#mottled lungfish

It's time for a family reunion featuring a very, very distant cousin (about 420 million years distant) you may have never met before. Today's Wet Beast Wednesday topic is about one of the last lobe-finned fish. Sort of. You see, all tetrapods, including yourself, are lobe-finned fish whose ancestors decided to leave the water and ended up becoming the first reptiles, amphibians, and mammals over the course of millions of years. Of the lobe-finned fish who stayed in the water, only two lineages have survived to the modern day: the coelacanths and the lungfish. I covered the coelacanth in a previous WBW, so now I'll look at lungfish too.

(Image: the West African lungfish Protopterus annectens lying on sand in an aquarium. It is a long, cylindrical, eel-like fish with wispy, noodle-like pectoral and pelvic fins. Its dorsal fin runs down the back and connects with the tail fin. Its head is rounded, with a short, blunt snout. Its body is brown with darker spots. End ID)

The lungfish are members of the class Dipnoi and there are 6 living species across three families: the Australian lungfish of family Neoceratodontidae, the South American lungfish of family Lepidosirenidae, and 4 species of African lungfish in family Protopteridae. They all share some common features, including a long, heavy, and cylindrical body, unique teeth, and lungs. The Australian lungfish has a single lung while all the others have two. The lungs allow them to breathe air. While there are other air-breathing fish around, they use a modified swim bladder as a lung that is a simple sac. The lungs of lungfish are still modified swim bladders, but they are divided into multiple changers that vastly increases the surface area available for gas exchange. This is far closer in structure to the lungs of tetrapods and represents an early stage in the evolution of lungs. Lungfish also have gills, but they are small and insufficient at providing all the oxygen the fish needs, forcing them to breathe air. The exception to this is the Australian lungfish, which has more developed gills and doesn't need to breathe air. Lungfish have unique teeth that from plates used to crush through hard-shelled prey. Lungfish are non-selective omnivores. Their digestive tracts are simple and lack true stomachs. Instead, the intestine contains a corkscrew-shaped section called a spiral valve that increases the surface area for nutrient absorption. Most species have very small, threadlike pelvic and pectoral fins. All living species are freshwater fish.

(Image: a marbled lungfish Protopterus aethiopicus. It has a similar body plan to the above picture, but its body is pale yellow color covered with large, grey spots. It is poking its snout into rocky sediment. End ID)

The Australian lungfish (Neoceratodus forsteri) or barramunda is the most primitive lungfish species and has changed very little in the past 100 million years. It is native to streams and pools in the Burnett and Mary river systems of Queensland, but has been introduced to other rivers. They are notable for having more robust bodies and more developed pectoral and pelvic fins than the other lungfish species. Their air breathing supplements their gills and allows them to survive dry seasons where their pools often become shallow and hypoxic. This gives the lungfish the ability to thrive in pools other animals struggle in. Unlike other lungfish, the barramunda cannot survive dry season on land. They can survive for several days out of the water as long as they stay wet, but will die if on land too long. Aussie lungfish are nocturnal and primarily carnivorous, feeding on frogs, fish, and various invertebrates, occasionally supplementing its diet with plants and algae. They can grow up to 150 cm (4.9 ft) and 43 kg (95 lbs) though most are smaller.

(image: an Australian lungfish lying on sand. It has a broader and more robust body than the other species, with more prominent scales. its body is a brown color. End ID)

Barramundas mate in spring and summer. They have an elaborate courtship ritual that occurs in three stages. In the first stage, they will swim in circles near aquatic plants while breathing more loudly than usual. In the second stage, one or more males will follow a female while nudging her with his snout. Sometimes the males will wave bits of plants around with their mouths. During the third stage, the female and her chosen mate enter plant patches to mate. The pair will lay on their sides or entwine themselves with each other. The female lays her eggs on plants and the male fertilizes them as they come out. They provide no parental care. The eggs hatch after a few weeks into larvae that look a lot like tadpoles but with internal gills. They develop slowly and eat plankton, small worms, and algae. It can take up to 20 years for a lungfish to reach sexual maturity and they can live for a very long time. The oldest aquarium fish on record was an Australian lungfish named Granddad, who died at 109. The oldest living aquarium fish is another Aussie lungfish named Methuselah who is estimated to be 93. She lives in the Steinhart Aquarium at the California Academy of Sciences. We stan a golden girl.

