Essential oils are volatile liquids with strong odor and fragrance extracted from different tissues of plants, such as leaves, flowers and roots. They are highly effective and safe botanical drugs with antibacterial, antiviral, antiparasitic, antifungal, enhancing immune function of animals, improving the ability of animals to resist stress and improving Good animal performance is one of the focuses and hotspots in the development of new feed additives. The application of plant essential oils in aquaculture is reviewed in this paper.
1 Effects of plant essential oils on growth performance of aquatic animals
In the study of essential oils related to oregano oil, Wang Mengqiang et al. demonstrated that adding vegetable essential oils (citrus oil, oregano oil, etc.) to the feed compensated for the growth retardation caused by the reduction of fish meal content in the feed (Wang Mengqiang et al., 2015). Adding oregano oil to the feed of Penaeus vannamei significantly increased the weight gain rate (Ching et al., 2008), while Kim et al. (2011) showed that oregano oil had no effect on the growth of Penaeus vannamei.
Ran et al. found that carvacrol and thymol (1:1) had no significant effect on the growth performance of tilapia feed (Ran et al., 2016), while thymol could significantly increase the growth rate and feed utilization rate (Shehata et al., 2013). Carvacrol and thymol (1:1) significantly decreased the dietary coefficient (P rez-S nchez et al. 2015). Oregano oil had significant effects on catfish growth, hepatopancreas index, viscera-body ratio and fatness (Zheng et al., 2009), weight gain and feed coefficient (Wu Qiangqiang et al., 2010).
Dietary carvacrol or thymol decreased the bait coefficient of rainbow trout (Giannenas et al., 2012). Similarly, dietary carvacrol-thymol and oregano oil significantly decreased the bait coefficient of rainbow trout (E. Ahmadifar et al., 2012; Diler et al., 2016). A yellow tail fat carp study showed that adding oregano oil to feed increased growth performance (Ferreira, 2014).
The results showed that adding orange peel oil to tilapia feed significantly promoted the growth effect (Acar, et al., 2014). Adding fennel oil or garlic oil to tilapia feed can increase survival rate, weight gain rate, specific growth rate, and reduce feed coefficient (Hassaan and Soltan, 2016), while catfish feed with Jiangteng oil has no effect on growth performance (Saccol, 2013).
2 Effect of plant essential oils on immune function of aquatic animals
Overall, oregano oil and other plant essential oils can improve the non-specific immune level of aquatic animals, antioxidant, anti-stress ability, but also can improve the ability of aquatic animals to infect pathogens (Table 1).
3 Effects of plant essential oils on intestinal microflora of aquatic animals
In Ran et al. study, the microbial structure of the anterior, middle and posterior intestines of tilapia fed with carvacrol and thymol (1:1) changed significantly, and the microbial diversity of intestinal contents increased significantly (Ran et al., 2016). In the study of adding carvacrol or thymol to rainbow trout feed, two essential oils could reduce the total anaerobic bacteria and increase the amount of Lactobacillus in the gut of rainbow trout (Giannenas et al., 2012).
4 Effect of plant essential oils on intestinal health of aquatic animals
The ultrastructure of midgut of Penaeus vannamei was observed by transmission electron microscopy. It was found that the addition of plant essential oil (citrus oil, oregano oil, etc.) in diet could significantly improve the villus shedding of intestine of Penaeus vannamei (Wang Mengqiang et al., 2015). In Ran et al. study, tilapia feed supplemented with carvacrol and thymol (1:1) showed that vegetable essential oils increased intestinal villi and intestinal epithelial immune cells (Ran et al., 2016). As shown in Table 2.