Fish strokes: skimming, horizontal skimming/horizontal hook, vertical, horizontal folding, horizontal, vertical, horizontal, horizontal.

Fish is the oldest vertebrate. They inhabit almost all aquatic environments on earth, from freshwater lakes and rivers to saltwater oceans.

Fish are mostly cold-blooded animals, and very few are warm-blooded animals. They breathe through their gills and have jaws and fins. The existing fish can be divided into two categories: cartilaginous fish (such as sharks) and bony fish (fish with linear fins and wavy fins).

Both groups of fish first appeared in the early Devonian. A more advanced group of linear finned fish, called bony fish, began to evolve in Jurassic and has become the most individualized fish. There are also several extinct fish.

Fish have good hearing. Scientists have found that although many fish do not have long ears outside, they have specially designed sound receivers that can transmit sound waves to the tubular structure filled with liquid in the inner ear.

These pipes have special fine hairs, called cilia, which can transmit sound pulses to the fish brain through a series of complex mechanisms and chemical reactions, and process them there. Otolith is a part of the auditory system, which is connected with sensory cells and plays an important role in the hearing/balance mechanism of teleost.

Otoliths are very valuable to scientists. They rely on otoliths to identify the species of fish and determine their age, because when fish grow, otoliths grow into a concentric circle every year. Under the microscope, scientists can see and count these concentric circles.