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Background color is a crucial factor influencing the growth performance and feed utilization in crustaceans. However, little is known about its impacts on the culture performance of Chinese mitten crab (Eriocheir sinensis). In this study, we examined the impacts of six different background colors (white-W, black-BK, green-G, red-R, yellow-Y, and blue-BL) on growth, inter-molt period, and coloration and total carotenoids content of carapace of juvenile E. sinensis. The culture experiment comprised two trials: a 68-day individual culture, and a 30-day group culture. Individual culture results: No significant effects of background color were found on weight gain rate (WGR), specific growth rate (SGR), molting interval, carapace length and width, and survival rate (SR) of the juvenile crabs (P > 0.05). However, during the second molting cycle, the BL group exhibited the highest WGR, and the G group showed the lowest hepatopancreas index (HSI). Meanwhile, markedly lower feed conversion ratio (FCR) was obtained for the W group compared to the G and BK groups. Group culture results: Significantly higher SR was recorded for the BK group, and the highest limb loss rate was detected for Y group indicating the significant effects of background color on survival and aggressiveness of juvenile E. sinensis. Moreover, background color impacted the carapace coloration indices where the W, BK and Y groups showed the highest brightness (L*), redness (a*), and yellowness (b*) values, respectively. Furthermore, correlation analysis revealed a positive linear relationship between the L* value and background color (y =0.1638x+27.961, R2=0.613). In conclusion, our results demonstrated that background color significantly affects culture performance and carapace color. Black background color was shown to enhance the survival rate and reduce the limb amputation rate under group culture condition. These results provide new insights on the optimization of farming technology for juvenile E. sinensis.

Seawater contains a variety of mineral ions, such as Na+, Mg2+, Ca2+, and K+. Although previous studies have shown that brackish water can promote ovarian development of the Chinese mitten crab (Eriocheir sinensis), it remains unclear whether these ions directly affect the ovarian development and vitellogenesis of E. sinensis. Therefore, this study was designed, using in vitro tissue incubation mediums with different concentrations of Na+ (280, 320, 360, and 400 mmol/L), Mg2+ (5, 10, and 15 mmol/L), Ca2+ (5, 10, 15, and 20 mmol/L), and K+ (5.5, 9, and 13 mmol/L), to investigate their effects on Vg/Vn contents and the expression of ovarian development-related genes in the hepatopancreas and ovaries of E. sinensis. The results showed that: (1) Different Na+ concentrations had no significant impact on vitellogenesis and the expression of ovarian development-related genes in both tissues, except for an enhancement of the esEcR2-mRNA levels in the hepatopancreas. (2) The Mg2+ (15 mmol/L) significantly increased the Vg/Vn content in hepatopancreas and ovary. However, the 10 mmol/L Mg2+ group decreased esVgR-mRNA levels and esEcR2-mRNA levels in the ovary compared to the control group. (3) Ca2+ (10 ∼ 15 mmol/L) significantly promoted Vg/Vn content and esVg-mRNA levels in both tissues, which also upregulated the expression of ovarian development-related genes (esERR, esEcR2, and esRXR). (4) Increasing K+ content significantly decreased the Vg/Vn content in hepatopancreas and ovaries. Furthermore, the K+ significantly reduced esRXR-mRNA levels in the ovaries but significantly increased esERR-mRNA levels in both tissues. In summary, under in vitro conditions, Na+ concentration had no significant effect on the vitellogenesis, while Ca2+ (10 ∼ 15 mmol/L) and Mg2+ (15 mmol/L) significantly promoted the vitellogenesis in the hepatopancreas and ovaries of E. sinensis. Additionally, Ca2+ significantly enhanced the expression of genes related to ovarian development, while K+ had a negative impact on the vitellogenesis.

