Effects of docosahexaenoic acid and phospholipids on stress tolerance of fish larvae.
The effects of dietary docosahexaenoic acid (DHA, 22:6n3) and phospholipids (soybean lecithin) on the stress tolerance in marine fish larvae were studied. Red seabream (Chrysophrys major) and marbled sole (Paralichthys olivaceus) larvae were fed on diets containing DHA and soybean lecithin for 20-30 days. After the feeding trials, their tolerance in various stress factors: thermal, salinity, low oxygen and air-exposure were determined.
Results showed that dietary DHA and soybean lecithin treatments increased the tolerance of red seabream and marbled sole larvae exposed to stress caused by changes in temperature, salinity, low oxygen and air-exposure.
(Faculty of Fisheries, Kagoshima University, Kagoshima 890, Japan)
Culture of phyllosomas of spiny lobster: a review.
Complete development of phyllosomas, was achieved for the palinurid spiny lobsters: Jasus lalandii, J. edwardsii, and J. verreauxi, temperate-water species of the southern hemisphere, Palinurus elephas, a temperate-water species in Europe and Palinurus japonicus, a warm-water species in Japan. Males and females were obtained from the wild and transported to Sanriku, Japan. They were bred for several successive years in captivity. Hatching occurred annually from spring to summer. Newly-hatched phyllosomas were cultured in a 15-30 l container with an upwelling system. The culture container was connected to a 200-500 l microalgae Nannochloropsis sp. culture tank. The culture water was recirculated between both tanks. Nauplii of Artemia were used as food during the initial period in combination with small bits of mussels Mytilus edulis. Mussels were fed exclusively after the 2nd-3rd instar. The culture water was changed every 2 weeks to maintain the COD level lower than 1.2 ppm.
Success of complete development of phyllosomas is probably related to culturing microalgae and feeding mussels in the culture system equipped with upwelling flow. Mussels were found attractive and nutritionally satisfactory for phyllosomas. Mussels were caught by phyllosomas with their elongated pereiopods. In the case of Nannochloropsis sp. propagation, the ammonia-N concentration was maintained at a negligible amount in the culture water. Without introduction of Nannochloropsis sp., communal culture of phyllosomas was difficult due to occasional attachment of fungi Saprolegnia sp., Protozoa Vorticella sp., etc.
A high survival rate of about 10% was obtained with J. verreauxi while it was the lowest for P. elephas with heavy mortality at the first instar. However, the latter is considered to be the most promising aquaculture species because its phyllosoma life is very short (132 days) while 200-300 days for other species. The environmental and nutritional requirements for P. elephas may differ from those of Jasus and Palinurus.
(Research Institute for Marine Biological Science, Institute for Science and Technology, The Science University of Tokyo, 74 Onnemoto, Nemuro, Hokkaido 087-01, Japan)
Why microdiets are still inadequate as a viable alternative to live zooplankters for developing marine fish larvae?
The use of micro or inert diets as an economical and feasible alternative to live food for rearing marine fish larvae has, to date, proved unsuccessful. Although some success has been reported using microdiets as a partial replacement for zooplankters such as rotifers, Brachionus plicatilis and Artemia nauplii, it is becoming increasingly clear that nutritional and other factors present in live feed but absent in inert diet preparations are limiting their full application. A series of studies were carried out to determine and isolate those factors active in Artemia nauplii that may influence the ingestion, digestion and assimilation of microdiets.
Experiments investigating how the presence of Artemia influences food consumption in fish larvae, demonstrated that both visual and chemical stimuli, separate and in concert, account for an increase in ingestion rate of up to 120%. The chemostimulants were found to be mainly free amino acids (arginine, alanine, glycine) and betaine secreted into the water by the nauplii. The addition of Artemia naupli, combined with a microdiet, enhances the peristaltic movement of the larvae digestive tract and thus increases the processing and digesting of the microdiet. This improvement of the peristaltic movement is correlated with increased activity of the digestive hormone (Bombesin) in the larval body. The source of this hormone may be endogenous secretion or an exogenous one as Artemia nauplii.
Due to the fact that the digestive tract of larval marine species develops over an extended period, hydrolytic enzymes and other substances may be lacking or in short supply, affecting the assimilation and absorption of the nutrient content of microdiets. Although less effective than feeding fish larvae with a combined ration of microdiet and Artemia nauplii, the addition of pancreatic enzymes as a supplement to microdiets improved its assimilation by 30% and doubled the larval growth. On the other hand, the inclusion of hydrolyzed protein was negatively correlated with weight increase. This result may be due to the low level of development of transport mechanism of nutrients through the larval gut wall.
Other studies showed that the addition of the constituent neutral and polar lipid classes and a non lipid fraction of Artemia nauplii to microdiets significantly increased its assimilation by 10-20%. This effect decreased with larval age. A similar effect on incorporation of free fatty acids in both larval neutral and phospholipid fractions was found when dietary lecithin was added to the microdiet.
These studies of the isolation and identification of factors present in Artemia nauplii and responsible for its superior performance as a larval food showed that their supplementation to a dry microdiet can, significantly, improve its ingestion, digestion and assimilation.
(The National Center for Mariculture, lsrael Oceanographic and Limnological Research, PO Box 11212, Eilat 88112, Israel)
