What do crabs eat at the seaside?

They rely on the mother crab to give birth to small crabs, and every time the mother crab will lay a large number of eggs, the number can reach more than one million. After these eggs hatch in the abdomen of the mother crab, the larvae can leave the mother and float around with the coastal current. After molting several times, it grows into a big-eyed larva, and after molting several times, it grows into a young crab. The appearance of a young crab is almost the same as that of an adult crab, and it becomes a crab after molting several times. Most sea crabs are directly discharged into the sea without hatching after their eggs mature.

The hard carapace of crabs can protect crabs from natural enemies, but the carapace will not expand with the growth of the body. Therefore, the growth of crabs is intermittent, that is, after a period of time, the body will continue to grow after the old shell falls off. The largest crab on the earth is the spider crab, with a foot width of 3.7 meters, and the smallest crab is the bean crab, with a diameter of less than half a centimeter.

Although the crab is small, it covers everything. After removing the hard shell of the crab, we can find that the body part of the crab is protected by a shell. These shield-like shells are called crustaceans by biologists. Crabs are symmetrical and can be divided into forehead area, eye area, heart area, liver area, stomach area, intestine area and gill area. The two sides of a crab are connected by appendages. The appendages of the head are called tentacles, which have the functions of touch and smell. Some appendages have the function of mouth, which is used to tear food and send it into the mouth.

Crab chest has five pairs of appendages, which are called chest feet. A pair of appendages at the front end are equipped with powerful claws, which can be used for foraging. The other four pairs of appendages are crab feet. Crabs rely on these four pairs of appendages to walk and move. The way they walk is unique and interesting, and most of them walk sideways rather than straight. Except monks and crabs, they all walk straight.

Crabs spend most of their time looking for food. They are not picky eaters and eat whatever their claws can catch. Small fish and shrimp are their favorite, but some crabs eat seaweed and even animal carcasses or plants.

Crabs eat other animals, and other animals may eat crabs. For example, humans regard crabs as delicious food. And waterfowl eat crabs! Some fish like to eat crab feet as much as humans. When young crabs and underage crabs float in the sea in groups, they may be devoured by other marine life, so crabs have to lay a lot of eggs when laying eggs.

Fiddler crabs are common crabs on the seashore. Why is it called fiddler crab? Because these crabs wave their claws as if they were waving in the tide, that's how they got the name "Zhao Chao". In addition, the giant claws of male fiddler crabs seem to play the violin, so they are also called fiddler crabs or fiddler crabs.

Crab culture technology and disease control technology

Eriocheir sinensis, the scientific name of Eriocheir sinensis, is a precious freshwater product with delicious taste, rich nutrition and high economic value.

Eriocheir sinensis is a large crustacean belonging to the genus Eriocheir, which is divided into 2 1 node. Because the head and chest segments are mutually healed, the whole body is divided into two parts: head, chest and abdomen. Adult crabs have a dark green back and a gray belly. The average length of the head breastplate is 6-7 cm and the width is 7.5 cm.

1. Biological characteristics of Eriocheir sinensis

1, life history

River crabs are crabs that grow in fresh water and breed in seawater. Before reproductive migration, they were all young crabs. Because their shells are yellow, they are called "yellow crabs". Their gonads are very small and basically undeveloped, and the weight of the liver is far greater than that of the gonads. When it molts for the last time, it develops into a "blue crab". The purchased dark green gonad is full, the male crab has dense hair and thick bristles, the female crab completely covers the ventral surface of the head and breastplate, and the bristles on the ventral edge are long and dense.

The life span of Eriocheir sinensis is very short. In its life history, it has to go through several stages, such as flea larva, megalopa larva, young crab and adult crab. In its lifetime, it takes 5 times for a larva to molt into a megalopa, and then it takes 13- 15 days to molt into a crab. It has only one reproductive cycle in its life, and when reproduction ends, its life ends. Generally speaking, the life span of Eriocheir sinensis is 65438.

