Causes of Yellow Bottom and Yellow Gills in Farmed Swimming Crab (Portunus trituberculatus) (with pictures)

Because the swimming crab (Portunus trituberculatus) has characteristics such as fast growth, large individual size and delicious meat, it has become one of the important economic species in pond aquaculture in China, and artificial farming is now fairly common, so what do pictures of the swimming crab look like? Why do yellow bottom and yellow gills appear when farming swimming crab?

1. What do pictures of the swimming crab look like?

1. The cephalothorax is shuttle-shaped and slightly raised. The surface has 3 prominent wart-like bumps, 1 in the gastric region and 2 in the cardiac region. Its body shape is elliptical, with both ends pointed like a weaving shuttle, hence the name swimming crab (Portunus trituberculatus). Each of the two anterolateral margins has 9 serrations, and the 9th serration is particularly long and extends to the left and right. The frontal margin has 4 small teeth. On both sides of the forehead there is 1 pair of movable stalked compound eyes. There are 5 pairs of thoracic legs. The chelipeds are well developed, the merus is prismatic, and the inner margin has blunt teeth. The dactylus of the 4th pair of walking legs is flat, wide and thin like a paddle, suitable for swimming. The abdomen is flat (commonly known as the crab navel); the abdomen of the male crab is triangular, and that of the female crab is round. The dorsal side of the male crab is tea-green, the female crab is purple, and the ventral side of both is grayish white.

2. The body color of the swimming crab (Portunus trituberculatus) varies with the surrounding environment. Individuals living on a sandy bottom have a light grayish green cephalothorax, with a round white spot in the anterior branchial region; most of the chelipeds are purplish red with white spots, and part or all of the ventral surface is white; the merus and carpus of the first 3 pairs of walking legs are also white, the palm is blue-white, the soft hairs are brown, the dactylus is purple-blue or purplish red, and the 4th pair of walking legs is green with white spots and purple-blue fingertips. Individuals living among seagrasses have darker body color. It is omnivorous, eating fish, shrimp, shellfish and algae, and even its own kind, and likes to eat animal carcasses.

2. Why do yellow bottom and yellow gills appear when farming swimming crab?

1. In the swimming crab (Portunus trituberculatus), the above may be due to yellow gills and edema, which lead to a decrease in its oxygen-carrying capacity, so the crab shows a condition similar to oxygen deprivation and moves toward the pond edge.

2. Yellow bottom in the swimming crab (Portunus trituberculatus) is mostly caused by poor substrate and long periods without molting.

(1) Causes of poor substrate

① The aquaculture pond is an old pond of many years, and at the beginning of the year no thorough dredging was carried out; only simple trimming of the ring ditch was done, so there is a lot of silt and the substrate is poor.

② In the middle and late stages, the feeding of pelleted feed and bait fish increased, and leftover feed and shrimp and crab feces also increased, while the frequency of bottom improvement was low, leading to deterioration of the pond substrate.

③ The use of low-quality water-fertilizing paste and chemical fertilizers, without attention to supplementing bacteria and regulating water, meant that much fertilizer not fully absorbed by algae settled in the pond; at the same time, the farming process did not pay attention to improving the substrate, leading to an increase in harmful substances and deterioration of the pond bottom.

(2) Causes of not molting

① The continuous high temperatures and low dissolved oxygen in mid-to-late July and early August may have caused some swimming crabs (Portunus trituberculatus) to molt one fewer time or even two fewer times, and crabs that have not molted for a long time are prone to yellow bottom.

② Insufficient feeding of animal bait (bait fish): some ponds mainly feed small shrimp, and insufficient feeding of bait fish easily causes a decline in the physical strength of the swimming crab and affects molting.

③ Usually no attention is paid to calcium and mineral supplementation. As shrimp and crabs grow, calcium, magnesium and some trace elements in the pond water are gradually used up; if not replenished in time, shrimp and crabs are more likely to have difficulty molting or even not molt at all later on.

Treatment suggestions:

1. Pay attention to strengthening the improvement of the pond substrate, and regularly use the chemical bottom improver “Diheian” or “Ozone Water An” together with the biological bottom improver “Digai” to improve the pond bottom.

2. In the middle and late stages of farming, pay attention to supplementing bacteria and reducing fertilizer supplementation. In fact, fertilizer in the pond is sufficient in the middle and late stages, and beneficial bacteria such as “Lishengsu” or “Rujiaosu” can be used to remove fertilizer and regulate algae with bacteria.

3. In ponds with a relatively large number of swimming crabs (Portunus trituberculatus), strengthen the feeding of animal bait to enhance nutrition, and at the same time promote fattening of the swimming crab; when the crab is fat, the price will be better.

4. Regularly supplement calcium and minerals to keep shrimp and crabs able to absorb enough calcium, promote molting, and reduce the probability of incomplete molting and failure to harden the shell.