Amphibians: Biology, Ecology and Conservation e-bog

Amphibians: Biology, Ecology and Conservation opens with a review of the current knowledge on the Harderian gland of anuran amphibians. The Harderian gland is an orbital gland present in all vertebrates possessing nictitating membrane, with the primordium of the Harderian gland appearing during metamorphosis throughout the development of the nictitating membrane. The authors go on to discuss D-Aspartate (D-Asp), an excitatory amino acid involved in the synthesis of sex steroid hormones. High concentrations of D-Asp were found in the gonads, in the brain and in the Harderian gland of the green frog, Pelophylax esculentus. The current knowledge on the D-Asp in gonads and in tissues during the seasonal reproductive cycle is reviewed. Since D-Asp modulates sex hormone synthesis, this book correlates the local amino acid levels with serum and local sex hormone concentrations. Next, the authors report on the existing work on the steroidogenic pathways activated in reproductive and post-reproductive phases in both testis and brain in the aforementioned green frog. Plasma testosterone levels are low in the non-breeding period and high during the breeding period, while 17β-estradiol levels illustrate a contradictory trend. This study indicates a close correlation between steroidogenic process in the testis and in the brain, suggesting a synergic mechanism of regulation. The concluding chapter presents three case studies that demonstrate how the life cycle of the giant toad, Rhinella marina, as well as its somatic and hepatosomatic index, voracious appetite and highly permeable skin make it a species susceptible to the bioaccumulation of organic and inorganic pollutants and their toxicological effects. Due to these characteristics, R. marina is considered to be a good biomonitor of environmental pollution.