The shape of living things is determined by genes, and genes are affected by the environment. Under the conditions of natural selection, genes with better adaptability can be retained. So biological forms are partly caused by the environment. In this article I will not discuss the effects of various environments on genes, this is not my expertise. From the perspective of mechanical environment, I will discuss the morphological characteristics of organisms in high-pressure environments. Anyway, in the deep sea it is ugly and no other fish can be seen.

1. Water pressure in deep sea environment

Water pressure is positively correlated with depth. The deeper the water depth, the greater the water pressure. The formula is as follows, in which density and gravitational acceleration g are also related to height, but this value does not change much and is ignored here. According to the formula, the deepest Yana Trench in Mali can reach a depth of 11 kilometers, and the water pressure at the bottom is 110 MPa. The normal air pressure environment of the human body is 1 atmosphere, which is 0.1MPa. It can be seen that the water pressure in the Yana Trench and Mali is 1,100 times the comfortable air pressure of the human body. In such a high pressure environment, the human body cannot bear it no matter what.

Internationally, water depths exceeding 500 meters are deep sea areas, where the water pressure is about 5 MPa, which is still 50 times the atmospheric pressure.

2. Morphological characteristics of deep-sea fish

Deep-sea fish are characterized by large mouths, big eyes and lightness. In appearance, it is extremely ugly. The deep sea angler fish is pictured below. It has jagged fangs, a "small lantern" standing on its forehead, and a big mouth on its cheek.

Of course, the above-mentioned deep-sea fish are located on the deeper sea floor and live below the middle layer of the deep sea (below 1,000 meters). Within this range, the sun cannot penetrate and the deep sea is pitch black. The water pressure exceeds 10 MPa, which is 100 times the standard atmospheric pressure.

3. The relationship between mechanical environment and morphology

In the deep sea where the water pressure exceeds 100 standard atmospheric pressures, the high-pressure environment is a prominent feature of deep-sea organisms. In a high-pressure environment, on the one hand, the cells of deep-sea organisms bear higher pressure, and on the other hand, the shape and structure of fish also ensure better endurance.

The picture above is a skeleton of a deep-sea fisherman. In fact, as far as bones are concerned, they have calcium and can bear a relatively large load. Relatively strong bones, such as the human femur, have a maximum compressive stress of 170MPa (below). Even if they are thrown into the Yana Trench, they will not be crushed. Therefore, there is no problem in terms of bone compression alone.

However, the high ultimate pressure of the bone material itself does not mean that the stress state of the structure itself is good. In terms of structure, stress concentration tends to occur at discontinuous places, so the cross-section of the fish bone is similar to a circular cross-section, and the pressure is distributed as evenly as possible. This influence is also reflected in the shape of the fish. For example, the fish's mouth is made of curved bones, not triangles, just to reduce stress concentration.

After the basic shape of the skeleton is determined, the fish is underneath. We found that those ugly deep-sea fish are fatter, as shown in the picture above. The round shape is actually to disperse the pressure of water on the body. Such a fat fish will not feel particularly tired in any part of the body (stress is dispersed). Even a skinny fish has an approximately circular cross-section. Of course, this fish has one big drawback: it doesn't swim fast. Unlike the flat fish, it can move quickly through the water, as shown above. This flattened body minimizes water resistance, allowing the fish to swim faster.

The fish in the deep sea probably do not hunt by speed, because it is impossible to see clearly in the dark deep sea. These fish will catch fish in another way. This is why many deep-sea fish carry their own lanterns. It uses an illuminator to attract food and then opens its mouth to swallow it.

4. Summary

Deep-sea fish live in the sea below 500 meters. The deeper the fish, the longer and uglier they become. Most of those ugly deep-sea fish live below 1,000 meters of water, and the water pressure exceeds 100 times the standard atmospheric pressure. Under such huge water pressure, the fish's body becomes as round as possible to evenly distribute the water pressure throughout the body.