**1.4 Vitamin C deficiency in fish**

Vitamin C plays a crucial role in the physiological function of various animal species, including fish [22]. The vitamin C requirements of fish are influenced by factors such as size, age, growth rate, environmental conditions, and nutritional needs. Different species of fish may require varying levels of vitamin C. Deficiency in vitamin C can lead to reduced survival rates, lipid accumulation, skeletal deformities, decreased activity of specific enzymes, reduced tissue vitamin storage, lower liver lipid content, decreased hepatosomatic index, and diminished expression of heat shock proteins, ultimately affecting growth performance [23].

Loss of vitamin C at certain levels can result in decreased vitality and nutrient absorption in fish. Vitamin C supplementation significantly reduces the risk of stress in fish and promotes reproductive success by aiding in the synthesis of fats necessary for vitellogenesis [24].

Vitamin C deficiency in fish impairs their ability to neutralize free radicals found in water, as vitamin C is crucial for preventing lipid peroxidation in blood plasma. This deficiency leads to a decline in the fish's resistance against bacterial attacks, as its immune system becomes compromised. Reduced levels of vitamin C significantly diminish phagocytic activity, antibody degradation, and lysozyme activity in fish [22].

In tilapia, vitamin C deficiency increases susceptibility to bacterial attacks, such as from *Aeromonas hydrophila*, reduces resistance to environmental changes, and increases stress levels, ultimately leading to higher mortality rates [25].

Deficiency of vitamin C in fish is influenced by factors such as temperature, oxygen levels, pH, light exposure, and diet composition. The nutrient requirement for vitamin C in tilapia is around 50 mg/kg-1 [26].

The effects of vitamin C deficiency in fish include weight loss, reduced specific growth rate, and decreased survival rates of Nile tilapia fingerlings. Additionally, deficiencies lower the quality of fish in terms of nutrient absorption from feed, increase the risk of hemorrhage during bacterial infections, reduce blood and plasma volume, lower protein conversion efficiency and protein efficiency ratio, and decrease oxygen consumption levels [26].

The absence of vitamin C can lead to damage in vital organs such as the gills, kidneys, liver, and scales, making fish more susceptible to injury. Furthermore, it can contribute to higher ammonia production in fish [27].

Vitamin C is essential for maintaining various functions in fish, including the absorption of ions in water, maintaining strong bone structure, and preventing skin irritations and bleeding. Skin irritations are often observed when there is a decrease in vitamin C levels in the blood, as it impairs the synthesis of collagen [28].

Moreover, vitamin C deficiency can lead to a decrease in hormones' self-defense activity and the proteomic activity of hemocytes. Additionally, deficiencies in vitamin C may lessen the function of enzymes such as phenoloxidase and reduce the effectiveness of antiviral agents, antimicrobial proteins, agglutinins, and hemolysins [29].

Deficiency of vitamin C can lead to various negative effects on fish health and immune function. Roosta et al. [30] reported that vitamin C deficiency can lower the activity of self-defense hormones and the proteomic activity of hemocytes. Additionally, it can reduce the function of enzymes such as phenoloxidase, antivirus agents, antimicrobial proteins, agglutinins, and hemolysins.

Gao et al. [22] highlighted that red sea bream (Pagrus major) experience decreased phagocytic activity, antibody levels, and lysozyme activity when their vitamin C needs are not met. Similarly, Ibrahem et al. [25] found that low levels of vitamin C result in decreased feed efficiency and metabolic assimilation of dietary nutrients in fish, leading to retarded growth, darkening of the skin, anorexia, and high mortality in fish such as parrotfish (*Oplegnathus fasciatus*) [31].

Furthermore, Wu et al. [32] observed that vitamin C deficiency in white shrimp (*Litopenaeus vannamei)* leads to a decrease in antioxidant levels and cofactors in

*Vitamin C Deficiency DOI: http://dx.doi.org/10.5772/intechopen.110892*

enzyme reactions, reducing the non-specific immune response against bacterial infections and the synthesis of collagen for tissue repair. In juvenile cobia (*Rachycentron canadum*), a deficiency in vitamin C leads to decreased activities of lysozyme serum, superoxide dismutase (SOD), alkaline phosphatase (AKP), and total immunoglobulin (Ig) [33].
