They depend on their mother crabs to give birth to their young, and each time she lays a large number of eggs, up to millions or more. After these eggs hatch in the mother's abdomen, the young ones can be detached from the mother's body and float around with the coastal currents. After a few retreats, they grow into large-eyed larvae, which then grow into juvenile crabs after a few retreats, which are almost identical in appearance to adult crabs, and after a few more retreats, they become crabs. Most of the marine crabs are eggs that mature and are discharged directly into the ocean without hatching.

The hard shell of the crab protects the crab from predators, but the shell does not expand as the body grows. So the crab growth is intermittent, that is, after a period of time, the old shell molted body will continue to grow. The largest crab on earth is the spider crab, their feet spread out as wide as 3.7 meters, the smallest crab is the bean crab, less than half a centimeter in diameter.

Though small, crabs have all the essentials. After the crab's hard shell is removed, we can find that the crab's body is partially protected by a layer of shell, these shield-like shells, biologists call the back armor (carapace). The body of the crab is symmetrical and can be divided into the frontal area, eye area, heart area, liver area, stomach area, intestinal area and gill area. The crab has appendages attached to both sides of the body. The appendages on the head are called antennae, which have tactile and olfactory functions, and some of the appendages have a mouth function, which is used to tear food and deliver it into the mouth.

The crab has five pairs of appendages in the thorax, called thoracic legs. A pair of appendages located in front of the strong chelae, can be used to forage for food, the remaining four pairs of appendages is the crab's feet, crabs walk and move to rely on these four pairs of appendages, they walk in a unique and interesting manner, most of them are walking horizontally rather than straight ahead. The exception is the monk crab, which walks straight ahead.

Crabs spend most of their time searching for food, and they are not picky eaters; they will eat whatever their chela can get their hands on. Small fish and shrimp are their favorites, but some crabs eat seaweed and even dead animals or plants.

Crabs eat other animals, and other animals may also eat crabs, for example, humans eat crabs as a delicacy. And water birds eat crabs too! Some fish also love to eat crab legs as much as humans do. When young, immature crabs swim in schools in the sea, they may be eaten by other sea creatures, which is why crabs lay so many eggs when they spawn.

The fiddler crab is a common crab on the beach, so why is it called a fiddler crab? Because these crabs wave their large chelae as if waving to the tide, the name of "Chaochao" is so called. In addition, the male fiddler crab's giant chelipeds look like violins, so they are also called fiddler crabs or fiddler crabs.

Crab culture technology and disease control technology

River crab is known as the Chinese mitten crab, is a valuable freshwater product, delicious flavor, rich in nutrients, has a high economic value.

The river crab belongs to the Crustacea, mitten crab genus, is a large crustacean, the body is divided into 21 sections, due to the head and thorax sections of the mutual fusion, so the whole body is divided into cephalothorax and abdomen two parts. Adult crabs are dark green on the back, grayish-white on the ventral surface, and the average length of the cephalothorax is 6-7cm and the width is 7.5cm.

I. Biological Characteristics of the River Crab

1. Life History

The river crab is a freshwater crab that grows and reproduces in seawater. Before the reproductive migration are young crabs, because of its shell color yellowish, common second "yellow crab", its gonads are very small, basically undeveloped, the weight of the liver is much greater than the weight of the reproductive glands. When it completed the last molt after the development of the "green crab", over the purchase of dark green gonads developed full, male crab chelicerae villi silk dense, foot setae thick and long, female crabs abdomen completely covering the ventral surface of the cephalothorax, abdominal edge of the setae long and dense.

The life of the river crab is short, in its life history through the flea-like larvae, big-eyed larvae, juvenile crabs and adult crabs and other stages, the life of the larvae need to molt 5 times to become big-eyed larvae, and then molt to become adult crabs in 13-15 days, it has only one reproductive cycle in its life, reproduction end of the life will also be terminated, in general, the life span of the river crab for the age of 1-3 weeks.

