Can Dolphins Live in Fresh Water?

Is it Possible for Dolphins to Live in Fresh Water?

We are very familiar with dolphins and fin-less porpoises, but dolphins and fin-less porpoises are both in the same family. 

Why is there a difference between finless porpoise and porpoise? 

Can dolphins follow the estuary and live in the big rivers?

 

Let me start with the answer: a small part can, the vast majority cannot. The reason is simply three sentences: can’t swim, eat, and live.

 

In detail, it may be longer and boring. You must be mentally prepared.

 

At present, whether it is fossil evidence or molecular biology research, it is still believed that cetaceans have a single origin, that is, the current whales and dolphins evolved from the same ancestor. 

Of course, various whales and dolphins have embarked on their own different evolutionary paths. To this day, more than 80 kinds of whales and dolphins with completely different body shapes, habits and distribution areas have been evolved. 

From the physical structure, we can divide them into toothed whales. And baleen whales, from the distribution area, we can also divide them into freshwater whales and marine whales.

 

From this perspective, there are indeed several species of whales and dolphins that can live in fresh water, or even only in fresh water, such as the familiar white whale and ordinary marine finless porpoise, the Indo-Pacific dolphins (Chinese white dolphin) ), and the unfamiliar Turku dolphins, Irrawaddy dolphins, and puffer dolphins. 

Although they mainly live in the ocean, they also enter freshwater life (mainly upstream in the estuary, and some are still far upstream For example, the marine finless porpoise was once found upstream in Laos, which is more than 700 kilometers away from the mouth of the Mekong River), and some cetaceans live only in fresh water, such as the familiar baiji and Yangtze finless porpoise. 

(It should be noted that the Yangtze finless porpoise and the marine finless porpoise are two subspecies of the same species), the sub-puffer porpoise and the Ganges porpoise of the Amazon River all live in freshwater rivers and lakes all year round.

 

Baiji dolphins that have adapted to freshwater life

But the whales and dolphins found in fresh water are just the above mentioned. Besides, most of the whales and dolphins do not set foot in fresh water at all. What causes this? Or from the perspective of this question, why can't the vast majority of cetaceans enter freshwater life?

 

In fact, it must be impossible, but it is really unwilling. There are many factors that cause most whales and dolphins to reluctant to get involved in fresh water, such as space restrictions (not swimming), food restrictions (not eating well), and most importantly-the difference in osmotic pressure between fresh water and sea water (not used to living) ).

 

Let's first look at space constraints. Frankly speaking, the water area of ​​many lakes is large enough and the water depth is deep enough, even if it is more than enough to accommodate a behemoth like a blue whale, but if you want to reach these lakes from the ocean, you have to go upstream through the river. The space limitation is very obvious. 

Let’s take the well-known big river, the Yangtze River as an example. Its downstream can barely reach a depth of 10M+ after artificial dredging. Going further upstream, it is often a depth of 3-5M. 

For large cetaceans such as blue whales and sperm whales, they face the danger of stranding at all times, let alone freely swimming and hunting. Therefore, we found that most of the people who like to live or settle down upstream to the rivers and lakes are mostly small whales and dolphins, such as the Yangtze finless porpoise and the baiji. The largest body is the white whale, which is 3-5 meters long. The small whale is out

 

If the humpback whale came so suddenly in the Danshui River, its head would break

Next we will talk about food restrictions. Cetacean recipes are far less abundant than ours. Many whales have very tricky tastes and their predatory habits have become specialized. Let's take the blue whale as an example.

 Even if we move the blue whale to a large enough freshwater lake with the great move of the universe, it is likely to starve to death. They can swallow krill in the ocean with great ease, but let it prey in the lake. Carp, I am afraid that it can neither be caught nor eat, this is the specialization of predatory habits. 

Let’s take another example of killer whales. Given the flexibility of killer whales, it’s probably not a problem to catch a carp, but in fact killer whales are very picky in taste. 

Killer whales living in the southern hemisphere can even be divided into several groups according to their taste. One of the populations eats almost only seals, and the other eats almost only minke whales. 

If you give it a carp, you probably don’t even look at it. This is the narrow diet. There are simply too many examples of this kind, and some cetaceans and dolphins take up both aspects, such as sperm whales, which feed on large cephalopods and have even evolved a series of physical characteristics adapted to extremely deep and extremely high pressure. 

But there is not even a kind of cephalopod in fresh water. What do you let it eat, and what does it do in fresh water...

 

Killer whales that prey on minke whales

However, what has been said above is not the most important thing. What really stops most whales and dolphins from freshwater is the difficulty in adapting to the difference in osmotic pressure between seawater and freshwater.

