Adaptation of the eel swimbladder to the spawning migration

The European eel Anguilla anguilla is a catadromic fish, spending most of his life in the European freshwater system and then completing his life cycle by a spawning migration to the Sargasso Sea. Returning to the sea requires physiological adaptations known as the silvering process. This change from a freshwater dwelling yellow eel to a sea water adapted silver eel comprises hormonal changes, osmotic adaptations to the marine environment, but also an enlargement of the swimbladder rete mirabile, a remarkable countercurrent system, and a thickening of the swimbladder epithelium, which in the eel consists of the so called gas gland cells. Although compelling evidence suggest that silvering includes an improvement of swimbladder function and that the swimbladder as a buoyancy organ is crucial for the successful spawning migration over a distance of 4000-5000 km to the Sargasso Sea, the metabolic and molecular changes in gas gland cells occurring during silvering are totally unexplored. Using Illumina Sequencing Technology the expression changes accompanying the silvering process in gas gland cells will be analyzed, focusing in particular on metabolic changes, changes in ion transport and regulation and in possible control mechanisms. Combined with primary cell culture experiments and biochemical studies looking at enzymes involved in the scavenging of reactive oxygen species ROS and at cellular antioxidants the question will be solved how the swimbladder tissue, which is chronically exposed to severe hyperoxia, is coping with ROS, which are generally produced under hyperoxic conditions. Within the last two decades the nematode Anguillicoloides crassus infected a large fraction of the European eel populations and significantly impaired swimbladder function and also swimming performance. Therefore Illumina Sequencing will also be used to analyze the impact of a nematode infection of the swimbladder on the silvering event and swimbladder function in silver eels. Due to the significant decline in the eel population a lot of effort is invested in an artificial, hormonal maturation of eels in order to be able to breed this species successfully. Therefore the molecular modifications of gas gland cells achieved with this hormonal maturation will be assessed and compared to the events observed in silver eels captured in the wild. This project will thus provide a detailed understanding of the molecular and metabolic adaptations of gas gland cells in silver eels and lay the ground for our understanding of swimbladder performance and function during the spawning migration. A proper understanding of swimbladder function in turn is essential for any assessment of a successful spawning migration, and this is essential for the survival of the species.