(image: Methuselah in her open-topped tank. An aquarium worker is inspecting her. One had is under her chin while the other is laying on top of her back. End ID)

The four species of African lungfish are very eel-like in appearance, much more slender than the Australian lungfish, with less prominent scales. Of them, not much is known about the gilled lungfish (Protopterus amphibius) or the spotted lungfish (Protopterus dolloi), though the gilled lungfish is the smallest, reaching only about 44 cm (17 in). The marbled lungfish (Protopterus aethiopicus) has grey to light pink skin covered with dark splotches, giving them a leopard-like pattern that helps them camouflage against light coming into the water. They likely primarily use their gills for respiration, with air-breathing being used as a supplement. The marbled lungfish has the largest genome of any animal and the 4th largest of any organism, with 133 billion base pairs. The marbled lungfish can get up to 2 m (6.6 ft) long. The most well-studied African lungfish is the West African lungfish (Protopterus annectens). They have a unique ability to use their pelvic fins to lift their bodies and walk around on the substrate. They can only do this when submerged, being too heavy to walk on land. They will still move over land though, wriggling between pools. It can reach up to 1 m (3.3 ft) long.

(Image: a spotted lungfish in a tank. It looks very similar to the West African lungfish, but with dark spots. End ID)

(Image: a gilled lungfish. It has the same body plan as the other African species, but is smaller and its body is a yellowish color, with dark speckles. End ID)

Traits all African Lungfish share is their method of reproduction and their ability to survive dry periods. During mating season, the male will make a nest for the female to lay her eggs in. After laying, the female leaves and the male will remain to guard the nest. He will continue to do so after the eggs hatch. Larval lungfish look like tadpoles, complete with external gills. They feed on worms, plankton, and algae as they grow. Once the larvae develop to the point of losing their external gills and growing internal ones, the father will leave them to fend for themselves. The process can take several months. All African lungfish have the ability to survive drought.. When their streams and ponds dry up, the lungfish can burrow into the mud and secrete a cocoon of mucus around themselves, including into their mouths. The mucus entering the mouth allows them to keep their gills wet. The cocoon will harden and the lungfish will enter a state called aestivation, which is similar to hibernation. During this period, metabolic activity drops considerably and the lungfish switches from producing ammonia as waste to the less toxic urea. Lungfish can survive for years in this state, only to emerge once the rains come back and their habitats are flooded again. There are stories of people cutting up dried mud to use as building material, only for lungfish to start crawling out of their walls once it rains.

(image: a CGI model of a lungfish in its cocoon. It is buried in dirt in a cavity with a tunnel leading to the surface. The lungfish is in the cavity and wrapped around itself with its head facing up. It is surrounded by slimy mucus. End ID. Source unknown)

Despite having a very wide distribution consisting of the Amazon, Paraguay, and lower Paraná river basins, very little is known about the South American lungfish (Lepidosiren paradoxa). It is more closely related to the African lungfishes than the Australian lungfish. It appears that the anatomy and lifestyle of the South American Lungfish is very similar to that of its African relatives, to the point some taxonomists have suggested both groups should be in the same family.

(image: a South American lungfish swimming. It looks very similar to the African species, but with yellowish spots around its head. End ID)

The Australian lungfish is classified as endangered by the IUCN, while the other species are all classified as least concern. The Australian Lungfish is threatened by damming and changes to the rivers it lives in, as well as the non-native Mozambique tilapia. The larger African species and the South American lungfish are fished in their range to be used as food, though apparently not to the extent that it is significantly harming their population. That being said, lungfish are slow-growing animals that can take a long time to reach sexual maturity. Species like that have a harder time repopulating after a population loss.