2. Living habits

(1) caves

Eriocheir sinensis likes to live in mud flats or caves on the beach in rivers and lakes, or hide in gravel and aquatic plants. Digging a hole is the instinct of the river crab, and it is also an adaptive way for the river crab to defend against enemy damage. Crabs usually burrow on steep slopes with hard soil, with bank slopes of 1:0.2 or 1:0.3, and rarely burrow on gentle slopes below 1: 1.5-2.5, let alone on flat land. This provides a basis for the construction of aquaculture ponds in China.

(2) Eating habits

Eriocheir sinensis has a variety of feeding habits. Under natural conditions, it mainly eats aquatic plants and humus, likes to eat animal carcasses, and also likes to eat snails, mussels, worms and insects. Occasionally they prey on small fish and shrimp. When food is scarce, it will even devour the eggs it holds, and sometimes it will cause similar food competition because of a dead fish. Under normal circumstances, Eriocheir sinensis can easily get plant food, which often constitutes the main component of crab stomach food.

River crabs usually hide in caves during the day and go out for food at night. On land, river crabs don't eat much, and often drag the food on the shore to the underwater or the edge of the cave before eating. On the anniversary, river crabs eat as usual even if they migrate in winter, but they don't eat low temperature for the time being. When the water quality is good, the water temperature is suitable and the bait is rich, the river crab can eat a lot. It can prey on several snails, newly molted soft-shelled crabs and disabled individuals, but river crabs have strong hunger tolerance. If it doesn't eat for 10 days or even half a month, the river crab won't starve to death. Under the condition of artificial culture, it is beneficial to master the "fine-coarse-fine" structure of feeding bait according to the needs of the growth and development of river crabs.

(3) molting and growth of Eriocheir sinensis

The growth process of Eriocheir sinensis is accompanied by the molting of larvae, young crabs or adult snails. Every time a larva molts and metamorphoses, it is divided into one stage. From the molting of megalopa to the first stage of juvenile crab, the body length and weight of each megalopa will advance by leaps and bounds every time it molts. The weight of each big-eyed crab will gradually increase to 250g, and it needs to molt for at least dozens of times, and each molt will pass the survival barrier.

Eriocheir sinensis needs to absorb a lot of water during molting, so its weight is obviously increased. In the later stage of growth, water loss is slow and gradually replaced by tissue growth. The growth rate of Eriocheir sinensis is restricted by environmental conditions, especially water temperature and bait. Usually, early young crabs often shed their skins. The megalopa just entering the lake will shed its skin every 5- 7 days and 7- 10 days, forming the second and third stage larvae. With continuous growth, the molting interval will be extended one after another. If the environmental conditions are not good, the molting growth will stop, which is also the reason why individuals of the same age have great differences in body shape under different conditions.

The growth of Eriocheir sinensis is closely related to calcium and phosphorus in water and food. Related experiments show that the weight of newly molted soft-shelled crabs is increased by 30%-40% compared with that before molting, and this period is as long as 1 hour, and it takes at least a few minutes, relying on gills to absorb a lot of water and inorganic salts in water. In natural ponds or lakes, the shells of soft-shelled crabs become hard after 1-2 days. If soft-shelled crabs are raised in distilled water, their ability to absorb calcium ions in water is stronger than that of calcium and phosphorus in compound feed. Before molting, the total amount of calcium in the shell is required to be equal to that in the body, and the total amount of phosphorus in the body is 52.2 times that in the shell.

In order to cooperate with the growth and molting of Eriocheir sinensis, 5 kg of calcium chloride and 2.5 kg of disodium hydrogen phosphate (or quicklime or calcium superphosphate) were applied per mu (average water depth 1 m) in intensive crab ponds every week. The ratio of calcium to phosphorus must be considered in the compound feed of Eriocheir sinensis, and the ratio of calcium chloride to disodium hydrogen phosphate is 2: 1, so as to ensure the material needs of Eriocheir sinensis for growth and molting.

