European eel larval ontogeny and physiology Insights at morphological and molecular level

Abstract

The research conducted within this PhD project contributes to filling gaps in knowledge about the enigmatic life cycle of European eel (Anguilla anguilla) by addressing biotic and abiotic factors influencing early larval stages. This involves experimental studies and utilization of state-of-the-art molecular tools elucidating links between morphology and molecular mechanisms in the quest to identify suitable rearing and feeding conditions for larviculture. As such, this thesis comprises six studies within three main topics: i) temperature, ii) salinity and iii) nutrition, influencing larval development and survival. The first three studies address the influence of temperature on early larval ontogeny. Here, Study 1 determined the thermal tolerance limits and identified an intermediate thermal environment for future larval culture with efficient growth and low frequency of deformities associated with high expression of growth hormone (improved growth) and low expression of heat shock proteins (decreased stress). Moreover, Study 2 revealed that expression of genes encoding thyroid hormone receptors and deiodinases, associated with the mediation of thyroid hormone action, show sensitivity to temperature and are involved in and during early larval development. Additionally, Study 3 shed light on the molecular ontogeny of the larval immune system under different thermal scenarios and identified an immune-compromised phase during which mortality is high and larvae are more vulnerable to pathogen infection. This will have important implications on rearing conditions and disease prevention protocols in eel hatcheries but also improve our understanding of ocean warming impacts on fish recruitment. Thereafter, experimental work focused on the salinity tolerance of these marine larvae. Here, Study 4 clearly demonstrated that culture regimes reducing salinity towards isoosmotic conditions facilitated enhanced European eel pre-leptocephalus development and survival revealing the existence of underlying, highly sensitive and regulated osmoregulation processes in the early larval stage. This novel insight gained by morphologically and molecularly defined physiological thermal and osmoregulatory tolerance limits and preferences gradually led to improved protocols for pre-leptocephalus larval culture and resulted in significantly increased numbers of healthier and stronger larvae reaching the first-feeding stages. Previous to these culture condition improvements, a pilot nutritional trial was performed. Here, Study 5 for the first time explored several diets, tested attractants and described behavioral feeding patterns of European eel larvae. Finally, with much enhanced numbers of larvae and using the previously identified benchmark diet as well as enhanced temperature condition, Study 6 revealed that initiation of exogenous feeding in European eel occurs concurrently with the onset of the genetically pre-programmed underlying hormonal control of ingestion and the enzymatic regulation of digestion, known to regulate physiological functions of feeding. The here gained knowledge, improved the understanding of an undisclosed phase of the European eel life cycle, which is the transition from yolk-sac pre-leptocephalus larvae to the exogenous feeding leptocephalus stage and constitutes essential information in order to develop efficient feeding strategies for future larviculture of this species. In conclusion, the conducted research elucidated molecular aspects of important biological processes in order to more closely understand the complexity of regulations involved in early European eel ontogeny and physiology. The gained knowledge contributes to our understanding of unknown mysterious aspects of the European eel life cycle and most importantly provides promising steps for eel aquaculture towards completing the life cycle in captivity of this socially and economically important as well as critically endangered fish species