Zoea-2 syndrome of Penaeus vannamei in shrimp hatcheries
At present, Indian shrimp hatcheries, solely depend on imported SPF P vannamei brood stocks for the seed production. In recent years, Zoea-2 syndrome emerged as a significant challenge to P.vannamei larviculture causing severe economic consequences to shrimp hatchery operators. During the study, out of fifteen commercial shrimp hatcheries in Tamil Nadu and Andhra Pradesh, nine hatcheries were affected with zoea-2 syndrome. It was observed that gradual progressive mortalities with cumulative occurrence reaching 30-100 % in the zoeal stages have been recurrent in the P. vannamei hatcheries in India. Generally, the most critical stage in larval rearing is nauplii VI – Zoea I. At Z I stage, the zoea start extensive feeding mainly on algae. Healthy zoea feed actively and have full gut with no abnormalities. After 36 – 48 hrs, of zoea I, larvae suddenly stop feeding and develop systemic abnormalities and suffer mortalities. Gross signs of non-feeding and lethargy in the affected larvae could be linked to lack of proper nutrition during the larval metamorphosis which may further hinder the successful larval development in the larval cycle and this might lead to poor survival (D’Souza and Loneragan, 1999, Jamali et al., 2015). Microscopic and ultrastructural studies revealed underlying pathological changes in the digestive system and associated organs, particularly hepatopancreas and intestine resulting the impairment in the nutrient absorption and starvation in the affected animals. This impairment in the nutrient absorption could be leading to the diminishing activity, delayed molting and mortality in zoea-2 syndrome affected larvae, as reported from Exopalaemon carinicauda larvae (Zhang et al., 2015), where starvation significantly affected the growth, survival and development. Sloughing of microvilli and epithelial cells in hepatopancreas, the inflammation and desquamation of epithelial cells in intestinal epithelium point to an underlying pathological process during zoea-2 syndrome. The ultrastructural studies and PCR did not reveal the presence of any pathogens including the known shrimp DNA and RNA viruses in the affected zoea.
During the microbial investigation, considerably higher association with V. alginolyticus was observed in the zoea-2 syndrome affected larvae. However its role as pathogen could not be resolved since it is also associated as natural flora of healthy larvae. Its association with zoea-2 syndrome was highlighted in many previous studies (Vandenberghe et al., 1998) but still their role in larval mortality due to zoea-2 syndrome is not clear. Several authors have described the association of Vibrio alginolyticus in penaeids and non penaeids (Lavilla Pitogo et al., 2000; Lee et al., 1996; Mohney et al., 1994) and Macrobrachium rosenbergii larval mortality due to antibiotic resistant V. alginolyticus (Jayaprakash et al., 2006). With the occurrences of vibriosis in both healthy and diseased animals, the hypothesis of opportunistic nature of vibriosis in penaeid shrimp is widely accepted (Egidius, 1987).
The present study has indicate that zoea-2 syndrome is possibly due to accrued conditions in the larval rearing tanks during the larval cycles even with constant and uniform water quality parameters and management practices. Further, zoea-2 syndrome appears to be related to prolonged production cycle in the hatcheries and continuous stocking more than three to four days in the same larval rearing units may probably cause zoea-2 syndrome incidence. Since zoea-2 syndrome was not seen in the fresh larval units of the hatchery, its incidence might have been caused by the cumulative effect of unknown factors accumulated during larval cycles in the affected hatcheries. It is also observed in this study that lack of proper disinfection during and between the larval production cycles and lack of separate units could be other predisposing factors in zoea-2 syndrome occurrence.
In almost all commercial shrimp hatcheries, despite following extensive water treatment with sand filters, cartridge filters, UV filters and chlorination, the bacterial load got increased during the larval production cycle. With some undefined negligent management practices, the vibrios being opportunistic pathogens could cause infections resulting in delayed molting for three to four days and mortality of zoea at Z-II stages. Ensuring strict good management practices and following proper disinfection and shutdown periods between the larval production cycles and reducing the number of days of stocking nauplii in less than 3- 4 days in the same unit and improved algal quality by serial dilution/ batch culture and ensuring separate re-circulatory systems, physically separate units for maturation, spawning, larval rearing, algal culture (Indoor and outdoor), and separate water and air supply units and separate workers and separate implements for individual unit would help reduce the incidences of zoea-2 syndrome.
Fig. Light microscopic observations of normal and Zoea-2 syndrome affected larvae A – Normal Zoea with full gut and fecal strands; B – Affected zoea with empty gut and absence of fecal strand; C& E – Normal zoea with full gut with no abnormalities, D, F, G – Infected zoea showing empty gut with inflammation like disruptions in intestinal epithelium;H- Infected zoea showing sloughed of epithelial cells as white balls or white sphere like structures(circle). FS - Fecal stands; FG- Full gut; EG- Empty gut: INF- Inflammation, WB- White ball or sphere like structures.
How can we control Zoae-2 syndrome?
Ensuring strict good management practices and following proper disinfection and shutdown periods between the larval production cycles and reducing the number of days of stocking nauplii in less than 3- 4 days in the same unit and improved algal quality by serial dilution/ batch culture and ensuring separate re-circulatory systems, physically separate units for maturation, spawning, larval rearing, algal culture (Indoor and outdoor), and separate water and air supply units and separate workers and separate implements for individual unit would help reduce the incidences of zoea-2 syndrome.