Second, the river crab culture technology

(1) Breeding of Eriocheir sinensis

In order to improve the survival rate of young crabs and increase economic benefits, crabs should be raised first. At present, in order to make the big-eyed larvae reach the commercial crab, most of them adopt temporary rearing technology in plastic greenhouse. Choose one with good water quality and sufficient water resources; Note: The pond with convenient drainage, no water leakage, no water seepage and little silt should not be too large, the water depth should not exceed 1.2m, the east-west direction of the pond is good, and the sunshine is sufficient. Please refer to the construction method of agricultural vegetable greenhouse for the construction of greenhouse.

15- 20 days before the crab seedlings are released, 75 kilograms of quicklime should be sprinkled on each mu for disinfection. After the medicinal properties disappear, it is necessary to feed water with an 80-mesh sieve to cultivate basic bait, transplant aquatic plants (which must be strictly disinfected), and set up necessary escape prevention facilities. Crab seedlings can be shallower when they enter the pool. Conducive to improving the water temperature and the growth of algae and aquatic plants in the water. Feed bait 2-3 times a day, such as soybean milk, fish, shrimp paste, etc. , and feed in the surrounding shallow waters, and feed at 4% of the body weight, and always pay attention to the weather, water quality sanctions, individual growth, and the number of organisms competing for food. With the growth of young crabs, we should also add some plant bait, such as duckweed and other high-quality aquatic plants. In daily management, we should pay attention to water quality regulation, grasp that water quality should be fat and cool, change water frequently, and change water less, so as not to cause drastic changes in water temperature and water level. After about 2 months of cultivation, crabs can be detained.

(2) Adult crab culture

Adult crabs like the clear and transparent water environment, rich in aquatic plants and rich in bait, which is most suitable for the growth of river crabs. At present, it is mostly cultivated in ponds, lakes, rivers and rice fields. Now give priority to with the pond for a brief introduction:

1, pond conditions

Clay, sandy soil or sub-sandy soil with sufficient water, convenient water intake and drainage, good water pollution and good ventilation are beneficial to the growth and reproduction of aquatic plants, benthic insects, snails and water worms. The old pond should be thoroughly dredged, the silt should not exceed 20cm, and the pond area should not be too small. The water depth of the pond is kept at about 0.6- 1.5m all the year round, and varies from place to place, with the shallowest point 10cm. Several mounds slightly above the water surface can be built in the pond, that is, crab island, where aquatic plants can be transplanted. The pond should not be too steep, and the slope ratio is generally below 1: 1.5, otherwise the river crabs will grow easily. You can also build artificial caves.

2. Transplant aquatic plants

The natural yield of Eriocheir sinensis mainly depends on the number of aquatic plants and benthic organisms (bait organisms) in the waters. In the process of aquaculture, it is an indispensable technical measure to plant aquatic plants well. Planting aquatic plants can not only feed crabs and supplement vitamins for ten days, but also support fighting and concealment, which is a powerful measure to improve the survival rate of river crabs in various periods. In addition, it can absorb harmful ammonia nitrogen, carbon dioxide and organic matter in the pool, release oxygen and stabilize water quality. The masses often say that "the size of crabs depends on aquatic plants." Therefore, river crabs cultured in ponds should make great efforts to plant aquatic plants. Aquatic plants mainly include duckweed, red duckweed, water hyacinth, water hyacinth, verticillium, Ceratophyllum, Sophora alopecuroides and Alternanthera philoxeroides. Attention should be paid to disinfection and prevention when transplanting.

3. Escape prevention facilities

Escape prevention facilities mostly use plastic film, but also useful cement board, depending on their own situation.

4, pond cleaning and disinfection

Generally, the pond is disinfected with quicklime half a month before the seedlings are released, and the dosage is 75 kg per mu. On the one hand, it can kill pests, on the other hand, it can improve the bottom of the pond, increase the content of calcium ions in the water and promote the molting growth of Eriocheir sinensis. Fertilize in time after receiving water to cultivate algae and basic bait, and the transparency is generally maintained at 40-50cm. If tadpoles or frog eggs are found, they should be taken out in time to avoid competing for food and hurting young crabs.