β-carotene has been shown to be beneficial for the color formulation, survival, antioxidation, immunity and stress resistance of many crustaceans. This study was conducted to investigate the effects of dietary supplementation of synthetic β-carotene on growth, antioxidant status, high pH resistance, color parameters, and carotenoids composition of pre-adult Chinese mitten crab Eriocheir sinensis. Three diets were formulated to contain 3.06, 52.40 and 103.59 mg/kg dry diets of synthetic β-carotene (defined as Diet 1 - 3). The results showed that: (1) There were no marked differences in body weights or feed utilization, suggesting that β-carotene was not particularly efficient in enhancing the growth of pre-adult E. sinensis. (2) Indicators of antioxidant capacity (T-AOC, MDA) and immunity (AKP, ACP) both got improved in the hepatopancreas and hemolymph with the supplementation of 50–100 mg/kg β-carotene in the diets. (3) The stress test with high pH indicated Diet 1 treatment had the highest mortality among all treatments, and the lowest mortality was found in Diet 3 treatment. (4) For color enhancement, the redness of the carapace and hepatopancreas increased significantly with the increase of dietary β-carotene supplementation at Day 30. Combined, the inclusion of β-carotene in the diet could enhance the antioxidative capability, non-specific immunity, stress resistance to high pH and tissue carotenoid contents, and the optimal level of dietary synthetic β-carotene supplementation was suggested to be around 50 mg/kg for pre-adult male E. sinensis.

Haematococcus pluvialis is an important source of natural astaxanthin, which plays a key role in the regulation of color, antioxidant capacity, and immunity in aquatic animals. However, it is unclear that the effects of long-term H. pluvialis astaxanthin feeding on the growth performance, coloration, and antioxidant capacity of commercial-sized rainbow trout Oncorhynchus mykiss. The commercial feed of O. mykiss was used as the basic feed, and 15, 30 mg/kg H. pluvialis astaxanthin were added to prepare isonitrogenous and isolipidic experimental diets, defined as HPA0, HPA15, and HPA30, respectively. Juvenile O. mykiss with an initial weight of 0.60 kg were fed with three experimental diets for 4 months, which was conducted to investigate the effects of long-term H. pluvialis astaxanthin feeding on the growth performance, coloration, antioxidant capacity, and biochemical composition. The results showed that the weight gain rate and specific growth rate of the HPA30 from 0 to 4 months were significantly higher than those of the other two diet groups. In the 4th month, the viscerosomatic index of O. mykiss significantly increased with increasing dietary astaxanthin levels. The redness (a*), DSM SalmoFan™ scores, and astaxanthin content of fillets significantly increased with increasing dietary astaxanthin levels during the whole feeding trial. The activities of total antioxidant capacity and glutathione peroxidase in the serum and liver, and serum immunoglobulin (IgM) content significantly increased with increasing dietary astaxanthin levels, while the levels of total antioxidant capacity, and malondialdehyde in the serum and liver showed a decreasing trend. The contents of total lipids in the fillet of O. mykiss increased significantly with increasing dietary astaxanthin levels, while the percentages of total saturated fatty acid (∑SFA), C18:3n3, and n-3/n-6 PUFA ratio showed a decreasing trend; the percentages of C18:2n6 and total n-6 polyunsaturated fatty acids (∑n-6PUFA) in the HPA15 and HPA30 were significantly higher than those of the HPA0. In conclusion, four months of feeding with 30 mg/kg H. pluvialis astaxanthin diet could improve the growth performance, fillet coloration, antioxidant capacity, and fillet total lipids content of commercial-sized O. mykiss.

In this study, the mating behavior of Portunus trituberculatus under indoor conditions and the role of shelters in its mating process were systematically studied by using behavioral observation methods. The results showed that the mating behavior of P. trituberculatus was divided into four stages: courtship and pre-copulatory guarding, reproductive moulting, copulation, and post-copulatory guarding. P. trituberculatus in the no shelter group (NSG) usually preferred to gather at the edges or corners of the cement tank, while P. trituberculatus in the shelter group (SG) preferred to hide in the shelters. During the whole experiment, the shelter occupancy rate of P. trituberculatus during the day (57.40%) was significantly higher than that at night (38.08%) (P