Effects Of Dietary Arachidonic Acid (ARA) On Growth Performance, Antioxidant Capacity, And Ovarian Development in Female Rice Field Eels (Monopterus Albus)

As an important economic freshwater fish in China, the artificial propagation of Monopterus albus faces a critical bottleneck due to unstable seedling quality. Nutritional regulation of broodstock is a core aspect to improve breeding efficiency, with arachidonic acid (ARA), an essential n-6 polyunsaturated fatty acid, playing an increasingly recognized role in fish gonadal development. A study published in 2025 in Animal Nutrition (DOI: 10.1016/j.aninu.2025.03.005) systematically explored the effects of dietary ARA on the growth, antioxidant capacity, and ovarian development of female M. albus broodstock, providing significant scientific evidence for optimizing feed formulations.

I. Research Background and Experimental Design

(1) Industry Challenges and Scientific Issues

M. albus exhibits natural sex reversal characteristics, where the quality of ovarian development during the female stage directly influences seedling yield. However, commercially available feeds lack targeted nutritional formulas, resulting in high dependency on wild broodstock. As a precursor to prostaglandins (PGE₂) and steroid hormones, ARA plays a crucial role in oocyte maturation and sex hormone secretion in fish, but the appropriate ARA requirement and action pathway for M. albus broodstock remain unclear.

(2) Experimental Methods

- Feed Design: Four groups of isonitrogenous (45.7% protein) and isolipidic (8.8% fat) feeds were formulated with ARA levels of 0% (control), 0.5%, 1.0%, and 1.5%, adjusting fatty acid composition using fish oil and ARA-enriched oil.

- Experimental Subjects: Six hundred female M. albus broodstock weighing 118±3.7 g were selected and randomly assigned to 12 tanks (50 per tank), with a breeding cycle of 10 weeks.

- Detection Indicators:

- Growth and Physiology: Weight gain rate (WGR), hepatosomatic index (HSI), gonadosomatic index (GSI).

- Antioxidant Capacity: Liver superoxide dismutase (SOD), malondialdehyde (MDA), serum alanine aminotransferase (ALT).

- Ovarian Development: Ovary crude fat, vitellogenin (VTG), prostaglandin E₂ (PGE₂) content, and expression of genes related to steroid synthesis (cyp19a1a, lhr, vtg, etc.).

II. Key Research Findings

(1) Bidirectional Regulation of Growth Performance by ARA

- Optimal Growth Dose: The WGR in the 0.5% and 1.0% ARA groups increased by 75% and 44% respectively compared to the control group (P<0.001), while the 1.5% ARA group showed a significant decrease in WGR due to metabolic burden from fat metabolism.

- Liver Health: Serum ALT activity decreased by 32%-35% in the 1.0% and 1.5% ARA groups (P<0.01), indicating that ARA can improve liver function, with HSI reduction possibly linked to enhanced lipid transport to the ovary.

(2) Dose-dependent Enhancement of Antioxidant Capacity

- Oxidative Stress Relief: With increasing ARA levels, liver SOD activity increased linearly (P<0.001), and MDA content decreased linearly (P<0.01), with the 1.5% ARA group showing a 32% reduction in MDA compared to the control. This suggests that ARA enhances antioxidant enzyme activity, reducing oxidative damage during the breeding period.

(3) Multi-dimensional Promotion of Ovarian Development

- Lipid Metabolism and Nutrient Deposition: Ovary crude fat content decreased linearly with increasing ARA levels (P<0.001), while the proportion of ARA in ovarian fatty acids increased from 1.7% in the control group to 4.4% in the 1.0% group (P<0.001). This indicates that ARA may optimize ovarian fatty acid composition by inhibiting fat synthesis and promoting conversion into active metabolites.

- Sex Hormones and Signal Pathway Regulation:

- Estrogen Synthesis: In the 1.5% ARA group, serum estradiol (E₂) levels increased by 67% compared to the control (P<0.05), with ovarian PGE₂ content increasing by 17%-18% (P<0.001), activating the PGE₂-LH signaling pathway to promote oocyte maturation.

- Vitellogenin (VTG): Both liver vtg gene expression and ovarian VTG content increased dose-dependently in ARA-treated groups (P<0.001), with the 1.0% ARA group reaching 43.9 ng/g, a 7% increase over the control, suggesting that ARA promotes yolk substance accumulation by enhancing estrogen receptor sensitivity.

- Synergistic Gene Expression Regulation:

- Ovarian cyp19a1a (cytochrome P450 aromatase) and lhr (luteinizing hormone receptor) gene expression were significantly upregulated in the 1.0% ARA group (P<0.05), promoting cholesterol conversion to estrogen.

- er (estrogen receptor) gene expression decreased with increasing ARA levels (P<0.001), likely maintaining hormonal balance through negative feedback mechanisms.

(4) Determination of Optimal ARA Addition Levels

Through comprehensive analysis of growth, antioxidant, and reproductive indicators, it was found that the 1.0% ARA group (10 g/kg diet) performed best in improving WGR, enhancing antioxidant capacity, and promoting ovarian maturity (GSI increased by 12%), without the metabolic burden seen in higher dose groups.

III. Mechanistic Insights and Industrial Implications

(1) Summary of Molecular Mechanisms

ARA regulates reproduction in M. albus broodstock through three pathways:

1. PGs Signaling Pathway: PGE₂ derived from ARA metabolism activates LH receptors, promoting E₂ synthesis and VTG transport, accelerating oocyte maturation.

2. Lipid Metabolism Reprogramming: Reducing liver fat deposition and promoting directed allocation of ARA to specific ovarian fatty acids (e.g., C20:4n-6).

3. Antioxidant Defense Reinforcement: Upregulating SOD activity and reducing MDA generation protects the ovary from oxidative damage.

(2) Recommendations for Feed Formulation Optimization

- Recommended Dosage: For female M. albus broodstock, an ARA addition level of 1.0% is recommended, achievable by blending fish oil (containing 1.1% ARA) with ARA-enriched oil (44.9% ARA).

- Nutritional Synergy: Combining n-3 polyunsaturated fatty acids (such as DHA, EPA) to adjust the n-6/n-3 ratio to 1.5-2.0 may further enhance reproductive efficiency (the n-6/n-3 ratio in this study's 1.0% ARA group was 1.9).

(3) Sustainable Aquaculture Applications

This research provides key technical parameters for intensive aquaculture of M. albus broodstock, enabling scale-up of seedling production by precisely regulating ARA levels, thereby reducing pressure on wild broodstock capture. Future studies could explore synergies between ARA and functional additives such as probiotics and phospholipids to develop environmentally friendly feed systems.

IV. Study Limitations and Prospects

While this study has clarified the positive effects of ARA on M. albus broodstock, its impact on fertilization rates and embryonic survival remains unverified. Additionally, variations in ARA requirements at different growth stages (e.g., during the critical period of sex reversal) need further investigation. Subsequent research combining transcriptomics and metabolomics could elucidate the interaction mechanisms between ARA and gut microbiota, providing theoretical support for integrated "nutrition-reproduction-gut" regulation models.

In summary, this study systematically reveals the multiple benefits of ARA in the cultivation of M. albus broodstock, not only providing scientific basis for sustainable development of the M. albus industry but also offering important references for reproductive nutrition research in other freshwater fish species.

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Reference:

Dietary arachidonic acid improves the growth performance, anti-oxidant capacity and ovary development of female rice field eel broodstocks (Monopterus albus) https://doi.org/10.1016/j.aninu.2025.03.005