The postlarval development, growth and nutrition of the Indian white prawn Penaeus indicus (H. Milne Edwards)
This study investigates the postlarval development of Penaeus indicus. Particular emphasis is given to characterisation of developmental morphology, growth, ontogenetic change in digestive enzymes, and assessment of energy requirements for postlarval substages. The morphology of the Penaeus indicus postlarvae (PL) changes continuously as consecutive substages (PL1-14) were reached by daily moults. After 22 ecdyses (typically 35 days) the PL22 substage is succeeded by the adult form. Most of the morphological differentiation is observed after 2 weeks at substage PL14, but the branchiae only reach full development from substage PL16. The rostrum teeth, telson spines and ratio of body segments are important characters for identification of Penaeus indicus PL stage. Growth of PL1 postlarvae was significantly slower when fed artificial diets rather than Artemia nauplii. Similarly 14-day postlarvae which were slow developers (PL9) also grew slowly on artificial diets whereas postlarvae of the same age (substage PL14) did not show this suppression. Trypsin and amylase digestive activities increased with PL development (P<0.001) but did not change significantly (P>0.05) with diet fed. Trypsin was low during early PL stages of development and a sharp increase in tryptic activity was only observed at substage PL20 (24 mm). Amylase increased from PL1 and exceeded that of trypsin for 2-3 weeks after metamorphosis. It appears that during early stages of development postlarvae are unable to efficiently digest artificial diets due to low digestive activity. For smaller 14-day postlarvae, poor performance is possible related to a genetic regulated constraint and not to digestive capacity since enzyme levels were similar to those in larger PL14. PL1 fed for 15 days on commercially dried low-hatch decapsulated Artemia cysts showed comparable growth and survival to that of PLs fed on Artemia nauplii, but significantly higher (P<0.05) than that supported by commercial granulated and flake diets and low-hatch decapsulated cysts processed into a granulated diet or dried at 90°C. Leaching of soluble protein and carbohydrates was high for all artificial diets but low-hatch decapsulated cysts were highly stable in water. Commercially dried low-hatch cysts retain a living membrane capable of osmoregulation and retaining highly digestible nutrients important for fast growth and development of postlarvae. Survival of postlarvae was negatively correlated (P<0.05) with leaching of soluble protein, but no correlation was observed for loss of soluble carbohydrates. Daily food ingestion and routine metabolism of postlarvae increased with PL development. Food metabolism (SDA) was low for early PL stages, but increased steadily up to stage PL16 and remained the same from this substage onwards. Assimilation efficiency decreased for early PL substages and remained low up to PL13, and then increased steadily. Different energy strategies seem to be adopted during postlarval development to cope with ontogenetic modifications after metamorphosis. During early development little energy is lost in metabolism, and so more energy is converted to growth to support fast development, with increase in predatory behaviour and development of digestive system. Later more energy is lost in metabolism and committed for maintenance. The ontogenetic changes in digestive activity, energy trend and assimilation efficiency latter in PL development seems to reflect the adaptation to benthic carnivorous existence and migration of postlarvae and juveniles form inshore nursery to deeper waters. Stocking density above 20 PLs 1- reduced growth and survival but increased size variability above the inherent range, for postlarvae PL1-18 days old. However, Penaeus indicus postlarvae showed low agonistic behaviour and tolerated relatively high densities similar to that of other penaeid species, which further enhances the potential and advantages of the white prawn for culture.