Granddad" at the Shedd Aquarium in Chicago was the oldest living fish in any Aquarium, and was already an adult when he was first placed on display in 1933; Granddad was estimated to be at least in his eighties, and possibly over one-hundred, at the time of his death on February 5, 2017.

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.

Reproduction and development