 

We all know that the stability of the environment in organisms is a basic prerequisite for survival, and the stability of osmotic pressure is a very important part of this. 

Cells in organisms must maintain biological activity under an osmotic pressure environment it adapts to. 

In order to maintain a stable osmotic pressure, organisms must adjust the osmotic pressure through a series of osmotic pressure adjustments. If the osmotic pressure in the body is lower than the normal level, that is, if there is more water, it needs to replenish salt or remove water.

If the osmotic pressure in the body is higher than the normal level, it needs to replenish water and remove salt.

 

For terrestrial animals like us, the adjustment of osmotic pressure is relatively simple. But for aquatic organisms, in addition to their own regulation, they also have to resist the interference of the osmotic pressure of the external environment. This is because the internal osmotic pressure of cetaceans (300-360 mOsm/Kg) and sea water (1000 mOsm/Kg) or the osmotic pressure of fresh water (1-25 mOsm/Kg) is still very different, and water always has a tendency to permeate from a low concentration to a high concentration. So, the osmotic pressure adjustment of aquatic organisms is even more important.

 With its regulating ability, a whale will be dehydrated in seawater and become a salted fish, and in fresh water, it will be soaked and swollen to death.

 

For cetaceans living in the ocean, what they urgently need to solve is the problem of replenishing water and removing salt in the body. Since there is no fresh water in the ocean, the water needed by whales and dolphins is mainly obtained from food, such as fish and shrimps with a water content of up to 60%-80%, and water produced by breaking down the fat, protein and sugars of these foods. 

In fact, it is sufficient for the water requirements of cetaceans, but it also brings a new problem: the osmotic pressure of fish and other vertebral animals is almost above 300 mOsm/Kg, and nitrogen is produced in the process of digesting food. 

Wastes-urea and the like, which will increase the osmotic pressure in the cetaceans, and the cetaceans need to excrete the salt and urea. This method is to pee. However, in the process of peeing, it will consume a certain amount of itself. 

How can we improve the efficiency of removing salt and urea during peeing while reducing water loss as much as possible? 

For this reason, whales and dolphins have evolved extremely unique kidneys. Unlike our humans, the kidneys of whales and dolphins are not a whole, but a grape-like shape. Each individual individual is called a small kidney, and the number depends on each type of whale. Dolphins are different. 

Generally speaking, the number of toothed whale kidneys in the ocean is relatively small, ranging from hundreds to thousands. This is because toothed whales mostly eat fish with an osmotic pressure of 300 mOsm/Kg. 

The number of small kidneys of baleen whales is amazing. There are even 6000-7000 large baleen whales. This is because their main food is invertebrates represented by krill, and the osmotic pressure of invertebrates is basically the same as that of sea water, reaching 1000. mOsm/Kg, baleen whales are much more difficult to regulate osmotic pressure than toothed whales, and their kidneys must be more powerful.

Blue whales that are preying on krill, because the osmotic pressure of krill is isotonic with sea water, the osmotic pressure of the blue whale is very heavy

 

Thanks to the powerful kidneys, whales and dolphins in the ocean can highly concentrate urine, and the urine osmotic pressure often reaches 2000-3000 mOsm/Kg, so as to eliminate as much salt and urea as possible while reducing water loss , In order to achieve the goal of osmotic pressure balance in the body.

 

However, cetaceans living in freshwater must adapt to a completely different environment. The osmotic pressure of freshwater is much lower than that of freshwater cetaceans. The problem facing freshwater cetaceans is to absorb and retain as much salt as possible. Exclude as much water as possible.

We can compare the two paragraphs above and guess how freshwater whales and dolphins do it. In fact, it is true. For whales and dolphins that live only in fresh water, the number of small kidneys is generally only a few dozen. A hundred or so, far fewer than the thousands of relatives in the ocean, and the osmotic pressure of urine discharged by freshwater whales and dolphins is also very low. 

The finless porpoise living in the Yellow Sea has a urine osmotic pressure of 1600 mOsm/Kg.

 

After talking about the kidneys, let's talk about some skin.

In humans, the skin is an important way to regulate osmotic pressure. For example, the dense sweat glands on our body can discharge sweat, a liquid with high osmotic pressure, and discharge salt. 

Students who exercise regularly know that the human body consumes a lot of water during exercise. In this way, the water in the body is reduced and the original osmotic pressure balance is broken. 

In order to maintain the osmotic pressure, the human body will start to sweat-a kind of High osmotic pressure liquid of salt.