5, put the seedlings

Crab seedlings produced in the Yangtze River system are the best seed selection, requiring neat specifications, complete steps, strong physique, active crawling and no injuries.

Sowing and density, pay attention to temperature difference when sowing. Fish fry bought from other places can't be directly put into the pond. They should be soaked in water for 2-3 minutes, taken out and put into the pond for 10 minute, and so on for 2-3 times. Young crabs gradually absorb enough water, adapt to the water temperature, and then put them into the pond, which can improve the survival rate.

Density: At present, it is mostly polyculture, especially polyculture of fish, shrimp and crab, which will bring higher economic benefits. The density can be controlled within 1500 crabs/mu, and the specification is 120- 150 crabs/kg. If the conditions are poor or shrimp farming is the main thing, the amount of seedlings released can be reduced appropriately.

6. Daily management

(1) Feeding: The whole process of intensive culture in ponds mainly depends on artificial feeding, so the type and quality of feed have a great influence on the growth and development of river crabs. When feeding, we should adhere to the principle of reasonable collocation of fine, green and coarse, with animal concentrate accounting for 40%, aquatic plants accounting for 35% and other plant feed accounting for 25%. There are three main types of feed: ① ② animals: small miscellaneous fish in seawater and fresh water, corpses of various animals, snails, mussels, blood of livestock and poultry, fish meal, silkworm chrysalis and so on. ; (3) Botany: aquatic plants, duckweed, Alternanthera philoxeroides, black algae, etc. Commodity feed: sweet potato, hemp potato, grain, bran, feed bran, etc.

Like pond culture, the feeding method of river crabs is "four observations and four determinations", that is, watching seasons, weather, water quality and crab activities, timing, fixed point, qualitative and quantitative.

It depends on the season: young crabs should be fed some live animal feed in spring, and the amount of animal feed should be appropriately increased in the middle growth period of river crabs, especially in May-August. However, the feed is mainly plant-based, and the crab needs a lot of nutrition to meet the gonad development in the later stage, so it is necessary to feed more animal feed, so that the weight of the crab will increase and the meat quality will be delicious. Feed distribution is generally 40% from March to June, 7-6540.

Look at the weather: feed more in sunny days, less in rainy days, stop feeding when it is sultry and windy before showers, and feed in foggy days.

Look at the water quality: the water quality is clear, it can be fed normally, and the water quality is strong. Reduce the feeding appropriately and change the water in time.

Look at the activity of crabs: generally, they are eaten up the next morning after feeding, and the feeding amount is appropriate, indicating that crabs have a bad appetite or too many. Therefore, it is necessary to analyze the reasons in time, reduce the feeding amount, and appropriately increase the feeding amount during molting.

Timing: Eriocheir sinensis has the habit of staying up late at night and going out for food at night. Feeding is carried out at 8: 00 and 9: 00 in the morning and in the afternoon and evening, and the feeding amount in the evening is 60-70% of the feeding amount in the whole day.

Fixed point: The feed to be fed should have fixed food, and the feed should be scattered on the feed table or on the slope near the shallow water level, so as to observe the feeding and activities of the river crab and increase or decrease the feed at any time. Chinese mitten-handed crabs have a strong competition for food, so it is necessary to set up more points so that Chinese mitten-handed crabs can eat evenly and avoid killing each other because some of them are small or weak and can't compete for feed.

Qualitative judgment: Eriocheir sinensis is sensitive to the taste of fragrance, sweetness, bitterness, saltiness and odor, and its feed must be fresh and palatable, and rich in protein.

Quantification: "If you don't eat fish for one day, it won't be bad for three days", so does the river crab. This requires that the feeding amount should be determined according to the size, density, different seasons, weather and activities of river crabs. Generally, the daily feeding amount is 8- 10% of the crab weight in the pond. If the feeding amount is small, it can only maintain life, and if it exceeds the timely range, it will also affect growth and increase feed coefficient.

