Changes in digestive enzyme activities during the early ontogeny of milkfish, Chanos chanos larvae.

Knowledge of the developmental ontogeny of the digestive system and nutritional requirements of marine fish larvae is a primary requisite for their successful rearing under an optimal feeding regime. In this context, we assessed the activity profile of key digestive enzymes viz., trypsin, chymotrypsin, leucine aminopeptidase, lipase, amylase, and alkaline phosphatase during the early ontogeny of milkfish, Chanos chanos (0 day, 3 days, 6 days, 9 days, 12 days, 15 days, 18 days, 21 days, 25 days, and 30 days post-hatch). Larvae for this study were obtained from the successful breeding of milkfish at ICAR-Central Institute of Brackishwater Aquaculture, India. Growth curves (length and weight) of the larvae indicated a positive morphological development under a standardized feeding regime that comprised Chlorella salina, Brachionus plicatilis, Artemia salina nauplii, and commercial weaning feed for different larval stages. With respect to protein digestion, the specific activity of pancreatic enzymes trypsin and chymotrypsin and intestinal brush border leucine aminopeptidase showed two peaks at 3 dph and 15 dph, following the introduction of rotifer and Artemia nauplii. Similar bimodal peaks were observed for alkaline phosphatase and amylase activities, with the first peak at 3 dph and the second peak at 18 dph and 21 dph, respectively. Whereas in the case of lipase, high activity levels were observed at 0 dph, 3 dph, and 18 dph, with subsequent decreases and fluctuations. Overall, as most of the enzymes were found to have peak activities at 15 to 21 dph, this period can be potentially considered as the developmental window for weaning larvae from live to formulated feeds in milkfish hatcheries.

The effect of acclimation temperature and optimal temperature gradient for egg and larvae of silver moony (Monodactylus argenteus) during the early ontogenesis.

The early life history of a fish species is regulated by temperature, the most critical environmental cue. Thus, identifying the gradient of temperature that optimises the early development of a species is a prerequisite for standardising hatchery technology. Silver moony, Monodactylus argenteus is a tropical brackishwater ornamental fish that holds scope for the Indian and Global ornamental fish industry. This study unravels the effect of water temperature increments (26, 28, 30, 32, and 34 °C) on embryonic development, hatching, and survival rate, as well as the growth profile and survival rate of larvae at 5 days post-hatch (5 dph). Experiments were conducted to find out the optimal temperature gradient for egg incubation and early larval rearing. The experiment results revealed that the embryogenesis was accelerated at increasing temperatures, especially after the gastrula stage, and apparent differences were evident in each stage. However, the morphological development profile at each embryonic stage was similar throughout the temperature range. The incubation period differed significantly (P < 0.05) between the temperature gradients. The highest rates of hatching (90-100%) and survival after 12 hph were observed at 28 ºC and 30 ºC. Hatched-out larvae demonstrated the highest total length (1.92 ± 0.02 µm) at 34 °C, and the total length decreased at lower temperature levels. The yolk sac volume of larvae was shrunken with an increase in temperature, and a significant difference was observed between the studied temperatures. However, the oil globule diameter did not differ between the different temperatures. The total length and growth rate of 5 dph larvae were significantly different among the temperature treatments and increased with increasing temperature. In contrast, the survival rate of 5 dph larvae was highest at the range of 26 ºC and 30 ºC. The results indicated that the change in temperature from the spawning temperature (29 ± 1 °C) negatively influenced embryogenesis and the early development of M. argenteus. Based on the experimental results, the growth and survival of embryo and larvae were found to be optimum at 28 to 30 ºC. This prediction is of great importance for the effective management in the hatchery production phase, and especially the temperature could be considered the critical environmental cue.

Resistance of pearlspot larvae, Etroplus suratensis, to redspotted grouper nervous necrosis virus by immersion challenge.

Viral nervous necrosis (VNN) affects more than 120 species mostly belonging to the order Perciformes. However, none of the brackishwater species belonging to the family Cichlidae under the order Perciformes are reported to be susceptible. Hence, the present experiment was undertaken to study the susceptibility of the brackishwater cichlid, pearlspot, Etroplus suratensis to NNV. Thirty-day-old pearlspot larvae were infected with NNV by immersion. Mortality was recorded till 14 days post-infection, and the infected larvae were subjected to nested RT-PCR and histology. The virus was isolated from infected larvae using SSN-1 cells. To study the replication of the virus in vitro, primary cultured brain cells of E. suratensis and IEK cells were infected with NNV. No mortality was observed in any of the control or experimentally infected larvae. However, the experimentally infected larvae were positive for NNV by nested RT-PCR and the virus was isolated using SSN-1 cells. Further, the infected pearlspot brain cells and IEK cells showed cytopathic effect at second and third passage of the virus and they were positive for NNV by nested RT-PCR. Pearlspot is relatively resistant to VNN although the virus could replicate in the larvae and in cell culture.