2, habits

(1) burrowing

River crabs like to live in the rivers, lakes, mud banks or mudflats in the cave, or hidden in the gravel and grass. The river crab digs burrows as its instinct, and it is also a way of adaptation for the crab to defend itself against enemies. Generally, river crabs dig their burrows on steep banks with hard soil and a slope of 1:0.2 or 1:0.3, and seldom dig their burrows on gentle slopes below 1:1.5-2.5, not to mention digging their burrows on flat land. This provides a basis for our breeding pond construction.

(2) food

The river crab has a very mixed diet, under natural conditions to eat water plants, humus-based, animal carcasses, but also like to eat snails, mussels, worms, insects, and occasionally prey on small fish, shrimp, food scarcity will also be killed by each other, and even devour their own eggs, and sometimes because of a dead fish and incurred the same kind of competition for food, in general, the river crab to obtain plant food to easy, often constituting the crab stomach food, but also the river crab to obtain plant food. In general, river crabs get plant food easily, often constitute the main component of the crab stomach food, the crab stomach and some sediment, which is a sign of the river crab feeding on benthic organisms and humus.

River crabs generally hide in the hole during the day, and come out of the hole at night to feed. On land, river crabs do not feed too much, often dragging the shore food to the underwater or cave side, and then feeding. Anniversary of the river crab in addition to low-temperature stinging temporarily do not eat, even if the winter migration is also feeding as usual. In good water quality, water temperature, bait when abundant, river crabs eat a lot of food, a day and night can be continuous predation of several snails, just moulted soft-shelled crabs, limbs, individuals, and often suffered from infringement, but river crabs are also very strong ability to withstand hunger, fasting for 10 days or even half a month without food, river crabs do not starve to death. In artificial culture conditions, according to the growth and development of river crabs, in the feeding of bait to master the "fine - coarse - fine" structure, is conducive to the growth of river crabs.

(3) river crab molt and growth

The growth process of the river crab is accompanied by larval molt, juvenile crabs or adult snails molt, larvae per molt on the metamorphosis of once, which is divided into a period. From the big eye larvae molt into the first phase of the baby crab, after each molt a shell its body length, weight are made a leap increase from each big eye body weight of 6-7mg gradually increased to 250g of the crab, at least need to molt dozens of times, and each molt is a shell is to pass through the survival of a big pass.

River crabs need to absorb a lot of water when moulting, so in the process of moulting the mass increased significantly, in the subsequent growth, the loss of water is slow, and gradually for the tissue growth instead of the rate of growth of river crabs by the environmental conditions, especially water temperature and bait constraints. Usually, the early young crabs molt more frequently, just into the lake big eye larvae, and then every 5-7 days, 7-10 days successively molt into the second and third period of the young crabs, with the continuous growth, molt intervals are prolonged one by one, if the environmental conditions are poor, molt growth stops, which is the same age individuals, under different conditions the reason for the huge difference in body size.

The growth of river crab is closely related to calcium and phosphorus in water and bait. Tests have shown that the soft-shelled crabs just moulted, the weight than before the moult increased by 30% -40%, this period of time more than one hour, less a few minutes, relying on the gills to absorb a large amount of water as well as inorganic salts in the water. In nature, in ponds or lakes, soft-shell crabs harden their shells in 1-2 days. If put into distilled water to raise soft-shell crabs, the ability of river crabs to absorb calcium ions in the water body is stronger than the ability to absorb calcium and phosphorus in the compound feed. River crab molting eve requires that the total amount of calcium in the shell is equal to the total amount of calcium in the body, while the total amount of phosphorus in the body of the river crab is 52.2 times the amount of phosphorus contained in the shell.

In order to cooperate with the better growth of river crabs, moulting, in the intensive crab pool per mu (average depth of 1 meter) weekly application of calcium chloride 5 kg, sodium hydrogen phosphate 2.5 kg (or lime or calcium superphosphate) river crabs must be considered in the proportion of calcium and phosphorus in the feed with calcium chloride and phosphorus disodium hydroxide ratio of 2:1, in order to ensure that the growth of river crabs, the moulting of the material needs.

