EFFECTS OF DIETARY GLYCYRRHIZIN ON GROWTH,IMMUNITY OF SEA CUCUMBER AND ITS RESISTANCE AGAINST VIBRIO SPLENDIDUS

A 8-week feeding experiment was conducted to evaluate the effects of glycyrrhizin on the growth,non-specific immunity of sea cucumber(Apostichopus japonicus Selenka) as well as its resistance against Vibrio splendidus.The fish meal and defatted soybean meal were used as main protein source and fish oil and lecithin as main lipid source.The basal diet(19.6% crude protein and 4.7% crude lipid) was used as control.Four practical diets were formulated containing four glycyrrhizin levels:0,50,100 and 200 mg/kg feed,respectively.Each diet was randomly allocated to triplicate groups of sea cucumber in indoor 200-L fiberglass tanks with circulating seawater and constant aeration.And each tank was stocked with 30 sea cucumbers [initial average weight(6.80±0.00) g].The water temperature was(17 ± 0.5) ℃ and the salinity was(25 ± 1)‰ during the experimental period.At the termination of the feeding experiment,6 sea cucumber of each replicate were dissected and the coelomic fluid was collected with the 1︰1 ratio of the anticoagulant solution and coelomic fluid volumes.For the measurement of immune parameters,cells were counted immediately after sampling using a hemocytometer and then rapidly adjusted to 2 × 106 cells/mL with isotonic buffer.The results showed that glycyrrhizin supplemented at 200 mg/kg significantly enhanced the specific growth rate of sea cucumber(P0.05),whereas other supplementation did not(P0.05).During the feeding trial,no death of sea cucumber was observed among diet treatments.Sea cucumbers fed diet with 200 mg/kg glycyrrhizin had significant higher activi-ties of nitric oxide synthase(NOS) and lysozyme(LZM) than those fed other diets(P0.05).The intracellular O2? production(COP),superoxide dismutase(SOD) and acid phosphatase(ACP) activities in sea cucumbers fed diets with 100 and 200 mg/kg glycyrrhizin were significantly higher when compared with the control and 50 mg/kg glycyrrhizin groups(P0.05).Increased NOS activity and LZM activity were also noticed in sea cucumber fed glycyrrhizin at 100 mg/kg(P0.05).However,no significant difference of the above immunological parameters was observed between the control and 50 mg/kg glycyrrhizin groups(P0.05).After 56 days of feeding with glycyrrhizin-diets or control diet,20 sea cucumber of each replicate(each dietary treatment has 60 sea cucumber) were injected into the body wall with the 1×109 cfu/mL live V.splendidus bacterial stock solution at a dose of 0.1mL per sea cucumber(based on the data of 10-day LD50 pre-test).The sea cucumber that fed the control diet were injected with saline(0.1 mL) served as the unchallenged control.The results showed that sea cucumbers fed the diet either with 100 or 200 mg/kg glycyrrhizin had significantly lower accumulative morbidity compared with the control and 50 mg/kg glycyrrhizin groups(P0.05),and no significant difference was observed between 100 and 200 mg/kg glycyrrhizin groups.No significant difference was observed in accumulative morbidity between control and 50 mg/kg glycyrrhizin groups(P0.05).According to these results,some conclusions are put forward as follows:(1) the oral administration of glycyrrhizin to A.japonicus Selenka at a dose of 200 mg/kg improves growth,enhances the immunity as well as increases disease resistance;(2) long-duration administration of glycyrrhizin is sufficiently safe for sea cucumber farming.