When feeding bait, animal and plant feeds should be matched, such as forage and grain in the morning and mussels, snails and silkworm chrysalis in the afternoon. Avoid feeding a single feed for a long time, otherwise it will cause anorexia, low feed utilization rate, affect growth and small specifications.

(2) Water quality management: Eriocheir sinensis has higher requirements for water quality than fish and is more sensitive to polluted water. The pond water quality is good, which is beneficial to the growth and development of Eriocheir sinensis, and the meat is delicious. The PH value of the crab pond water should be kept at 7-9, preferably 7.5-8.5, and the dissolved oxygen in the pond should be kept above 5MG/ 1 and below 5MG/65438+. Crab will die when it is below 2MG/ 1. Adjust the PH value of the pool water. At present, the water quality of newly dug ponds is mostly acidic, so it is necessary to spray quicklime slurry regularly to improve the water quality, which can improve the utilization rate of feed for crabs and help them molt smoothly. Generally once a month in spring, once a day in summer and autumn 15 -20, and the water depth is 65438 per mu.

Pay attention to changing water frequently and keep the water level stable. Change the water once a week in spring and once every three days in summer. In case of continuous high temperature, change the water every day. The weather in autumn is hot, often sultry weather, and the water quality is easy to deteriorate. Pay special attention to changing water once every two days. Water exchange accounts for 1/3 of the total water area.

Eriocheir sinensis is not tolerant of fat and water. In order to control the water quality and keep it from becoming fat, 200-300 species of silver carp are planted per mu, and aquatic plants are planted around the crab pond.

Different seasons have different requirements for the water level of crab ponds. In order to raise the water temperature and promote the growth of Eriocheir sinensis in spring, the water level can be controlled at 0.5-0.7m, the water temperature is higher in summer and autumn, and the pond depth should be kept at1-1.5m. ..

(3) Escape prevention: A special person is on duty to patrol the embankment, especially when it rains and the weather changes.

(4) Patrol the pond: observe the color of the water, pay attention to the activities of crabs, don't hurt crabs, and keep the water environment stable. When the gonads are well developed and the price is appropriate, the pool water can be released and fished up.

Third, disease control.

1, shell rot

The toes of the sick crab were damaged and turned into black ulcers, and then white spots appeared on each foot joint, carapace and chest plate, and gradually became black ulcers. In severe cases, the carapace is eroded into a hole, and muscles or skin membranes can be seen, resulting in the death of crabs. This disease is caused by toe damage and bacterial infection of crabs.

Prevention methods:

(1) Thoroughly clean the pond with quicklime to keep the water clean, and replenish fresh water frequently in summer to keep 5- 10 cm silt at the bottom of the pond.

(2) In the pond with symptoms, the whole pond is splashed with quicklime mixed with water, showing 15-20PPM.

Treatment method: spray 2PPM bleaching powder all over the pond, add sulfonamides to the feed, and add 0. 1-0.2 g per kilogram of feed for 3-5 days.

2. Black gill disease

The gill of the sick crab is discolored by infection. When it is light, the left and right gills are dark gray or black. When it is heavy, all gills turn black. Sick crabs move slowly and have difficulty breathing, commonly known as sighing. The disease mostly occurs in the late stage of adult crab culture, and the deterioration of water environment conditions is the main cause of the disease. Prevent the same rot.

3, rotten limb disease

The abdomen and appendages of the sick crab rot. During the wintering period, the incidence of the disease is high, and the appendages appear spotted rot, and the spots change from yellow to gray and then to black. This disease is caused by being injured in the process of fishing, transportation and stocking, or being injured by the enemy during the growth process.

Prevention methods:

(1) Don't hurt crabs during fishing, transportation and stocking.

(2) Before stocking, put the river crab in 2PPM furazolidone solution for a few minutes and then put it in.

Treatment:

(1) oxytetracycline or furacilin is sprayed at 0.5- 1PPM.

(2) Furazolidone overflowed from the whole pond was 0. 1-0.2PPM.