Second, the river crab aquaculture technology

(a) buckle crab cultivation

In order to improve the survival of juvenile crabs, increase economic benefits, the first to cultivate buckle crab. In order to the year's big eye larvae to reach the commercial crab, most of the current use of plastic greenhouse temporary culture technology. Choose good water quality, sufficient water; injection, drainage is convenient, no leakage, no seepage, less silt pond, the area should not be too large, the depth of water does not exceed 1.2 meters, pool shape east-west as well, sunny, greenhouse construction can refer to the construction of agricultural vegetable greenhouse construction methods.

15-20 days before putting crab seedlings, each mu with 75 kilograms of quicklime splash disinfection, to be drug disappeared, with 80 mesh filter into the water, cultivate basic bait, transplantation of water plants (must be strictly disinfected), and set up the necessary anti-escape facilities. Crab seedlings can be appropriately shallow when entering the pool. It is conducive to raising the water temperature and the growth of algae and water plants in the water. Feeding bait 2-3 times a day, feeding soybean milk, fish, shrimp mince, etc., feeding into should be more cast in the shallow areas around, cast bait by 4% of the body weight, and always pay attention to the weather, water sanction, individual growth, and compete for the bait organisms depending on the amount of bait. With the growth of young crabs should also increase some plant bait, such as: duckweed and other high-quality water plants, in the daily management of water quality regulation, to master the water quality fat and fresh, water change should be changed diligently less change, do not cause drastic changes in water temperature and water level. After about 2 months of cultivation can reach the buckle crab.

(B) adult crab culture

Adult crabs like clean water, transparency of the water environment, water plants, bait rich, the most suitable for the growth of river crabs, most of the current ponds, lakes, river swings and rice paddy farming. Now the pond as a simple introduction:

1, pond conditions

Sufficient water, easy access to drainage, good water quality and pollution, choose clay, sandy soil or sandy soil, good ventilation, conducive to the growth and reproduction of aquatic grasses and benthic insects, snails and mussels, earthworms and other growth and reproduction of water, the old pond should be thoroughly desilted, silt is not more than 20cm as well as the area of the pond should not be too small. Pond water depth all year round to maintain in 0.6-1.5 meters, the depth of water varies from place to place, the shallowest place 10cm, the pond can be built a number of slightly above the water surface of the mound, that is, the crab island, the island can be transplanted aquatic plants, ponds should not be too steep, the slope ratio is generally in 1:1.5 or less, otherwise, the river crab is easy to dig holes and is not conducive to the evening climb out of the water activities. Artificial burrows can also be constructed.

2, transplanting water plants

The natural yield of river crab mainly depends on the amount of water plants and benthic organisms (bait organisms) in the waters. It is an indispensable technical measure to plant water plants in the process of culture. Planting water plants in addition to vitamin supplementation for crab feeding ten years, but also can be raised to fight each other, the role of hiding, is to improve the survival rate of river crabs during the period of a powerful measure, in addition to absorbing harmful ammonia nitrogen in the pool, carbon dioxide, organic matter, the release of oxygen, stabilizing the role of the water quality. The masses often say "crab size, look at the water plants". Therefore, for the pond culture of river crabs must make great efforts to plant good water plants, water plant species are mainly duckweed, full of red, water hyacinth, water flora, rotunda black algae, goldfish algae, bitter grass, aquatic peanuts, etc., and pay attention to the transplantation of disinfection and prevention of pests.

3, escape facilities

Escape facilities are mostly plastic film, there are also cement boards, depending on the respective situation.

4, pond disinfection

Generally half a month before the release of seedlings with lime pond disinfection, the dosage of 75 kilograms per mu. On the one hand, it can kill hostile organisms, on the other hand, it can improve the bottom of the pond, increase the content of calcium ions in the water, and promote the growth of moulting of river crabs. Na water should be fertilized in time, cultivate algae and basic bait, transparency is generally maintained at 40-50cm is appropriate, if you find a tadpole or frog eggs should be removed in time, so as not to compete for food, for the harm of young crabs.

5, put seedling

Seedling purchase, to the Yangtze River water system production of crab seedlings is preferred, the requirements of neat specifications, complete footsteps, strong body, crawling active, no injury and no disease.

