Full length articleThe effect of rearing density on immune responses of hepatopancreas and intestine in Litopenaeus vananmei against Vibrio paraheamolyticus E1 challenge
Introduction
Litopenaeus vannamei is one of the most commonly cultured shrimp species widely distributed in the world [1]. With the aim of increasing profitability, intensive farming mode, based on the utilization of high rearing density (HD) and artificially formulated feed to achieve continuous increase in their production, is employed in aquaculture practice and becoming prevalent [2,3]. Nevertheless, persistent HD culture could exert adverse effects on shrimp growth, immune function, reproduction and increase disease susceptibility [4,5]. Therefore, studying the underlying biological mechanisms caused by rearing density stress and mitigating its negative effects through corresponding aquaculture practices will help improve animal welfare and productivity. Studies on rearing densities have been conducted in many shrimp species, such as L. vannamei [6], Fenneropenaeus chinensis [7], Penaeus monodon [8], Lysmata seticaudata [9], Palaemonetes varians [10], and Penaeus esculentus [11]. These studies mainly focused on growth performance, antioxidant enzymes, and the expression of certain genes. But, how such stress impact transcriptional responses to pathogen challenge is still limited and systematic and in-depth research is needed.
For the past few years, stress, pathogens, and parasites were the most devastating and virulent agents threatening aquaculture culture industry, causing substantial economic losses [[12], [13], [14], [15]]. It is unclear whether the challenge level and mortality of shrimp against pathogens are related to increased pathogen susceptibility or decreased immune response capacity caused by environmental stress. Thus, to understand the stress of rearing density on the immune response of shrimp and their resistance to pathogens is pre-requisite. Vibrio parahaemolyticus, as one of the most hazardous pathogens in shrimp aquaculture, could induce acute hepatopancreatic necrosis syndrome (AHPNS) or early mortality syndrome (EMS), which have been responsible for widespread mortality of farmed white shrimp in South East Asia [16,17]. Although several previous studies have analyzed such responses to a combination of high density and pathogen challenge. For example, Lin et al. reported exposure to extreme high densities (10–40 shrimp/L) during up to 12 h decrease resistance to Vibrio alginolyticus and white-spot syndrome virus (WSSV) mainly by a depressed immune capacity [4]. Liu et al. reported that the immune status and welfare of white shrimp can be seriously impaired in the high rearing density condition (500 shrimp/m3) in biofloc systems [18]. Still, the biological mechanism between stress of rearing density and the shrimp susceptibility to pathogen, especially V. parahaemolyticus, were rarely reported.
The hepatopancreas and intestine, playing an important role in immune defense, digestive, nutrient absorption and metabolism, are the two most critical organs in marine invertebrates [19,20]. The healthy hepatopancreas and intestine are critical for the growth, metabolism and immunity of hosts. However, the hepatopancreas and intestine of hosts are vulnerable to environmental stress and pathogens, resulting in the dysfunction of hosts physiochemical activities, and lead to diseases and death [4,21]. Therefore, the relationship between environmental stress and disease susceptibility can be through analyzing potential morphological, biochemical, and microarray changes in the hepatopancreas and intestine of host in response to pathogen challenge between different rearing densities.
In this study, to systematically investigate the potential mechanisms for density-related differences between disease susceptibility and rearing densities, we conducted comparative transcriptome analysis of the hepatopancreas and intestine of L. vannamei under two different rearing densities (800- and 400- shrimp/m3) for 15 d and further analyzed the differences in immune response to V. parahaemolyticus E1 (VPE1) raised under two density conditions. Meanwhile, we determined growth performance, histopathology and the activities of several important antioxidant enzymes in the intestines and hepatopancreas. The results of this study not only provide a significant in-depth transcriptomic resource for future studies of rearing stress and immunity in shrimp, but offer a foundation for finding potential and feasible practical strategies for health management and disease prevention in shrimp aquaculture.
Section snippets
Shrimp rearing conditions
Before the start of the feeding trial, apparently healthy L. vannamei obtained from a commercial shrimp farm Ruizi Seafood Development Co. Ltd. (Qingdao, China) were acclimatized for a week before the experiments, at 28 ± 2 °C in oxygenated seawater (30–31‰ salinity). In the study, we conducted two rearing densities according to the shrimp culture practice under field conditions. The common tank size is generally about 670 m2 in shrimp farms, rearing amounts ranges from 300,000 to 500,000
Effect of rearing density and VPE1 challenge on shrimp grows and survival
After 15 d of breeding experiments, a significant difference in weight gain ratio of shrimp was observed between the LD (63.20 ± 1.67%) and HD (18.73 ± 3.35%) groups (P < 0.05). Then, susceptibility to VPE1 challenge 48 h was studied in shrimp raised under two density conditions. Overall survival rate of shrimp was 77.50 ± 1.62% and 83.75 ± 2.23% in LD and HD VPE1-challenged treatments, respectively.
Effects of rearing density and VPE1 challenge on histopathology of hepatopancreas and intestine of L. vannamei
Shrimp from HD groups exhibited markedly histological alterations in the hepatopancreas (Fig. 1
Discussion
Rearing density and disease management are considered pivotal factors determining shrimp farm productivity and profitability [11,28]. HD, as one of the vital factors disrupts shrimp's homeostasis and affects their immunocompetence and disease susceptibility, inducing disease with a gradual decline of reared shrimp stock quality and profitability, thus causing a significant problem in aquaculture practices [18,23]. In the present study, HD exhibited a negative effect on growth performance and
Conclusion
To summarize, this study investigated the complex functional genomic effects of rearing density on shrimp immune responses and disease susceptibility. We demonstrated that high densities that likely induce increased social stress in shrimp result in altered immune responses to VPE1 challenge, decreased weight gain rate (63.20 ± 1.67% and 18.73 ± 3.35% in the HD and LD groups, respectively), severely destroyed the histopathology, inhibited the antioxidant enzymes activities, disordered the
Acknowledgements
The authors are grateful to all the laboratory members for their technical advice and helpful discussion. This study was financially supported by the Marine S&T Fund of Shandong Province for Pilot National Laboratory for Marine Science and Technology (Qingdao) (2018SDKJ0502-2), and the Science and Technology Development Fund Project of Shinan district, Qingdao city, China (2018-4-001-ZH).
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