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Physiology of a Fishman

Numerous ancient traditions speak of fish men . But would it really be possible for a human being to
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Numerous ancient traditions speak of "fish men". But would it really be possible for a human being to live underwater? And if so, how might his body work?

The fish man is a rather widespread figure in mythology: the Indian Matsya, the Philistine Dagon and the Nommo of the Dogon are just a few examples. But the best known of these beings is certainly the Sumerian-Babylonian Oannes, who in ancient times emerged from the sea to bring civilization to men.

In a previous article we had retraced the main myths about fishmen, also reporting some modern sightings of similar creatures and sketching a hypothesis on their origin. However, we had not explored in depth a question of fundamental importance: is the existence of living beings halfway between man and fish (as in the case of Giants) physiologically plausible?

We could answer this question rather briefly: if fishmen existed, it means that they could have existed. But to many readers such an answer might – rightly – appear unsatisfactory. We will therefore try to examine the topic in detail, reconstructing as far as possible the anatomy and physiology that these singular creatures could have had.


  • From land to sea
  • See and hear underwater
  • Gills or lungs?
  • Other adaptations

From land to sea

Oannes is described in quite detail in the History of Babylon by the Chaldean historian Berossus, dating back to the first half of the 3rd century BC. We report here the passage in question, which will be the starting point for our research.

“In the first year [of the first antediluvian king] a beast named Oannes appeared from the Erythraean Sea [the Persian Gulf] in a place near Babylon. His entire body was that of a fish, but a man's head had grown beneath the fish's head, and similarly human feet had grown from fish's tails. He also had a human voice. An image of him is still preserved today. He spent his days with the men, but did not take food. He gave men the knowledge of letters, sciences and arts of all kinds. He also taught them how to found cities, build temples, introduce laws, and measure the land. He also revealed how to sow and reap the fruits, and in general gave man everything pertaining to civilized life. Since the time of that beast nothing else new has been discovered. But when the sun went down, this beast, Oannes, dived back into the sea and spent the nights in the depths, because he was amphibious.”

Oannes' unusual appearance is certainly what stands out most in this story. However, there is also another oddity: how is it possible that an aquatic being had so much knowledge about typically "terrestrial" activities such as architecture and agriculture? It is unlikely that he could have learned them in the depths of the sea: much more likely, he and his peers (Berossos mentions six other fishmen) descended from men born and lived on land.

In support of this hypothesis we could cite the Aztec myth according to which, at the end of the fourth age of the world (the one preceding ours), men were swept away by a great flood and only a few survived, transforming into fish. These survivors, among whom there may have been Oannes' ancestors, would have preserved and handed down the knowledge acquired before the catastrophe in order to then be able to transmit it to the survivors remaining on the mainland, who in the meantime had regressed to the Stone Age.

We had already formulated a hypothesis regarding the ways in which the "transformation" would take place; this topic, however, would require a separate article. For the moment let's focus on the simple possibility of man adapting to underwater life. Although - obviously - adaptations that make the human organism similar to that of a fish have never been observed, there are some less "extreme" ones that are well documented.

One of them is that of the Moken who populate the Mergui archipelago, which extends along the western coast of the Malay peninsula. The Moken, also known as "sea gypsies" due to their nomadic lifestyle, make frequent dives to obtain fish, molluscs and crustaceans. They can spot them without problems, despite the fact that underwater visual acuity should be significantly reduced. Physiological investigations have revealed ( that the underwater vision of the Moken, more than double that of Europeans, is achieved by pushing pupil constriction and accommodation to the limit. A subsequent study ( showed that this ability does not depend on genetic factors: after a month of training, even European children managed to achieve the same visual acuity.

The pupil of a Moken child (A), smaller than that of a European child (B), promotes better underwate
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The pupil of a Moken child (A), smaller than that of a European child (B), promotes better underwater vision. The bars at the bottom right correspond to 4 mm.

Other adaptations, this time due to genetic mutations, have been documented in the Bajau, another tribe of "sea nomads" widespread in various parts of Indonesia. This population of divers, as highlighted by rather recent research (, has a spleen that is approximately 50% larger than normal. This increase in size, due to a variation in the PDE10A gene, allows the Bajau to store a greater quantity of oxygenated blood, which can be released into circulation during free diving. This occurs through the contraction of the smooth muscles of the spleen, regulated by the PDE10A gene. The study also identified mutations in other genes involved in the physiological responses to diving: among these there are BDKRB2 and FAM178B, the first involved in peripheral vasoconstriction (fundamental for conserving body heat underwater), the second in the regulation of blood pH (which tends to decrease during diving).

These two cases show that the human body is capable of adapting to a (partially) underwater life. We will now try to establish if (and how) it could adapt to an almost totally underwater life, such as that of the mythological fishmen.