The seedling and density, put the seedling should pay attention to the temperature difference. Seedlings purchased from the field can not be put directly into the pool, should be soaked in water for 2-3 minutes, removed into the set for 10 minutes, so repeat 2-3 times, to be gradually absorbed by young crabs full of water and adapt to the water temperature, put into the pool, you can improve the survival rate.

Density: At present, most of them are mainly mixed culture, especially fish, shrimp and crab mixed culture, the economic benefits will be higher. Density can be controlled at 1500 only/mu or less, with specifications of 120-150 only/kg of button crab. If the conditions are poor or to raise shrimp can be appropriate to reduce the amount of seedlings.

6, daily management

(1) baiting: the whole process of pond finishing, mainly by artificial feeding feed, so the type of feed, the advantages and disadvantages and how much of the growth and development of river crabs have a great impact, baiting should adhere to the principle of fine, green, coarse reasonable mix, animal concentrates accounted for 40%, 35% of the watercress, other plant feed accounted for 25% of the type of feed, the main types of feed are Three categories (a) full-price river crab special feed; (b) animal: sea, freshwater fish, animal carcasses, snails, mussels, livestock and poultry blood, fishmeal, silkworm pupae, etc.; (c) plant: aquatic grasses, duckweed, aquatic peanuts, bitter grass, rotifers, etc., the commercial feed category: taro, mare's yam, cereal grains, gluten, bran, such as chaff.

The feeding method of river crab like pond fish, do "four look four", namely shield season, look at the weather, look at the water quality, look at the crab's activities, timing, fixed point, quality, quantity.

Look at the season: spring young crabs should be fed some live animal feed, river crab growth in the middle of the month, especially May-August, to appropriately increase the amount of animal feed feeding, but mainly plant-based feed, the late river crab needs a lot of nutrition to meet the gonadal development, should be fed more animal feed, so that the river crab weight increase, meat flavor, feed feeding seasonal distribution is generally 40% of the March-June, 60% of the July-October, the feed is generally 40% of the July-October, the feed is generally 40% of the July-October, the feed is generally 40% of the July-October, the feed is generally 60% of the July-October. The water temperature is below 10℃, the crab's activity is less, the intake is not big, you can feed less, feed every other day, when the water temperature is 3-5℃, you can not feed.

Look at the weather: feed more when the weather is sunny, feed less on cloudy and rainy days, stop feeding on hot and muggy days without wind before rain showers, and on foggy days, wait for the fog to close before feeding.

Look at the water quality: the water quality is clear, can be normal baiting, the water quality is thick, appropriate to reduce feeding, timely water change.

Look at the crab's activity: generally eat all the bait in the morning of the next day after feeding, the amount of bait is appropriate, not eat all the bait, indicating that the crab's appetite is not strong or the number of too many, you should analyze the reasons in time to reduce the amount of feeding, and the crab in the moult period should be appropriate to increase the amount of feeding.

Timing: river crabs have the habit of day and night, and go out at night to forage for food, feeding in the morning at 8, 9 o'clock and in the afternoon evening two times, the evening feeding amount of the whole day 60-70% of the amount of feeding.

Fixed point: feeding feed to have a fixed food, feed spread on the feed table or choose to be close to the water level line in shallow water on the slope, in order to observe the river crabs eating, activities, increase or decrease the feed at any time. River crabs have a strong competition for food, so we need to set up more points, so that the river crabs eat evenly, to avoid a part of the whole small or weak physique can not compete for the feed and cause each other to kill.

Quality: river crabs are sensitive to flavor, sweet, bitter, salty, odor and other flavors, the feed must be fresh and palatable and rich in protein.

Quantity: "Fish one day do not eat, three days do not grow", river crabs are the same, which requires the size of river crabs, density, different seasons, weather, activity to determine the feeding volume, the general daily feeding for the stock of crabs 8-10% of body weight, feeding less than life-sustaining, more than the right time to affect the growth of the range, but also to increase the bait coefficient. The bait coefficient is also increased.