 

However, this kind of skin has advantages and disadvantages. Although it can assist in regulating osmotic pressure, in the environment of aquatic animals, it also has to face the problem of water penetration through the skin (we humans naturally do not need to face this problem).

If the sea whale has the same skin as ours, then the water in its body will quickly lose through the skin, if the freshwater whale has the same skin as ours, then the external moisture will penetrate through the skin in large quantities. 

Not the ending that whales and dolphins want. Therefore, on the path of evolution, the skin of whales and dolphins becomes extremely special, firstly due to the degeneration of the glands, and secondly by the abnormal thickening of the epidermis, which all play a role in blocking the flow of water.

 

However, the barrier mechanism of this skin to water seems to be different on various whales and dolphins. Seawater whales and dolphins seem to mainly focus on preventing water from escaping from the body, and freshwater whales and dolphins seem to mainly focus on preventing water from entering from the outside world. 

The principle of action The difference has not been studied in detail so far, but there are still some phenomena to support this inference:

 

A typical example is the beluga whale commonly found in aquariums. When beluga whales are in the wild, they live in sea water most of the year, but they migrate to freshwater estuaries or lower rivers with relatively high water temperature in winter and spring every year. 

After they reach the freshwater environment, their original skin will quickly With aging folds, beluga whales are about to molt and change skins. We can see such a grand occasion in these estuaries-large numbers of beluga wriggling on the sand and gravel at the bottom of the river to remove dead skin. 

Is there a structural difference between the skin of beluga whales living in seawater and that of freshwater? 

Is it true that the skin in seawater can only prevent the water from losing from the inside out, while the skin in fresh water can only prevent water from seeping in from the outside to the inside?

 

Beluga whale dermabrasion on the riverbed

A similar example is southern right whales. Newly born small southern right whales will shed their skin after one week of birth. Is this because the osmotic pressure of the baby whale in the uterine environment of the body is different from that of sea water, and the baby whales shed their skin for this.

What about the skin with the osmotic pressure of the uterus and the skin adapted to the osmotic pressure of seawater?

However, this ability to shed skin to adapt to different osmotic pressures is not available in every species of cetacean. In 1974, some scholars put bottlenose dolphins in a freshwater environment. 

After 72 hours, the skin appeared obvious. Swelling, and has never actively changed to the kind of skin that can adapt to the freshwater environment, which is enough to prove that the extremely flexible adaptation method of beluga is not suitable for every cetacean.

 

Then we can summarize it roughly. On the long evolutionary road, most marine whales and dolphins are highly adapted to their living environment, whether it is predatory habits, dietary tastes, kidney adjustment ability or skin, they can all be competent to live in the marine environment. But this height Specialized evolutionary results, once placed in a completely different environment, become out of place, or very poorly adaptable. 

We can understand that if an F1 car designed to be turned into a track is taken to run on an unpaved mountain road, it will be difficult to move. Representatives of this category are most marine cetaceans represented by blue whales, sperm whales, and bottle dolphins.

 

There are some whales and dolphins that came to the fresh water along the river a long time ago (the baiji came to the Yangtze River about 25 million years ago). Perhaps after paying a heavy price, some highly adaptable individuals survived. 

And further evolved to be more adapted to this environment, their urine became lighter, the skin from the ocean blocking from the inside to the outside changed to blocking from the outside to the inside, and even their eyes were also turbid due to fresh water. It degenerates without function, and their sonar system becomes more developed. 

Representatives of this category are the baiji and puffer porpoise, and the Yangtze finless porpoise is walking on this road.

 

Then there are some whales and dolphins, which have excellent adaptability and can actively change their body functions in a short period of time-such as moulting, such as omnivorous eating, which can make them comfortable in freshwater and seawater environments. Come and go freely. 

Representatives of this category are beluga whales and marine finless porpoises.

 

Therefore, it is not that whales and dolphins cannot live in fresh water. In fact, we have also seen that there are several kinds of whales and dolphins that can gradually adapt to the freshwater environment through long evolution. 

But if the subject asks, let a sea live It is very difficult for the whales and dolphins to quickly adapt to freshwater life, even life-threatening.

 

Some Explanation: 

The Yangtze finless porpoise and the marine finless porpoise that appear many times in the article are actually two subspecies of the same species. The Yangtze finless porpoise is only distributed in the Yangtze River, while the marine finless porpoise is generally distributed in the ocean, but it often goes to the estuary and even goes upstream. Up to the river for short-term life.

 

Author: Naval Kishore

Education: Masters Degree in Actuary

The author is an Actuarial expert with 20 Years of Work experience in Fisheries work