Shulou(Shulou.com)11/24 Report--

He Jiabei

Graduated from Yantai Marine Economic Research Institute

Mainly engaged in the research on the breeding of economic snails in the north of China

Wang Han

Graduated from Yantai Coastal Zone Research Institute, Chinese Academy of Sciences

Mainly engaged in the research on the breeding of economic snails in the north of China

When it comes to snails, most of us think of the slow guys on the lawn, vegetable field and orchard in the community after the rain, but in our country, the halo of the snail protagonist is almost full. What snail-hare race story, snail grapevine story, and even Jay Chou wrote a song "Snail" to praise it. However, few of us know how snails give birth to children. Today, let's talk about how snails give birth.

Fig. 1 snails in fact, our common land snails are only a small part of them. Aquatic snails are the absolute core of the family, which is what we often call snails. Today, we mainly talk about the reproductive process of several aquatic snails in northern China.

Reproductive strategy organisms mainly have two reproductive strategies, r strategy and K strategy. Popularly speaking, the former is crazy to have children, win by quantity, regardless of management. Many lower small creatures adopt this way of reproduction, such as our common sea cucumber, scallop chlamys farreri, Penaeus chinensis and so on. The latter, on the contrary, wins by quality, and parents take the route of eugenics and rearing their offspring. Many large higher animals take this approach, such as Asian elephants, whale sharks and so on.

Snails are classified as molluscs and gastropods, which are relatively inferior animals. in theory, they should also manage and raise them, but there are mavericks everywhere, and snails are no exception. Let's talk about it in detail below.

Haliotis discus hannai is actually a kind of aquatic snail, but the snail is not very obvious. If you take a closer look at its shell, or eat it with internal organs, it will be easy to find that it is really a snail.

Fig. 2 abalone Haliotis discus hannai is a common abalone in northern China. In fact, it is a hybrid between Japanese abalone and our native abalone. Because of its stronger adaptability, it has now replaced the local abalone and become the most important species of abalone farmed and caught in northern China.

Haliotis discus hannai is dioecious. In the breeding season, the female gonad is dark green and the male gonad is milky yellow, which can be clearly distinguished from the appearance. In vitro fertilization, the number of female abalone spawning is related to size, nutrition and other factors. Under the normal culture condition, the average female abalone lays 80 million eggs in one reproductive season.

During the breeding season, technicians illuminate clean sea water with ultraviolet lights in advance for a period of time, and then put male and female abalone in different breeding tanks to stimulate spawning and ovulation, and then mix the sea water containing sperm and eggs to complete the process of in vitro fertilization. In the whole process, male and female abalone do not need to meet, do not know each other and fall in love with each other, and even the joy of bridal chamber candles is omitted.

Under natural conditions, usually male and female abalone meet, the male first ovulates, the female feels the stimulation, begins to ovulate, and the sperm and eggs combine in the water to complete the extension of life. Three to four days after fertilization, the fertilized egg ends the planktonic larval stage and develops into a creeping larva and enters the benthic stage through the processes of trochophore larva and faceplate larva, and then after about 40 days, the creeping larva metamorphoses into a juvenile snail, and the child-rearing process is almost over.

The whole process of abalone parents did not make much effort to prolong their lives, and the whole process was managed by breeders or Mother Nature. In nature, the survival rate of abalone larvae is very low, typically winning by quantity.

Red snail is a common marine snail in northern China and even in the world. The breeding season is every summer. Its reproductive season has a clustering habit (figure 5), Yantai has a common saying "wheat on the court, conch on the bed," it is precisely this habit.

Unlike abalone, in vivo fertilization (figure 4) adopts a polyandrogenic strategy, that is, children born to the same mother have many fathers. Similarly, the father of the red snail does not put his eggs in one basket, and the children of the same father can be born by several mothers.

Fig. 4 unlike the abalone, the red snail lays the fertilized egg in a long knife-shaped egg sac, the fertilized egg is wrapped in protein, and the overall structure is similar to that of a yellowish egg. A single oocyst contains thousands of fertilized eggs, and the base of multiple oocysts are flattened by snail mothers with abdominal feet and glued together, and the clusters of oocysts are glued to undersea reefs or oyster shells. The newly laid eggs are bright yellow and look like a blooming chrysanthemum in the water, commonly known as "sea chrysanthemum".

Fig. 5 after about 15 days, the fertilized eggs in the oocyst went through the stages of trochophore larva and facet larva, and the color changed gradually from bright yellow to black (figure 6). The larva drilled out of the small hole in the head of the oocyst and entered the stage of planktonic larva.