Baiting should be done with animal and plant feeds, such as feeding grass and cereals in the morning and mussels, snails and pupae in the afternoon. Avoid feeding a single feed for a long time, otherwise it will cause spilling anorexia, low feed utilization, affecting growth, and small specifications out of the pond.

(2) water quality management: river crabs on the water quality conditions than the requirements of fish high, especially on the polluted water with greater sensitivity, pool water quality is good, conducive to the growth and development of river crabs and meat flavor, crab pond water PH value should be maintained at 7-9, the most suitable 7.5-8.5, pool water dissolved oxygen to maintain more than 5MG / 1, less than 5MG / 1 to affect the growth of river crabs, the following will be the death of river crabs, in the 2MG / 1 The following will be the death of river crabs, adjust the PH value of the pond water, at present the newly excavated pond, the water quality is mostly acidic, we should regularly use lime slurry splashing to improve the water quality, so as to improve the utilization rate of river crabs on the feed, and is conducive to the smooth moulting of river crabs, generally once a month in the spring, summer and autumn commissions every 15 -20 days, every mu of water depth of 1 m with quicklime 7-10 kg.

Pay attention to change the water diligently, keep the water level stable, change the water once a week in the spring, change the water once every 3 days in the summer, such as in case of persistent high temperature, should be insisted on changing the water every day, the weather is hot in the autumn, and often appear hot weather, water quality is easy to become bad, pay special attention to change the water, change the water every 2 days, change the amount of water accounted for the whole area of 1/3 of the water.

The crab is not tolerant of fertilizer, in order to control the fertilization of water quality, put 200-300 pcs of water per acre.

Different seasons of the crab pool on the water level requirements are not the same, the spring in order to improve the water temperature, to promote the growth of river crabs, the water level can be controlled at 0.5-0.7m, summer and fall seasons, the water temperature is higher, the depth of the pool water should be maintained at 1-1.5m.

(3) anti-escape: a person on duty to inspect the embankment, especially on rainy days and change time to be more careful.

(4)Pond patrol:Observe the water color, pay attention to the activities of the crab, do not hurt the river crab, and maintain the stability of the water environment. In the gonad development plump price is appropriate when you can drain the pond water to pick up.

Third, disease prevention and control

1, shell rot

The tip of the diseased crab's step foot is broken into black ulcers and decay, then the step foot sections and dorsal armor, thoracic plate appear white spots and gradually turn into black ulcers, in severe cases, the shell is eroded into a hole, visible muscle or dermatome, resulting in the death of river crabs, the disease is due to the tip of the step foot of the river crab is damaged by infection of the germs.

Prevention:

(1) Thoroughly clear the pond with quicklime, keep the water clean, often refill new water in summer, and keep the bottom of the pond with 5-10 centimeters of silt.

(2)In the pond with the harbinger of disease, use quicklime to splash the whole pond with water to present 15-20PPM.

Treatment:Sprinkle the whole pond with bleach 2PPM, swallow in the feed with sulfonamides, 0.1-0.2 grams of medicine per kilogram of feed, and feed for 3-5 days.

2, black gill disease

Diseased crab gills infected discoloration, light around the gill filaments part of the dark gray or black, heavy gill filaments all become black, slow action, breathing difficulties, commonly known as sighing disease, the disease occurs in the later stages of adult crab culture, the deterioration of the water environment conditions is the main causative factor for the occurrence of the disease. Prevention is the same as rotting shell disease.

3, rotting limb disease

Diseased crab abdomen and appendage rot, during the winter, the incidence of this disease is higher, pay limbs appear spotted rot, spots from yellow to gray to black, the disease is due to cuttings, transportation, stocking process injuries or growth of hostile infections caused by germs.

Prevention:

(1) Don't injure the crab body during cutting, transportation and stocking.

(2) Soak the crabs in 2PPM furazolidone solution for a few minutes before stocking.

Treatment:

(1)Splash 0.5-1PPM of hygromycin or furacilin in the whole pool.

(2)Splash 0.1-0.2PPM of furazolidone in the whole pool.

(3)Splash 15-20PPM of quicklime in the whole pool, and apply it for 2-3 times.

