The effect of food, light intensity and tank volume on resting eggs production in Daphnia carinata.

Sexual reproduction is an important stage of cyclical parthenogenesis Daphnia, these cyclic parthenogens need to time their shift from asexual to sexual reproduction correctly, based on environmental cues. We investigated the effect of food quality and quantity, light intensity, and culture tank volume on the resting eggs production of a tropical Daphnia carinata clone of Vietnam. The results showed that sexual ephippia production of the D. carinata was strongly induced when fed with green algae Scenedesmus sp., under moderate light intensity condition. The experiments also showed that food limitation is not an obligate requirement of shifting from parthenogenetic reproduction to sexual ephippia reproduction in D. carinata. Beside of that, culture volume seemed to be an important determinant of successfully forming of embryos inside ephippium. These findings here were from cultures in contaminant free condition, thus they are important basics for further studies of additional effects of environment toxicants on D. carinata species of Vietnam.

Chronic effects of cyanobacterial toxins on Daphnia magna and their offspring.

The zooplankton grazer Daphnia magna endures living in water bodies up to moderate densities of cyanobacteria, such as Microcystis spp., known for producing toxic secondary metabolites. Although daphnids are affected via decreased food filtering, inhibition of digestive proteases and lethality, development of tolerance against cyanobacterial toxins has also been observed. Aim of our study was to investigate in detail chronic effects of cyanobacterial toxins, with emphasis on microcystin, on D. magna. The animals were exposed chronically for two generations to either microcystin-LR in 5 or 50 microg L(-1), or to cyanobacterial crude extract containing the same amount of total microcystin, starting at neonate stadium. Survival, growth, maturation and fecundity were observed for the first generation during two months. In the offspring survival, maturation, and growth were followed for the first week. Low concentration of microcystin-LR slightly affected the growth and reproduction of parent daphnids. Survivorship decreased during chronic exposure with increasing microcystin concentration. Age to maturity of the offspring increased and their survival decreased after parent generation was exposed to the toxin, even if the offspring were raised in control medium. Besides, cessation of the eggs/embryos was observed and malformation of neonates caused by cyanobacterial toxins was firstly recorded.

Tropical ecotoxicity testing with Ceriodaphnia cornuta.

The Sai Gon-Dong Nai river system in southern Vietnam is of great social and economic importance yet receives a large amount of industrial, domestic, and agricultural discharges. Toxicity assessment has started to become an issue in Vietnam, and it is important to employ a test system that is appropriate for typical Vietnamese conditions with a species that is representative of the invertebrates living in its aquatic ecosystems. The aim of this study was to develop and to validate an ecotoxicity test with an autochthonous organism. The microcrustacean Ceriodaphnia cornuta (Cladocera) was isolated from the Sai Gon River. A protocol was developed for the culturing of the organisms, and satisfactory results were obtained for long-term reproduction of C. cornuta. Quality control criteria were established. The toxicity of two relevant pesticides (diazinon and methyl parathion) and two metals (chromium and mercury) to C. cornuta was evaluated and compared to the standard organisms for ecotoxicological testing: Daphnia magna and Vibrio fischeri. Nonlinear regression models were applied to estimate such parameters as EC(50). The results of acute toxicity tests showed that C. cornuta was less tolerant than D. magna and V. fischeri to salinity and that C. cornuta was more sensitive than D. magna and V. fischeri to potassium dichromate, diazinon, methyl parathion, and mercury.