See and hear underwater

One of the first needs of a living being is to perceive what is happening around him. For this purpose, man mainly uses the senses of sight and hearing. But it goes without saying that underwater our eyes and ears could not work as well as on land. We just saw this with the Moken: the human eye must adapt to underwater vision, since it is not designed for this. And, as we will see, the ear will also have to undergo appropriate changes.

Before going into detail, let's focus for a moment on Berossus' statement that Oannes had two heads, one of a man and one of a fish. We could hypothesize that each of them contained its own brain connected to the respective sense organs, some adapted to life on land, the others to underwater life. But, in my opinion, it is much more likely (as well as biologically sensible) that there was only one head, the "human" one: the skull of the latter, perhaps particularly developed upwards to make the body more hydrodynamic, could be been mistaken for a fish head positioned above the first. As a result, Oannes had to have only one pair of eyes and ears, just like any human, but both organs had to be adapted to both terrestrial and aquatic environments.

Our eye, as we have already said, is not capable of seeing clearly underwater, and this is for various reasons. First of all, human visual acuity is maximum when there is a lot of light, while underwater there is little light. Depending on the wavelength, it can penetrate more or less deeply (the blue-green one for example penetrates more than the red one), but around 200 meters it has almost completely disappeared, and at 1000 meters complete darkness reigns. Secondly, water has more or less the same refractive index as the cornea: this means that in water the cornea will lose its refractive power (based precisely on the difference between its refractive index and that of the air), making vision blurry.

So what was a fishman's eye supposed to look like? To understand this we must look at animals with amphibious habits, which must be able to see well both in water and in the air. Among them there are penguins: although they reproduce and raise their young on land, penguins hunt underwater, managing to reach, depending on the species, from a few dozen to over 500 meters of depth. Their cornea, unlike the human one, is rather flat, which, together with a more spherical lens, cancels the refractive errors that would occur in water. Although, in theory, penguins should be short-sighted on land, it has been shown that –at least for some species – vision is good in both environments.

A fishman's eye may have been similar to that of a penguin, whose flattened cornea allows for good u
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A fishman's eye may have been similar to that of a penguin, whose flattened cornea allows for good underwater vision. On land, such an eye would be short-sighted, but some research has disproved this belief.

It is possible, therefore, that the eye of a fishman was similar in structure to that of penguins. As regards the ability to see in the depths of the sea, where the brightness is very low, one possibility is that fishmen had a retina particularly rich in rods, the photoreceptors most sensitive to light, and perhaps also a tapetum lucidum, a reflective structure typical of many nocturnal animals. It may also be that these creatures preferably remained in the euphotic zone, i.e. within 200 meters, where light can penetrate more easily. In this regard, we note that the Persian Gulf (from which Oannes emerged) has an average depth of 50 meters and a maximum of 90. Finally, we cannot even rule out the use of some type of artificial lighting (after all, these civilisers emerged from the sea there was certainly no lack of ingenuity).

What about the ear instead? Again, we can get an idea of ​​what it might have looked like by observing animals with an amphibious lifestyle. Let's take seals for example: first of all, their auricle, as in all aquatic animals, is poorly developed or completely absent. On land, however, their ear works like ours: sound is transmitted to the cochlea through the external auditory canal, the eardrum, and the chain of ossicles. In water, however, the external auditory canal is closed to prevent the pressure from damaging the eardrum, and the conduction of sounds occurs through the bones and soft tissues of the skull. It is likely that fishmen had developed a similar strategy for hearing sounds both in and out of water.

Gills or lungs?

We now come to one of the most crucial issues: that of breathing. How could a fishman breathe? Did he have gills, lungs, or both? The fact that Oannes could also live on land would lead us to exclude that he did not have lungs, but it is not sufficient to answer our question. In fact, fishmen could have breathed like cetaceans, which have lungs but not gills (and are therefore forced to come back to the surface every now and then, given that they are practically free-diving underwater), or like lungfish, fish equipped with both of gills and lungs, capable of breathing in both water and air.

Before attempting to answer this question, it is necessary to delve deeper into an aspect of fundamental importance: the need for oxygen. As we all know, our body requires a constant supply of oxygen, absorbed in the lungs and transported through the blood to all tissues. Cells use it to completely oxidize glucose into carbon dioxide and water, depending on the reaction

C6 H12 O6 (glucose) + 6 O2 (oxygen) —> 6 CO2 (carbon dioxide) + 6 H2 O (water)

During this process (aerobic glycolysis), 32 molecules of ATP are produced for each molecule of glucose, which the cell uses to obtain energy. However, when oxygen is not available, glucose is not completely oxidized (anaerobic glycolysis) and only 2 ATP molecules are produced. It is evident, therefore, how the presence of oxygen allows us to obtain much more energy, which is fundamental for sustaining the activity of organs such as the brain (which in fact alone consumes around 20% of the requirement).

