Fish scale method
The most common and simplest method is the fish scale determination method. For example, for the carp that we are familiar with, this method can be used to measure its age.
The traces of the years are carved on the scales of the fish. If we look closely at the fish scales, we will find that the surface texture is uniform and dense. This is because the growth of the fish’s body in the four seasons of the year is uneven, vigorous in spring and summer, and gradually slow in autumn and winter. Reflected on the scales, it causes periodic changes in the texture. For example, the scale texture of small yellow croaker has loose rings in spring and summer, but tight rings in autumn and winter; in the second year, the rings become loose again in spring and summer, so there is a clear boundary between a loose and tight cycle. Annual rings”. By recording the number of annual rings, the age of the fish can be inferred.
What if it is a scaleless fish like catfish? At this time, otoliths are often needed to determine the age of such fish.
Otolith
The so-called otoliths are the “small rocks” in the cranial cavity of the fish. They are composed of secretions from the inner ear, and the main components are calcium carbonate and fibrous organic substances. Different fish species have different sizes and shapes of otoliths. For example, the Tochiocephalidae family, where the famous large yellow croaker is located, is named for its large otoliths. In the same way, fish grow at different speeds throughout the year. The spring and summer seasons have sufficient food and higher water temperature. The fish grow faster, and the inner ear secretes more, so the otoliths increase quickly; the situation after autumn and winter On the contrary, otoliths increase slowly. If a fish has been exposed to heat and cold, its otoliths will form small lumps wrapped in thin layers of secretions of different thicknesses.
Since the discovery of annual rings on otoliths in 1899, otoliths have been used as an important material for identifying the age of fish. Take out the otoliths of the fish, select the appropriate type of otoliths, observe and record the number of annual rings on the otoliths from an optimal angle after processing, and use this to infer the age of the fish. If it happens that it is difficult to observe the otoliths of fish species whose age needs to be determined, the age can also be determined by the calcium parts such as bones and fin spines.
Eye lens
Although the above methods are simple and ingenious, they still have certain limitations: they are only suitable for bony fishes, and for cartilaginous fishes such as sharks, they have neither scales that can read age nor calcium otoliths in their skulls. , So it is difficult to determine its age. But people are not helpless. Although some large sharks such as whale sharks are cartilaginous fishes, their vertebrae are still calcified to a certain degree, so growth rings are also formed during the development process, and people can calculate the age of the sharks based on this.
However, this seemingly only method has many drawbacks: on the one hand, people have found that the growth rings of some sharks have become sparse during growth, which also means that the age estimation from the vertebral growth rings is likely to be inaccurate; on the other hand, On the other hand, if the vertebral calcification of the age test object is low, then its vertebrae are likely to be smooth and non-stripe, such as the Greenland shark. Scientists racked their brains and finally found a scientific way to judge-measuring the eye lens of the Greenland shark.
The lens of the Greenland shark’s eye is the small white ball. During its growth and development, the lens will wrap on it like an onion. When each layer is formed, it will record the chemical characteristics of the fish’s habitat, and even the diet of the fish in each habitat. People often use stable isotopes to analyze these “logs” hidden in the eyes of the fish.
Due to the large-scale nuclear test during the Cold War in the middle of the last century, a large amount of C-14 entered the marine environment and was “in the eyes” of marine animals. As a radioisotope with an ultra-long half-life, people passed the radioisotope in the eyes of sharks. By measuring, the age of these sharks can be estimated more accurately. It turns out that the Greenland shark has an amazing longevity: the largest shark in the experimental sample has an estimated lifespan of about 392 years, but there is a fluctuation of about 120 years. This means that this shark is at least 272 years old and up to 512 years old. In other words, it may be a shark born in the Ming Dynasty. This discovery allowed the Greenland shark to surpass the bowhead whale in one fell swoop and become the longest known vertebrate!