4, edema disease: the sick crab abdomen, abdominal umbilicus and dorsal shell below the swelling, the sick crab creeping poolside, refusing to eat, and finally died in the shallow water at the edge of the pool, the disease is due to the river crab in the aquaculture process of its abdominal injury caused by infection.

Prevention: In the process of breeding, especially during moulting, minimize the disturbance of river crabs to prevent them from being injured.

Treatment:

(1)Sprinkle 0.5-1PPM of hygromycin or furacilin in the whole pool.

(2)Sprinkle 0.1-0.2PPM of furazolidone in the whole pool.

5.Water mold disease

The flocculent filaments on the surface of sick crabs, especially in the wound site, the sick crabs are slow to move, feeding is reduced, and the wounds do not heal. The disease is caused by the injury of river crab and the invasion of mold into the wound.

Prevention:

(1) Don't make the crab body injured during the operation of cuttings, transportation and stocking.

(2) Increase the animal feed before mass molting.

Treatment:

(1) Malachite green is splashed in the whole pool at 0.25PPM, and applied once after 5 days.

(2)Soak the diseased crabs with 3-5% salt water for 5 minutes and apply 5% phosphorus to the affected area.

6, ciliopathy

The diseased crab can attach ciliated protozoa on the joints, step feet, dorsal shell, forehead, appendages and gills. Diseased crabs have more dirt on the surface of the body, activity and feeding ability is weakened, and the worst case can die before dawn. The disease is due to over-fertilization of pool water, long-term non-exchange, ciliate reproduction caused by too much.

Preventive methods: often change the pool water, keep the water quality fresh.

Treatment:

(1) Splash the whole pool with 30% aldehyde for 5-10PPM.

(2) Splash the whole pool with copper sulfate and ferrous sulfate (5:2) for 0.7PPM.

(3) Dip the sick crabs with a mixture of 0.5-1PPM Neosporin and 5-10PPM potassium permanganate.

(4) Use 0.2-0.3PPM malachite green to splash along the edge of the pool where the water plants and other dense crabs.

7, crab slave disease

The sick crab abdomen slightly bloated, open the umbilical cover visible 2-5 mm, about 1 mm thick milky white or translucent granular insect parasites in the appendage or thoracic plate, the growth of sick crabs slow, gonads do not hair good, by the crab slaves serious parasitism of river crabs, the meat taste foul, can not be eaten, the disease occurs mainly because of the high salinity of the pool water, crab slaves reproducing in large quantities, the larvae diffuse The main reason for the occurrence of this disease is the high salinity of the pool water, the proliferation of crab slaves and the spread of larvae.

Preventive methods

(1) Thoroughly clear the pond, kill the crab slave larvae in the pond, common drugs are bleach, trichlorfon, formaldehyde and so on.

(2) Mixing a certain amount of carp in the crab pond can inhibit the number of crab slave larvae.

(3) In ponds with premonitory signs of the onset of the disease, complete replacement of pond water, injection of new water (salinity less than 1 ‰), or move the infected crab slaves of sick crabs to freshwater, you can inhibit the development of the proliferation of crab slaves.

Treatment

(1) Dip the disease with 5PPM copper sulfate solution for 10-20 minutes.

(2) Dip and wash the sick crab with 20PPM potassium permanganate solution for 10-20 minutes.

(3) Copper sulfate, ferrous sulfate (5:2) splash the whole pond was 0.7PPM.

8, molt attempt disease

The disease of the crab's cephalothorax posterior edge of the junction of the junction with the abdomen appeared cracked, but can not molt the old shell, the sick crab around the body of the black, and finally the column death. The disease is a common disease in pond crabs, and the cause of the disease is related to the lack of certain mineral elements in the growth process.

9, polycondylosis

Generally more parasitic crab seedlings, a large number of parasites, so that the crab seedlings can not molt metamorphosis and death, juvenile crabs and adult crabs outside the gill parasites, can also cause death.

Method of control:Soak the crab body with 10PPM malachite green for 45 minutes, or use IPPM Neosporin solution to soak the crab body until the polyps fall off.

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