No abstract

The present study describes the morphological changes of the mouthparts and foregut of the freshwater prawn M. acanthurus that occur during the development of the larvae and first juvenile. The results indicate that the zoeae I have mouthparts with reduced setae and a structureless foregut that indicates obligatory lecithotrophic behaviour. There is an increase in the number of setae in these structures between the zoea II and the juvenile stage, indicating the adaptation of the organism for feeding. More complex structural alterations were observed in the first juvenile, which acquires benthonic habits, which ensure the capture and ingestion of a diversity of feeding resources found in the substrate.

In the culture of crustacean larvae, the best survival rates are observed at optimal salinity due to the lower expenditure of energy for osmoregulation. In optimal conditions, greater longevity is expected under conditions of complete starvation. In the present study, larvae were obtained from ovigerous females of the freshwater prawn Macrobrachium amazonicum captured in two distinct regions of the Amazon coast of Pará, representing Amazonian várzea (Abaetetuba) and estuarine (Bragança) habitats. The larvae were maintained under starvation conditions, and from the zoea I to the zoea VII stages, they were exposed to salinities of 0, 2, 5, 10, 20 and 30 ppt. Based on the results of these trials, the larvae (100 larvae/L) were then reared in a dynamic closed system in 2L tanks at salinities of 5, 10 and 20 ppt, being fed with Artemia nauplii. In the larvae captured in estuarine habitats, longevity was greatest at a salinity of 10 ppt (12.00±0.79 days) and lowest in freshwater (6.77±3.47 days). In the várzea larvae, longevity was greatest at salinities of 2 (10.33±1.97 days), 5 (10.80±2.07 days) and 10 ppt (10.23±1.07 days). Other important differences were observed between the two groups of larvae. None of the estuarine larvae molted to the PL stage in freshwater and productivity was greatly reduced (43.2±7.8 PL/L) at a salinity of 5 ppt. By contrast, the várzea larvae developed into post-larvae (PL) normally in freshwater and could be reared at salinity 5 ppt with no loss of productivity. This permitted the minimization of management costs.

The hatchery of Macrobrachium rosenbergii is a successful activity that supports freshwater prawn farming operations worldwide, although feeding schedules require improvement. As M. rosenbergii is assumed to forage at night, Artemia nauplii (AN) are normally offered in the afternoon or at night. However, at present, there is neither consensus on the optimum schedule nor the amount of nauplii that should be provided daily during the larval phase. In the present study, two aspects of the provisioning of the M. rosenbergii larvae were tested experimentally – (i) the timing of the presentation of the Artemia nauplii (08:00 h and 20:00 h), and (ii) the density of the nauplii (5, 10, 20 AN mL-1). The results showed that M. rosenbergii larvae feed Artemia nauplii preferentially during the daylight hours, and that provisioning during this period supports a 25% increase in productivity. The results also showed that it is unnecessary to provide Artemia nauplii at a density up to 5 Na mL-1, not exceeding 10 AN mL-1.