Physiology of decapod crustacean larvae with special reference to diet
This study investigates the factors affecting larval and postlarval survival and growth of some de capod crustaceans with special emphasis on diets. Investigations were concentrated on the influence o live and artificial diets on larval growth, survival, development and trypsM activity of a cominerciall, important marine penaeid shrimp Penaeus indicus and a freshwater prawn Macrobrachium rosenber gij. In addition, feeding behaviour, gastroevacuation time, trypsM activity of other decapod species wen also studied. Live mixed microalgae Tetraselmis chuii and Skeletonema costatum at 60-70 cells PI-1 promote( highest larval survival, fastest growth and development in P. indicus in comparison to single algal spe cies. Rhinomonas reticulata neither alone nor in combination with other algal species was suitable a! food for the shrimp larvae, A water salinity (S) of 25 ppt was optimal for larval and postlarval cultun of this penaeid species. Postlarvae (PL) of P. indicus reared at lower salinities between PL7 and PL& (20-30 ppt) had a significantly (P<0.05) higher survival and a better growth than those at higher wate salinities. Early PL resisted sudden salinity change of 10 ppt, but required an adaptation period fo greater salinity changes, 10 ppt S was lethal to animals at around PL40-45. A free-living nematode Panagrellus redivivus was found to be a suitable alternative for live algai and Artemia in the culture of P. indicus. The nematodes gave good survival, but lower growth than al gaelArtemia from PZI to PLI. Larval growth and survival were significantly improved when the larvai were fed on either nematodes plus algal co-feeds or liPid-enriched nematodes. Pigmented- (astaxanthin nematodes also improved survival and colour of P. indicus larvae in comparison to non-pigmented ones Conventional live diets were also completely replaced using microencapsulated diets (MED) fo the culture of P. indicus, Like the nematodes, MED as a sole feed resulted In lower survival, slowe growth and development in comparison to algaelArtemia. Addition of 15 cells gl-1 frozen algae signifi cantly improved growth and survival during larval development. The larvae fed MED plus algal co feeds had significantly (P<0.05) higher trypsm activity than those fed MED as a sole feed. Similarly provision of 15 cells gl-' algae with nematodes for only 24h or 48h resulted in significant increase H trypsin activity and improved survival and growth to levels comparable to those obtained from al gaelArtemia. It appears that the presence of an algal diet is necessary to induce larval trypsin activity Mi P. indicus at early protozoeal stages, but algae do not influence trypsin at mysts stages. Results sugges that both nematodes and formulated diets lack gut enzyme stimulants and are less digestible than al gaelArtemia diets. When freeze-dried algal materials were incorporated into MED, it was found tha algal substances which trigger larval digestive enzymes were retained within the capsules. Whether thi will improve growth and survival of penaeid larvae remains to be examined. In contrast to penaeid larvae, a complete replacement of live Artemia with nematodes or artificia diets was not possible for the culture of caridean M rosenbergii and PaIdemon elegans larvae. Fo both species, only a partial replacement was achieved from Z4/5 to metamorphosis by using formulateA diets. It was found that these larvae have very low trypsin activity levels between ZI and Z4/5, but th, levels increase sharply afterwards, coinciding with a vast increase in the hepatopancreas. This sharl increase in digestive enzyme activities and longer food retention time enable these larvae to survive oj less digestible formulated diets. A comparison of specific trypsin activity in several larval decapod crustaceans shows a pattein with high levels in herbivores, low levels in carnivores and intermediate levels M omnivores. Herbivor penaeid larvae (P. indicus) and copepods (Temora longicornis and Centropages typicus) rely on hig digestive enzyme activities to extract nutrients from less digestible algae, whereas carnivorous larvaE the lobsters (Homarus gammarus and Nephrops norvegicus) and carideans (M. rosenbergii and .1 elegans) have limited enzymatic capacity and hence require large and easily digestible prey, but rests long starvation periods. Omnivorous mysis penaeid larvae and Carcinus maenas have intermediat levels of digestive enzymes and are able to transfer from herbivorous to omnivorous feeding. To datE only decapod larvae which show high trypsm activity can be successfidly reared to metamorphosis o: formulated feeds. Inclusion of algal material, as a gut enzyme stimulant, for penaeid protozoeal stage and pre-digested ingredients for later stages into feeds are proposed.