Observation of the embryonic development process of Neosalanx taihuensis

In order to master the artificial breeding technology of Neosalanx taihuensis and improve the success rate of stock enhancement and release of icefish, we conducted systematic observation experiments on the development process of fertilized eggs of Neosalanx taihuensis. The experiment report is as follows:

1. Materials and Methods

1. Materials: The fertilized eggs of Neosalanx taihuensis used in the experiment were obtained from the artificial breeding base of Neosalanx taihuensis at Xintang Bridge, Zhicheng Town, Changxing County, and the incubation water was filtered water from Basan Reservoir.

2. Time: April 30 to May 13, 1999, and April 20 to May 15, 2003.

3. Methods: Artificial breeding adopted the dry method of stripping sperm first and then collecting eggs for fertilization. Fertilized eggs were placed in culture dishes for incubation. An Olympus system biological microscope (BHS-313) was used for observation, timing and drawing. A fully automatic microscope camera (PM-10AD) was used in a timely manner to photograph the morphology at each stage. Samples were numbered and grouped, and the observation results were averaged.

2. Results

1. Fertilized eggs: The fertilized eggs were light cream-yellow, nearly spherical, with a diameter of 0.42-0.75 mm. After fertilization, the eggs absorbed water and swelled, forming a perivitelline space. The eggs were demersal and adhered to the bottom of the culture dish by means of free egg membrane filaments. Under the microscope, it could be observed that most fertilized eggs did not float freely due to the adhesive effect of the egg membrane filaments. The egg membrane filaments accompanied the eggs all the way until the fry hatched.

2. Early embryonic development: At a water temperature between 17.5-23°C, embryonic development took 99 hours and 30 minutes. The developmental sequence and main characteristics of each stage.

3. Late embryonic development: After hatching, Neosalanx taihuensis had the following characteristics:

(1) Newly hatched larvae had a body length of 2.7-3.4 mm, a yolk sac length of less than 2 mm, pear-shaped, with 55-60 vertebrae. Some brown-black pigment appeared above the eyes, and pectoral fin buds were visible.

(2) In the beaker, the larvae initially lay on their sides at the bottom, only intermittently turning in place. After 2 hours, they moved short distances. After 4 hours, with vigorous beating of the pectoral fins and twisting of the body, they swam intermittently in the water layer. Later, horizontally swimming larvae appeared, and most horizontally swimming larvae appeared after 18 hours post-hatching.

(3) In beakers with the same number of larvae, they could survive for a maximum of 9 days without any food. After the yolk sac was completely absorbed, 3-4 days later, if egg yolk or algae were fed, the larval digestive tract showed yellow or green within 2 hours, indicating that the larvae would eat in the beaker. However, due to water pollution, the survival time of fed larvae was instead shortened, and most could not live to 9 days.

(4) Three to four days after hatching, the yolk sac was basically completely absorbed, the fins were fully differentiated, and the body shape was basically similar to that of adult fish.

3. Discussion

1. Immature eggs generally have a lower specific gravity than mature eggs and fertilized eggs. Therefore, most immature eggs can be removed by rinsing. There is also another type of egg that is mature but unfertilized; it also initially shows a perivitelline space and cleavage, but around 24 hours it begins to turn white and the cellular structure becomes blurred. In view of the above two situations, we believe that the appropriate time to determine the fertilization rate of Neosalanx taihuensis is when embryonic development reaches the mid-to-late gastrula stage; a microscope should be used instead of visual observation, and it is hoped that egg-supplying and egg-introducing units will take note of this.

2. Because the egg membrane filaments produced during the incubation of fertilized eggs of Neosalanx taihuensis have a strong adhesive effect, and the fertilized eggs of icefish are relatively small and fragile; April to May is also a period of vigorous feeding by various aquatic organisms. Therefore, incubation should be carried out indoors or in reservoir cages as much as possible. If directly placed into a reservoir, the release area should be sought where dissolved oxygen is sufficient, there is no sediment scouring, and there are few wild trash fish. Whether incubating indoors or in outdoor cages, it is necessary to take into account the large day-night temperature variation in April to May and prevent sharp changes in incubation water temperature from causing embryonic development to stop or abnormal metamorphosis.

3. According to the characteristics of late embryonic development, combined with our practice in recent years in transplanting Taihu icefish, both transplanting fertilized eggs and fry are viable options. The key is to properly manage the developmental timing of icefish fertilized eggs and fry during transportation. We suggest that transportation of fertilized eggs is best carried out 24 hours after fertilization, while transportation of fry is best carried out after the third day post-hatching, so that when the fry arrive at the reservoir, their digestive tract has recently opened; at this time the fry are in good physical condition and have strong stress resistance. In addition, icefish are more delicate and smaller than fry of farmed fish, so transportation must be smooth and fast, and the density of fry in oxygenated nylon bags should not be too high, generally controlled within 50,000 fry per bag.

4. Whether transplanting eggs or fry, the release water body is required to be rich in zooplankton, with few wild trash fish and sufficient dissolved oxygen. Therefore, before release, appropriate fertilization should be actively carried out in the target water body to cultivate water quality, so that when the larvae enter this water body they can capture suitable food. For transplanting fertilized eggs, water bodies with little bottom sediment also need to be considered.