For 30 years, a unique fish eluded scientists: its slime made it "invisible" (photos included).

In 1836, European scientists discovered an unusual animal in the Amazon River that they struggled to identify. The body of this unknown species resembled that of a lamprey and measured approximately 60 centimeters in length. The air-filled lungs convinced the researchers that it must be a reptile, as reported by Science Focus.

A year later, another specimen was found in Africa. Scientists examined its heart structure and declared it an amphibian. It wasn't until 30 years later that a scientific consensus was reached: the prevailing hypothesis is that these are fish, but they differ from others. Instead of breathing underwater through gills, they possess lungs.

Researchers found that if fish lack access to the water's surface to inhale air, the West African lungfish (Protopterus annectens) will drown. The issue is that the dried gills of this species are too small to provide sufficient oxygen. However, lungfish have another unusual ability that helps them survive in their natural habitat—seasonally drying swamps and rivers.

Lungfish burrow into the mud and construct a sort of chamber filled with mucus, where they curl up in a dormant state, abstaining from food and movement—this can last until the rains return. It is known that in the wild, this can last for 7 to 8 months, while lungfish can sleep for up to four years.

Scientists have already determined that lungfish first appeared over 400 million years ago. However, despite the species' age, it still holds an incredible number of mysteries. For instance, researchers have not yet thoroughly explored the connection between lungs and the unique feature of fish: the swim bladder, which most fish use to swim underwater, as well as to hear and produce sounds.

It is worth noting that lungfish are not the only type of fish with lungs—these organs have also been found in knifefish and eels, but none of them possess both lungs and swim bladders. One organ might be a version of the other, but which one came first? This question remains difficult to answer.

It is known that in fish embryos, the swim bladder and lungs develop from a pocket in the intestine. Scientists have scanned both organs and discovered that both are connected to the circulatory system; however, lungs are considered the more ancient organ. The results also suggest that swim bladders appeared later and are essentially modified lungs.

Another long-standing question that has puzzled scientists is whether lungfish are the closest living relatives of all terrestrial tetrapods, meaning all mammals, birds, reptiles, and amphibians.

Recent studies suggest that this is indeed the case. Researchers believe that lungfish are more distantly related to bony fish, such as tuna and herring, than to terrestrial vertebrates, including humans. Currently, six species of lungfish are known, including four from Africa. The female Australian lungfish, known as Methuselah, holds the title of the oldest fish in captivity.