(3) The quicklime is sprayed in the whole pond at the rate of 15-20PPM, and applied in 2-3 times.

4. Edema: The abdomen, navel and the lower part of the back shell of the sick crab are swollen. The sick crab crawls by the pool, refuses to eat, and finally dies in the shallow water by the pool. This disease is caused by abdominal injury and infection of Eriocheir sinensis during culture.

Preventive measures: In the process of breeding, especially when molting, try to reduce the disturbance to the river crab and avoid its injury.

Treatment:

(1) oxytetracycline or furacilin is sprayed at 0.5- 1PPM.

(2) Furazolidone is sprayed in the whole pond at the rate of 0. 1-0.2PPM.

5. Hydromycetes

The flocculent silk on the body surface of the sick crab, especially on the wound, moves slowly, eats less, and the wound does not heal, which leads to the ulceration and spread of the wound tissue and leads to death. This disease is caused by the injury of river crab and the invasion of mold into the wound.

Prevention methods:

(1) Don't hurt crabs during fishing, transportation and stocking.

(2) Add a lot of animal feed before molting.

Treatment:

(1) malachite green 0.25PPM is sprayed in the whole pool, and it is applied once after 5 days.

(2) Soak the sick crab in 3-5% salt solution for 5 minutes, and smear the affected area with 5% phosphorus.

6. Ciliary diseases

Ciliated protozoa can be attached to the joints, feet, dorsal shell, forehead, appendages and gills of sick crabs. There is more dirt on the body surface of the sick crab, and its activity and feeding ability are weakened. In severe cases, they will die before dawn. The disease is caused by excessive fertilization of pool water, long-term non-replacement, and excessive ciliate reproduction.

Prevention method: change the pool water frequently to keep the water fresh.

Treatment:

(1) Spray 5- 10PPM with 30% acetaldehyde.

(2) Sprinkle 0.7PPM copper sulfate and ferrous sulfate (5:2) in the whole pool.

(3) Soak the sick crabs in the mixed solution of 0.5- 1PPM bromogeramine and 5- 10PPM potassium permanganate.

(4) Use 0.2-0.3PPM of malachite green to splash along the crowded places of river crabs such as aquatic plants beside the pool.

7. Crab slave disease

The sick crab's abdomen is slightly swollen. When the umbilical cover is opened, it can be seen that the milky white or translucent granular worms with a thickness of about 65438±0mm are parasitic on the accessory branches or chest plate. Sick crabs grow slowly, and their gonads are poor. The river crabs seriously parasitized by crab slaves have a bad smell and are inedible. The main causes of this disease are the high salt content in the pond water, the large number of crab slaves and the spread of larvae.

preventive

(1) Clean the pond thoroughly and kill the crab slave larvae in the pond. Common drugs include bleaching powder, trichlorfon and formaldehyde.

(2) Mixed culture of a certain amount of carp in crab ponds can inhibit the number of crab slave larvae.

(3) In ponds with signs of disease, the development and spread of crab slaves can be suppressed by completely replacing the pond water, injecting fresh water (salinity is less than 1‰) or moving sick crabs infected with crab slaves into fresh water.

therapeutic method

(1) soak the disease in 5PPM copper sulfate solution 10-20 minutes.

(2) Soak the sick crab 10-20 minutes with 20PPM potassium permanganate solution.

(3) The whole pond overflowed with 0.7PPM copper sulfate and ferrous sulfate (5∶2).

8. Incomplete molting

A crack appeared at the junction of the head, chest and abdomen of the sick crab, but the old shell could not fall off, and the sick crab became black all over and finally died. The common diseases of raising crabs in ponds are related to the lack of some mineral elements in the growth process.

9. Polyconchocerciasis

Generally, there are many parasites in the crab seedling stage. When there are many parasites, the crab seedlings can't peel off and die. When there are many parasites in the gills of young crabs and adult crabs, it will also lead to death.

Control method: soak the crab body with 10PPM malachite green for 45 minutes, or soak the crab body with IPPM bromogeramine solution until the tail falls off.

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