Fig. 6 the hatching rate of the gradually blackened "sea chrysanthemum" red snail larvae is so high that almost all fertilized eggs can break the membrane and enter the planktonic larval stage (see video). At this time, the larva has begun to take the shape of a snail, which is called a snail layer larva. After 28 days of floating life in the water, the snail layer gradually enriches and eventually metamorphoses into juvenile snails.

In the whole process of nurturing young snails, parents provided their children with a house (oocyst) and some property (protein liquid in the sac) as early support, and the larvae also had a shell after hatching from the sac, compared with abalone larvae. Can better protect themselves, it can be seen that the concept of eugenics has gradually spread in the snail world. But generally speaking, the survival rate of Oncomelania hupensis larvae is also very low, and its reproductive strategy is still not enough.

Oncomelania hupensis is a kind of cool-water marine snails, and the breeding season is spring and summer. Similar to the red snail, the snail is also fertilized in vivo. Thousands of fertilized eggs are laid in the oocyst, and the base of the oocyst is collapsed and bonded together to form a tower-shaped egg group, commonly known as "sea bract rice". (figure 7)

Fig. 7 the spawning snail and the snail also adopts the multi-male mating mode, talks about many relationships, and likes to carry out "multi-snail movement". However, the snail is also very different from the red snail, the snail egg group will not turn into black bract rice, from the beginning to the end will not become a panicle blackened bract rice.

The snail has no planktonic larva stage, and there is no pore at the top of the oocyst for the larva to break the membrane. After about 2 months, the fertilized egg directly hatches into the juvenile snail after the stages of trochophore larva and facet larva. What is more peculiar is that only one or two eggs in the same egg sac can eventually hatch into young snails, and the other eggs are used as the nutritional supply of this young snail, which is called nutritious eggs. Thousands of eggs hatch only one life, with an amazing degree of entrainment.

Fig. 8 the nutritive eggs of snail cubs that have not broken the membrane in the capsule are not rare in gastropods and hirsutes. In China, there are snails in the north and snails in the south, all of which are typical non-planktonic larvae. the mode of reproduction with nutritious eggs in the capsule.

However, researchers at home and abroad have different opinions on the cause of vegetative eggs, generally speaking, there are some scientific hypotheses, such as unfertilized nutritious eggs, abnormal fertilized eggs, parental environment selection and so on. However, regardless of the specific mechanism, snails provide a large house for future generations to live in for a long time, and reserve enough food as the living capital of young snails, and their offspring live up to the kindness of their parents. The hatching rate of snails is extremely high, which can reach more than 90%. This breeding strategy can be said to be very K.

The Chinese Yuantian snail thought that the aquatic snail had been very considerate in taking care of the snail's offspring. After all, having a house and money was the cub's highest pursuit in the human world, but the Chinese snail once again refreshed the height of K.

As a low-class animal, the Chinese round field snail successfully matched the target mammal and realized egg viviparity. The young snail developed in the maternal womb and did not break away from the maternal protection until it had the ability to live independently and hatched successfully from the maternal body.

We usually eat snails sometimes creak, in fact, it is not sand, but …

Although maternal arrest may be wiped out, adults are always more likely to survive than babies, aren't they? In this regard, I can only say, China Yuantian snail, you are old K.

In our human world, we tend to think that eugenics is good, but there are too few offspring, and we always want to protect them well, which often limits their ability to break through their parents. Although creatures such as abalone and red snails that adopt the r model have little protection for their offspring, under the influence of ocean currents, future generations have the opportunity to go farther away to explore a world that their parents have never explored. Leave it to Mother Nature to judge.

References:

Pan Yang, Qiu Tianlong, Zhang Tao, etc. Morphological observation on the early development of red snail [J]. Journal of Fisheries, 2013, 37, (10): 1503-1512.

[2] Hao ZL, Liu HY and Dong XY et al., Reproductive characteristics and variations in the biochemical composition of Neptunea arthritica cumingii crosse through embryonic development [J], Aquaculture Research, 2020; 00: 1-11.

Wang Rucai, Wang Zhaoping, etc. Marine Shellfish Culture, Ocean University Press of China, 2008.

Shen Guoying et al., Marine Ecology, Science Press, 2010.

This article comes from the official account of Wechat: Institute of Physics, Chinese Academy of Sciences (ID:cas-iop). By he Jiabei and Wang Han.

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