The growth response, non-specific immune activities and disease resistance were measured in sea cucumber, Apostichopus japonicus Selenka (initial average weight 6.80 ± 0.10 g), when fed diets supplemented with graded levels of guanosine from the guanosine-5'-monophosphate disodium (GMP) at 0 (control), 0.3, 0.6 and 1.2 g/kg for 8 weeks. The results showed that GMP supplemented at 0.6 and 1.2 g/kg significantly enhanced the growth of sea cucumber. Sea cucumber fed a diet with 0.6 g/kg of GMP had significantly higher intracellular superoxide anion production, nitric oxide synthase activity, lysozyme activity and the total superoxide dismutase (T-SOD) activity than those in control group (p < .05). Increased lysozyme activity and T-SOD activity were also found in sea cucumber fed GMP at 1.2 g/kg. Moreover, there was significantly lower cumulative mortality after the disease challenge in sea cucumber fed the diets with 0.6 and 1.2 g/kg GMP than that in control and 0.3 g/kg GMP groups (p < .05), and no significant difference was observed between 0.6 and 1.2 g/kg GMP groups. These results suggested that feeding GMP at a dose of 0.6 g/kg could enhance growth, non-specific immunity of sea cucumber as well as its resistance against Vibrio splendidus.
K E Y W O R D Sgrowth, immunity, immunostimulant, nucleotide, sea cucumber
| INTRODUCTIONSea cucumber Apostichopus japonicus Selenka is a common echinoderm in Japan, Korea and China and believed to have hygienical and curative properties, which results in its high commercial value (Liao, 1997). The rapid increase in market demand has resulted in the overexploitation of natural sea cucumber populations all over the world. As a result, sustainable industries with modern hatchery techniques have been established in several countries (Conand, 2004). However, the quick spread of intensified farming has led to an increase in various pathogenic diseases, which has become a major limiting factor in the industry. For example, the acute peristome oedema disease and the highly contagious skin ulceration disease in A. japonicus Selenka had broken out in China and caused great economic loss (Wang, Chang, Yu, Li, &Rong, 2004). In addition, the highly contagious skin ulceration disease has also been recorded on Isostichopus fuscus in Equator, on Holothuria scabra in Madagascar, in Australia and in New Caledonia (Becker et al., 2004). Artificial infection test proved that the causative pathogen associated with this disease was V. splendidus (Zhang, Wang, & Rong, 2006 antibiotic-resistant bacteria, thus posing potential threats to human beings and environment (Smith, 2008). Searching for an alternative for antibiotics has become urgent. Non-specific immune system acts to fight against disease in most aquatic animals (Zhang et al., 2014).Immunostimulants, which increase resistance to infectious disease by enhancing non-specific defence mechanisms, are considered to be safe and effective against various pathogens in aquaculture (Sakai, 1999). Dietary nucleotides have been shown to bene...