Lungfish

Lungfish, also called dipnoans, are special freshwater vertebrates that belong to the class Dipnoi. They are famous for their unique way of breathing, as they can breathe air using lungs, just like humans. This clever way of breathing helps them survive in water that may not have much oxygen.

Lungfish are very important because they are the closest living relatives of tetrapods, which include animals like amphibians, reptiles, birds, and mammals. Their bodies have special features, like tooth plates in their mouths that they use to crush hard shelled organisms.

Today, there are only six known kinds of lungfish, living in places like Africa, South America, and Australia. Long ago, they lived all over the world, and fossils show that they have been around since the Early Devonian period, over 410 million years ago. Early lungfish lived in the ocean, but most of them now live in freshwater areas.

Etymology

The word "lungfish" comes from several languages. In English, it's called Dipnoan. In Modern Latin, it's Dipnoi, and in Modern Greek, it's δίπνοο (dípnoo). All these names come from an ancient Greek word, δίπνοος (dípnoos), which means "with two breathing structures". This ancient word is made from two parts: δι- (di-) meaning "twice" and πνοή (pnoḗ) meaning "breathing, breath".

Anatomy and morphology

Lungfish have a special backbone made of cartilage and strong teeth on their upper jaw. Older groups of lungfish have more bones, but modern lungfish have fewer bones and a softer, cartlike braincase. The top of their skull used to have a hard covering, but this is gone in modern lungfish.

During breeding, the South American lungfish grows special feathery parts from its pelvic fins to help its eggs breathe. Lungfish have evolved structures for breathing air that are similar to those in animals that walk on land. Their teeth arrange in rows to create a flat surface for crushing food.

Modern lungfish keep some features from their youth throughout their lives. They also have the largest genetic code of any backboned animal. These fish have long bodies with strong paired front and back fins and a single tail fin instead of the usual dorsal, caudal, and anal fins found in most fish.

Main article: notochord
Main articles: palatal dentition [Basal] [primitive]
Main article: skull roof
Main article: mineralized tissue
Main article: cosmine
Main article: Devonian
Main article: homology
Main article: ray-finned fishes
Main article: tetrapods
Main article: South American lungfish
Main article: convergent evolution
Main article: choana
Main article: Lissamphibian
Main article: Latimeria
Main article: vertebrate
Main article: Odontodes
Main article: occlusion
Main article: lepidosireniformes
Main article: paedomorphosis
Main article: genome
Main article: pectoral
Main article: pelvic
Main article: caudal fin
Main article: dorsal
Main article: anal

Lungs

Lungfish have a special way of breathing that makes them unique. Their lungs connect directly to their throat without using a windpipe, unlike most other fish. While many fish can breathe air using a simple sac, lungfish have lungs divided into many small air spaces, which helps them exchange gases better.

Most lungfish have two lungs, but the Australian lungfish has just one. Their lungs are similar to those of animals that live on land. Like these animals and some other fish, their lungs start from the bottom side of their throat and stomach area.

When lungfish breathe through water using their gills, their blood flow works much like other fish. However, only the Australian lungfish can get all the air it needs from its gills. Other lungfish need to breathe air because their gills are not strong enough. When they breathe air, their body changes how blood flows to use the oxygen from their lungs better. Even while breathing air, they still use their sixth gill to help remove carbon dioxide from their blood.

Ecology and life history

Lungfish eat a variety of foods including fish, insects, small animals, worms, and plants. Instead of having a stomach, they have something called an intestinal spiral valve to help digest their food.

Some lungfish, like those in Africa and South America, can survive when their homes dry up by burrowing into the mud and going into a resting state called estivation. During this time, their body processes slow down a lot, and they change how they handle waste to stay alive until water returns.

Lungfish can live for a very long time. One famous lungfish named "Granddad" lived at an aquarium in Chicago from 1933 until 2017, reaching the age of 109. Another lungfish, known as "Methuselah," is thought to be over 90 years old and is possibly the oldest fish in any aquarium in the world.

Evolution

About 420 million years ago, during the Devonian, the ancestors of lungfish and tetrapods began to evolve separately. This means lungfish and animals like amphibians, reptiles, birds, and mammals started to develop along different paths a very long time ago. The earliest known lungfish lived around the same time and were able to eat hard-shelled food very early on. Most lungfish today live in freshwater, unlike their ancient marine ancestors.

Extant lungfish

The Queensland lungfish, Neoceratodus forsteri, lives only in Australia. Fossils show that lungfish have been around for about 380 million years, making them one of the oldest living fish groups. This special fish can stay alive out of water for several days if it stays moist.

The South American lungfish, Lepidosiren paradoxa, lives in the swamps and slow waters of the Amazon, Paraguay, and Paraná River areas in South America. It must breathe air to survive and has very small gills as an adult. Its body is long and eel-like, and it can grow up to 125 centimetres long.

The marbled lungfish, Protopterus aethiopicus, is found in Africa and is the largest of the African lungfish, growing up to 200 cm long. It has a unique pattern of dark spots on its body. The gilled lungfish, Protopterus amphibius, is the smallest living lungfish, reaching only about 44 centimetres long and found in East Africa.

The west African lungfish, Protopterus annectens, has a long, eel-like body and can grow up to 100 cm. The spotted lungfish, Protopterus dolloi, lives in the rivers of Africa and can survive on land by wrapping itself in a layer of dried mucus, reaching up to 130 cm in length.

Taxonomy

Lungfishes are closely related to a group called Powichthys, and then to the Porolepiformes. Together, these form the Dipnomorpha, which is closely related to the Tetrapodomorpha. All of these groups are part of the Rhipidistia, which is also related to the coelacanths.

Recent studies using molecules from the animals’ bodies show that lungfishes and tetrapods are very close relatives, with coelacanths branching off a little earlier. Understanding the relationships among different lungfish groups is harder because, after the Devonian period, we mostly only have parts of their skulls to study, and the rest of their skulls were made of flexible material. Also, some of the groups we know about might not all come from one common ancestor.