However, if we move underwater, problems begin. In fact, if the air we breathe contains a good 20% of oxygen, in water this very precious gas, in its absorbable form (O2), is present in negligible quantities. Its concentration depends on various factors, such as temperature and salinity, but is usually less than 1%. This means that aquatic organisms will have much less oxygen available than terrestrial ones, and will have to adjust their physiology accordingly.

Variations in oxygen concentration with depth in the Pacific and Atlantic oceans.
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Variations in oxygen concentration with depth in the Pacific and Atlantic oceans.

One of the strategies to deal with oxygen scarcity is to reduce its need. Fish achieve this in different ways: first of all, they are almost allectothermic ("cold-blooded"), given that the regulation of body temperature is one of the most energetically expensive activities. Their metabolism is slower than that of terrestrial vertebrates, and in some species, such as the coelacanth, it is anaerobic in all tissues.

Cetaceans, for their part, despite living underwater, return to the surface to breathe: in this way they are able to ensure a sufficient supply of oxygen to maintain a metabolic rate in line with that of terrestrial vertebrates. Compared to the latter, cetaceans have a greater blood volume and muscles richer inmyoglobin, a protein that stores oxygen: this makes it possible for them to remain apnea for long periods. Myoglobin, moreover, contains iron, responsible for the red color of meat: fish, which have less need for oxygen, almost always have little myoglobin in their muscles and this is why their meat appears pale.

The physiology of lungfish deserves a separate discussion. These fish with archaic characteristics, as we mentioned earlier, have the ability to breathe both in water and in air; their name in fact derives from the Greek dipnoos, which means "with double breathing". Their gills, in reality, are rather rudimentary and are mainly used to expel carbon dioxide; only the Australian lungfish is able to use them effectively to absorb oxygen.

The double breathing of lungfish is made possible by a peculiar circulatory system. Fish usually have simple circulation: the heart, equipped with only two chambers (an atrium and a ventricle), receives the deoxygenated blood coming from the tissues and pumps it up to the gills, where it again receives the oxygen to transport to the rest of the body. In higher vertebrates (birds and mammals) there is instead a double circulation: the heart has four chambers (two atria and two ventricles), which allow the separation of the pulmonary circulation, reserved for the oxygenation of the blood, from the systemic one, which transports tissues blood rich in oxygen.

In lungfish the situation is intermediate. The heart has two atria and a ventricle: the deoxygenated blood coming from the tissues reaches the right atrium, and the oxygenated blood coming from the lungs reaches the left atrium. During underwater breathing, circulation occurs as in the rest of fish, but when the fish breathes air some valves close the gill vessels, diverting the blood towards the lungs. The oxygenated blood then reaches the left atrium and from there to the ventricle, which, being partially divided in two, almost completely prevents mixing with the deoxygenated blood coming from the right atrium. The latter is conveyed to the pulmonary artery to be supplied with oxygen, while the oxygenated one passes into the aorta, which transports it to the various tissues.

Circulatory systems of fish, lungfish and higher vertebrates.
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Circulatory systems of fish, lungfish and higher vertebrates.

So how did fishmen breathe? The fact that they had intelligence comparable to that of humans indicates that their brain was highly developed: this would be more compatible with pulmonary respiration, capable of providing enough oxygen to support the metabolic needs of a brain similar to ours. On the other hand, Berossus' statement according to which Oannes spent the whole night at sea does not fit well with this hypothesis, given that a cetacean is unlikely to be able to remain submerged for more than a few hours.

Perhaps, therefore, the fishmen could also breathe underwater: in short, they had both gills and lungs. Consequently, their circulatory system may also have been similar to that of lungfish. To maximize oxygen absorption and optimize its use, their body could have implemented some of the strategies we have seen so far: slow and preferably anaerobic metabolism, high amount of myoglobin in the muscles, and so on. Furthermore, the fact that the fishmen's metabolism was decidedly slow can be deduced from the fact that Oannes never ate food.

But what about the brain? As we have already noted, theoretically there is too little dissolved oxygen in water to support the needs of a complex brain. This assumption, however, is partially refuted by the existence of the elephant fish, whose cerebellum has a relative size comparable to that of the human brain. And if the latter consumes 20% of the oxygen requirement, the cerebellum of the elephant fish even reaches 60% ! It is probable, therefore, that the fishmen managed to keep their "grey matter" active by allocating most of the oxygen absorbed to its sustenance.

