By collecting and counting the number of sperm released during separate matings in three batches of zebrafish Danio rerio, aged 3-4, 4-5 and 5-6 months, males were observed to release sperm before the female started laying their eggs. After the female left the nest, the number and motility of sperm and life span of sperm of younger fish were higher than those of older fish in water samples collected under the nest and at the surface of the tank. Sperm were released in the form of sperm trails laid on the nest surface, subsequently active spermatozoa left the trails and moved in the water for several minutes. Sperm trails consisted of bands of viscous material in which the sperm were embedded. In most cases eggs were not laid directly over the sperm trail, suggesting that sperm may contact the eggs after the latter are released into the water. In all the three tested groups there was no significant difference (P > 0Á05) between the number of sperm collected on some portions of the acetate sheets which lined the nest ceiling. This result demonstrated that the greater activity of younger fish accelerated the sperm dispersal in water. Male sperm duct glands, seminal vesicles, known to secrete mucosubstances are probably involved in the production of sperm trails. The possible influence of insemination on the mating style of zebrafish is discussed. # 2004 The Fisheries Society of the British Isles

Long-term toxicity growth tests were conducted with DDT using zebrafish (Danio rerio). The fish were exposed to concentrations of 0.05 µg/l, 0.5 µg/l, 5 µg/l and 50 µg/l DDT under flow-through conditions. The effect of the compound on the ecologically important parameters hatching rate, duration of the developmental stages and growth were the focus of interest. When exposed to 50 µg/l DDT, the hatching rate was 2 to 3 times lower than in embryos exposed to 0.05 µg/l DDT. This study demonstrates that the rearing zebrafish in the pesticide causes a reduction in body length and the durations of the life cycle stages (LCS) of the offspring are extended. These effects are increased when the fish are exposed to higher DDT concentrations.Key words ---Danio rerio, early life stage, life cycle stages, growth, F 1 -generation, DDT able up to now. The zebrafish [Danio rerio (D. rerio)] is a suitable model for full life cycle assessments because zebrafish takes only 3-4 months to reach the sexual maturity. Moreover, they can be maintained in a laboratory environment at low cost and remain sexually active throughout the year.The test employed in this study involved exposure of zebrafish to the toxin from the fertilized egg up to the mature animal. The effect on reproduction was assessed by examination of mortality, hatching rate, body length and development of the offspring. This study encompassed the examination of two generations of fish over a period of approximately 8 months. This was regarded necessary to obtain sufficient information on the duration of each life stage and the deleterious effect of the chemical on the subsequent generation. The parameters were also investigated in the offspring of exposed parents that were subsequently reared in a toxin-free environment. The second generation was assessed up to six weeks and the difference with offspring which remained exposed to DDT was determined.

No abstract

This study describes a long-term toxicity test within a period of 2 months using zebrafish (Brachydanio rerio) as the test species and concentrations of 0.05, 0.5, 5 and 50 µg/l DDT as a model substance. By collecting and counting the number of sperm released during separate mating events we observed that gametes are released asynchronously. Sperm is released in the form of sperm trails laid on the nest surface; subsequently active spermatozoa leave the trails and move in the water for several minutes. Sperm trails consists of bands of viscous material in which sperm is embedded. The water samples for the estimation of sperm presence were collected gradually within 180 min after 24 hr, 2 weeks, 1 month and 2 months of exposure. It was established that the reduction in count, activity of sperm and the average life span of sperm trails were significant (p < 0.05) at the tested concentrations 5 µg/l and 50 µg/l DDT after 1 month of exposure. In conclusion, this study demonstrates that long exposure time and higher DDT tested levels accelerate the occurrence of negative effect on the number and activity of sperm released as well as the life span of their trails.

No abstract