International Journal of Aquaculture, 2025, Vol.15, No.5, 229-239 http://www.aquapublisher.com/index.php/ija 230 2013; Chen et al., 2022). GWAS uses a large number of markers across the genome-wide range to find trait-related alleles at the population level, with higher resolution and application range than traditional QTLs. In shrimp, early QTL studies were able to roughly localize regions containing thousands of genes due to their limited marker density. With the development of high-throughput sequencing, tens of thousands of SNP markers have been developed and applied to map construction and association analysis, improving positioning accuracy and efficiency. 2 Biological Basis of Shrimp Growth Traits 2.1 Main growth trait indicators: body length, weight, shell thickness, survival rate Shrimp growth traits are usually characterized by several indicators that are phenotypically measurable, among which the most commonly used are body length and weight. Body length includes the full length and body length, etc., which can reflect the individual's morphological development. In genetic assessment, the average weight of a specific day age, daily weight gain or weight gain rate for a specific period is often used as a measure of growth rate. Shell thickness (shell hardness) is also a related trait. Although it does not directly represent growth rate, the thickness and weight of the shell will affect the net meat output ratio of shrimp, which has also been paid attention to in some breeding programs. In addition, survival rates are often regarded as important breeding traits and are closely related to growth (Yuan et al., 2018): Only under the premise of similar survival can it be practical to improve growth. Therefore, survival rate is sometimes used as an auxiliary indicator of growth tests. At the same time, there have been researches to include bait conversion rate, plumpness, etc. into the evaluation system of shrimp growth traits to more comprehensively measure growth performance. In QTL and GWAS analysis, these traits can be analyzed individually, or the overall growth phenotype can be extracted by comprehensive analysis methods such as principal components. 2.2 Physiological and metabolic regulation mechanisms The growth of shrimp is regulated by a variety of endocrine and metabolic factors. From an endocrine point of view, shrimp lack growth hormones similar to vertebrates, but have hormone-like molecules such as insulin-like peptides (ILPs). The ILP1 gene of vannabinoid shrimp was cloned and found that it is widely expressed in various tissues of the shrimp body, especially the highest among neuroendocrine organs (eye stems, etc.) (Su et al., 2024). This is similar to the role of insulin-like growth factors in regulating the growth and development of vertebrates, suggesting that ILP1 has an important function in the growth and development of shrimps. In addition, ecdysterone (20E) and ecdysterone (MIH) in shrimps indirectly affect growth rate by regulating the molting cycle: shrimps only increase in volume and weight after molting, and shortening molting intervals can improve growth rate (Naidu et al., 2013). In terms of metabolism, growth speed is often related to nutritional metabolism and energy distribution. Studies have compared the molecular differences of fast-growing and slow-growing shrimps, and found that the genome-wide DNA methylation level of slow-growing individuals has significantly increased and is accompanied by upregulation of metabolic-related gene expression. These differential genes are enriched in carbohydrate and fatty acid metabolism pathways, and it is speculated that slow-growing shrimp will use more nutrients to maintain basal metabolism and stress rather than somatic growth. This finding suggests that epigenetic modifications (such as DNA methylation) participate in the regulation of shrimp growth phenotype by affecting the expression of metabolic genes. 2.3 Effect of environmental factors on growth traits Environmental conditions largely determine the development of shrimp growth potential. Among them, water temperature is the most direct environmental factor affecting shrimp growth. The optimal growth temperature of vannabinoid prawns is about 28 °C~32 °C. Too low will reduce feeding and metabolic rates, and growth will slow down significantly, while too high may trigger a stress response and also inhibit growth (Heriyati et al., 2024). Studies have shown that under low temperature stress, the antioxidant enzyme activity and energy metabolism in shrimps change, and long-term low temperatures will lead to growth stagnation and even death. Dissolved oxygen levels are also crucial. Adequate dissolved oxygen keeps shrimps high intake and high metabolism. Conversely, hypoxia causes anorexia, growth stagnation, and weak shrimps are more susceptible to disease (Akbarurrasyid et
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