Other adaptations

In addition to those we have talked about so far, the organism of a fishman would have had to present further adaptations. Of these, one of the most important is undoubtedly pressure. The pressure that the air exerts at sea level is equivalent to 1 atmosphere: as we descend underwater, we have an increase of 1 atmosphere every 10 metres. As can be imagined, this would gradually lead to the crushing of the tissues and, above all, the collapse of the body cavities containing air, in particular the lungs. Marine mammals, however, manage to avoid this inconvenience: their rather flexible ribcage can bend to "squeeze" all the air from the lungs, avoiding the forced collapse that would occur underwater.

Further problems, well known to divers, are linked to nitrogen. According to Henry's law, the greater the pressure a gas exerts on a solution, the greater its solubility in it. Under water, the increase in pressure leads to greater solubility of the nitrogen contained in the inspired air, which dissolves in the blood. This can cause so-called “nitrogen narcosis”, a disorder with symptoms similar to those of alcohol intoxication. But even more insidious is the decompression that occurs during the ascent: the decrease in pressure, in fact, reduces the solubility of nitrogen, which - especially if the return to the surface occurs too abruptly - can form bubbles in the blood. Well, marine mammals have also solved these problems: in fact, by compressing the lungs and confining the air in the upper airways, where gaseous exchanges do not take place, they directly prevent nitrogen from entering the circulation.

Is it possible that fishmen had developed similar adaptations? If they had their "base" in the Persian Gulf, a maximum depth of 90 meters, they had to withstand no more than 10 atmospheres. It may seem like little if we consider that some cetaceans, such as the sperm whale, are capable of diving to over 2000 meters. And it may not seem like much even considering the numerous records set by free divers beyond 100 meters of depth. However, in humans nitrogen narcosis can already occur at around 30 meters, so fish-men (as they are capable of breathing air) would certainly have had to take precautions to live at those depths.

The type of thermoregulation that fishmen might have had is not easy to establish. As we have seen, they probably had a very slow metabolism, which is compatible with the fact that they were cold-blooded. However, if on the one hand ectothermy allows for significant energy savings, on the other hand it leaves the functioning of the organism at the mercy of the environmental temperature. It is no coincidence that the most advanced organisms are endothermic ("warm-blooded"): although they consume more energy, they have the advantage of always having an optimal body temperature for metabolic processes.

There is, however, a third possibility, and that is that fishmen were mesotherms. Mesothermy is characterized by the ability to keep body temperature relatively stable, although in the absence of the thermoregulatory mechanisms typical of endotherms. In some large animals, for example, body size would in itself prevent excessive heat dissipation, due to the low surface/volume ratio. This phenomenon, defined as "gigantothermy", may also have been present in dinosaurs, which in fact were probably mesothermic. Living mesotherms such as tuna and white sharks are instead able to conserve body heat mainly thanks to specific circulatory adaptations. In short, mesothermy may have been a good compromise between the low energy consumption of ectotherms and the high metabolic efficiency of endotherms.

Ectothermic animals (in blue) consume much less energy than endothermic ones (in red), while mesothe
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Ectothermic animals (in blue) consume much less energy than endothermic ones (in red), while mesotherms (including fishmen?) have an intermediate consumption.

A fishman would also have to develop mechanisms to maintain an adequate hydro-salt balance. Because body fluids contain fewer salts than seawater, cells will tend to lose water through osmosis; on the contrary, the salts will tend to enter the cells. The organism of a marine animal will therefore have to make sure to conserve water and eliminate excess salts. Many fish achieve this by drinking seawater and excreting sodium and chlorine (the components of sea salt) through their gills. Marine mammals, on the other hand, use their kidneys, with their characteristic multilobed structure, to produce highly concentrated urine. The fishmen could have adopted similar strategies.

Finally, what about reproduction? In my opinion, it is highly unlikely that fishmen developed from eggs dispersed and fertilized in the water, like most fish. More plausible is that the embryo grew inside the maternal womb, as in cetaceans and some shark species. On the other hand, this reproductive mode is typical of the most advanced organisms (including humans), as it guarantees the embryo a protected environment suited to its physiological needs, thus allowing it to develop more slowly and reach greater complexity.

And with this I would say that we can consider our research concluded. Of course, many mysteries still remain about fishmen. How did they spend their daily lives? How was their society structured? But above all, what happened to them? If they haven't disappeared, where are they hiding? Have they evolved (biologically, technologically, or both) from Oannes' time to the present?

The questions could continue for a long time, but in the meantime one of the most crucial ones has finally been answered: yes, the existence of fishmen - in light of our knowledge of physiology - is theoretically possible.

This article is a translation of the article "Fisiologia di un Uomo Pesce" published by Merlo Bianco at

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