Nature is so fond of symmetry
Some maverick guys
But he likes to break the rules
flatfish
Biological classification
Realm: Animal kingdom Animalia
Phylum: Chordata phylum Chordata
Order: Rayfin fish Actinopterygii
Order: Flounder Pleuronectiformes
English name: Flatfish
In nature, the bodies of both plants and animals are almost axisymmetric or centrally radiatively symmetrical. Today we will talk about a representative of an asymmetrical animal - the flounder. Their special body structure and evolutionary processes have attracted countless scientists who are fascinated by them and want to reveal their mysteries. The apparent asymmetry between the left and right of the flounder's appearance is formed during metamorphosis. Asymmetry is mainly manifested in the fact that one eye will move from one side of the head to the other, the pigment distribution on both sides of the body is obviously asymmetrical, and the body is tilted by 90°.
△ The asymmetrical head of the flounder, the picture comes from the network
Flounder refers to all carnivorous benthic fish of the order Flounder, with more than 700 species extant. Typical features are that their bodies are flat, some diamond-shaped, some oval or oblong-shaped, with two eyes on the same side of the body. There are three families in the order Flounder: turbots, flounders, and flounders, which we will cover next.
△ Different species of flounder, the picture comes from the Internet
Flounder mostly live in marine waters, inhabiting coastal shelf waters of moderate depths from the tropics to the boreal zone, with a few living in freshwater. The side of the body with two eyes is the back, the color is darker, and some will simulate the color of the seabed sand as a protective color, blending with the environment and not being discovered by the enemy. On the other side is the ventral surface, and the pigment has almost faded to a white color. Along the dorsal ventral margin there are elongated dorsal and fins, respectively. Lying flat on the bottom of the sea during rest, part of the body is often buried in the mud and sand, ambushing prey that swims overhead.
△ Flounder hiding in the sand, the picture comes from the Internet
△ The ventral surface of the flounder pigment faded and the darker back, the picture comes from the Internet
<h1 class="pgc-h-arrow-right" > Turbot family</h1>
△ Image from Wikipedia
Turbot is divided into 20 genera: Turbot, Turbot, Turbot, Turbot, etc. Each fin of this family has no hard spines, the dorsal fin starts above or in front of the eye, and the dorsal fin and the posterior end of the fin are mostly not connected to the caudal fin. The two eyes are on the left side of the body, also known as the left mouth fish. Turbots usually have larger mouths, sharper teeth, and greater flapping ability. Turbots are commonly found in turbots, turbots, turbots, etc. The turbot we are familiar with is also called turbot, which belongs to the turbot family.
<h1 class="pgc-h-arrow-right" > flounder family</h1>
In contrast to turbots, flounders in the flounder family move their eyes to the right side of their bodies, hence the name right-mouthed fish. They lie on the bottom of the sea with the right side of the body, the dorsal fin is even and continuous with the caudal fin, the dorsal fin extends towards the head, and the caudal fin is obvious. Compared with turbots, flounders are more cherry-savvy and like to hide in the sand to ambush and ambush prey.
<h1 class="pgc-h-arrow-right" > family</h1>
Flounders under the family Troutidae have slender bodies, both eyes are located on the right side of the head, a small mouth, a jaw that is not prominent, and a snout that is sometimes hooked and bent. The edge of the anterior gill cap does not swim away. The dorsal and fins are elongated, often connected to the caudal fins, with or without pectoral fins. Lie on your side on the mud and sand on the bottom of the sea, preying on small fish. Common ones include egg rays and striped rays, also known as "rays". The dragonfish (semi-slippery tongue fish) that we often eat also belong to this family.
In fact, the flounder larvae are as symmetrical to the left and right as normal fish, and the eyes are symmetrically grown on both sides of the head. They live in the upper layers of the sea and often swim near the surface of the water. As it grows, its form gradually begins to change, and one eye gradually moves to the opposite side through the upper edge of the head, until it is close to the other eye, and then stops moving. Different species of flounder eyes move differently in terms of methods and routes. The movement of the eyes of the flounder changes their internal structure and organs. The skull of the flounder is made of cartilage, and when the eyes of the flounder begin to move, the cartilage between the eyes of the flounder is first absorbed by the body, so that the eyes can move over the barrier of the skull to the other side. The body also gradually tilts during the movement of the eyes, eventually sticking to the bottom of the water to live.
△ Eye movement during flounder development, picture from Schreiber, 2006
So why exactly do flounder's eyes move, and what regulates eye movement?
Many hypotheses have emerged to explain these problems as early as the 19th century. In 1987, British scholars proposed their own "skull deformation hypothesis" by observing the skull changes in the development process of a variety of flounders. He believes that the movement of the eyes of the flounder is caused by deformation of the skull, which mainly consists of the torsion of the frontal bone and the asymmetrical enlargement of the left and right side of the sieve bone. But later the hypothesis was also questioned, such as that the frontal bone is located above the eye and is not in direct contact with the eye, and using the force generated by its torsion to drive the eye upwards is obviously not an optimal option. It's like having a heavier box, without any tools, to get it to move forward, the most labor-saving way is to push it in the back, rather than turning around in front of it in an attempt to drive the surrounding wind to roll the box forward.
Then there is a second "cell squeeze hypothesis": crowding between dividing cells in a finite area under the eye, driving the eye above to move upwards. During the movement of the eyes of flounder, semi-slippery tongue fish, Senegalese ray and other three kinds of flounder, it is indeed found that there are significantly more cells dividing in the limited area under the moving eye than above, and the non-moving eye on the other side is just the opposite. This means that the eyes of a flounder are squeezed by the division of cells under one eye, and which eye moves depends on which of the two eyes divides more cells. Two Japanese scholars, inspired by thyroid hormones regulating tadpole metamorphosis, found that flounder eye movements are also regulated by thyroid hormones.
△ Flounder hiding in the sand, the picture comes from the Internet
Some flounders are also color change masters, able to perceive changes in the surrounding environment and change color, the reason why flounder can change the color of the body is that it can use some parts of the body to feel the color of the external environment and be stimulated. These stimuli change the color of the skin through the nervous system by changing the arrangement of pigment particles contained in skin cells.
Special thanks▽
Marine Bivi Hu
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