Regional variation of gene regulation associated with storage lipid metabolism in American glass eels (Anguilla rostrata)

Glass eel (Anguilla anguilla L. 1758) feeding behaviour during upstream migration in an artificial waterway

The transition from marine to fresh water is a challenging task for juvenile eels. This critical step in the early eels' life is preceded by a metamorphosis from the oceanic larval to the continental glass eel stage, requiring major energy-demanding morphological, physiological and behavioural modifications during which time these animals do not feed. The success of the glass eels' inland migration after metamorphosis will largely depend on remaining energy levels, which can be supplemented only by resuming food uptake. Although it is crucial for their survival and the maintenance of the population, the feeding behaviour of glass eels is still an understudied aspect of the eels' complex life cycle. Many uncertainties about the phenology, diet, potential prey preferences and their relation with migration modus (migratory vs. sedentary) still remain. In this study, the authors analysed the stomach and gut contents of 458 European glass eels (Anguilla anguilla L. 1758) captured in a drainage canal connecting a small mesotidal estuary with an adjacent polder area during the spring migration seasons of 2016 and 2017. They demonstrated that although glass eels started feeding briefly upon arrival in the estuary, food uptake for early arrivals was restricted to a minority that sparsely feed on detritus and some worm-like benthic invertebrates. Along the season, food uptake intensified eventually engaging all glass eels and their dietary palette diversified including a wide array of planktonic and benthic organisms. Crustacean plankton (mainly cyclopoid copepods) was an important part of the glass eel diet, whereas benthic oligochaetes were less abundant as food source in spite of their high presence in the sediments. No clear differences in feeding behaviour could be observed between migratory and sedentary glass eels. This study showed that glass eels can use highly artificial and dynamic drainage canals as feeding ground during their critical marine/freshwater transition. This outcome is also a plea to improve the accessibility of alternative (unnatural) migration routes between the ocean and suitable freshwater growth habitats for the European eel.

Ecology and evolution of migration in the freshwater eels of the genus Anguilla Schrank, 1798

Scientists have long sought to uncover the secrets of the migration of anguillid eels, genus Anguilla. As catadromous fishes, anguillid eels spend most of their lives in freshwater until they return to their spawning grounds in the tropics, although part of the population never enters freshwater and instead resides in brackish and marine areas close to coastlines. Molecular phylogenetic research suggests that anguillid eels originated from deep-ocean midwater marine anguilliform species and that tropical eels originating from the Indo-Pacific region are the most basal species of anguillid eels. Anguillid eels left the tropical ocean to colonize temperate areas. The yearly spawning of tropical species and constant larval growth throughout the year extend to periods of recruitment in continental habitats to last all year for tropical eels. Tropical eels such as A. celebesensis and A. borneensis have relatively short migrations periods of less than 100 km to their spawning grounds. Conversely, the temperate European eel A. anguilla travels the longest distances and migrates more than 5000 km across the Atlantic Ocean to spawn in the Sargasso Sea. The ancestral state of migration in the genus Anguilla may have been local, short-scale and nonseasonal spawning migration throughout the year as defined in tropical eels. With the expansion of dispersion of global oceanic migration across the world, migration scales can gradually change. Temperate anguillid eels migrate thousands of kilometres from spawning areas to coastal and inland water habitats while retaining spawning areas in tropical areas, accompanied by seasonal downstream and spawning migrations with consequences for seasonal recruitment. Recent advances and the availability of electronic tags such as pop-up satellite archival tag could reconstruct the entire spawning migration from continental growth habitats to spawning sites with detailed migration behaviours and routes. Migration ecology and mechanisms throughout the life of anguillid eels have gradually been revealed in recent decades.

Weaning European glass eels (Anguilla anguilla) with plant protein-based diets and its effects on intestinal maturation

Weaning glass eels with compound diets (36% proteins, 16% lipids) differing in their fishmeal (FM) level (50, 75 and 100% FM replaced by a blend of plant proteins, PP) was compared to a group fed cod roe. Weaning lasted for 20 days and then, eels were fed compound diets for 70 days, whereas the other group was only fed cod roe (90 days). Diets were tested with four replicates and evaluated in terms of growth, survival, glass eels metamorphosis into elvers, oxidative stress status and activity of digestive enzymes. Although glass eels are traditionally fed with fish roe and progressively weaned onto compound diets, results revealed that this strategy should not be prolonged for a long time, since feeding glass eels with cod roe for 90 days negatively affected their growth (2 times lower than fish fed compound diets), delayed their metamorphosis, as well as the maturation of their digestive function as the ratio of alkaline phosphatase and leucine-alanine peptidase indicated. Weaning glass eels onto compound diets differing in their FM levels did not affect their growth, metamorphic stage nor the activity of pancreatic enzymes (total alkaline proteases, trypsin, bile salt-activated lipase and α-amylase), although 75% FM replacement by PP sources delayed the level of intestinal maturation in eels. In comparison to glass eels fed the 100% FM diet, survival was negatively affected in groups fed diets with 50 and 75% FM replacement by PP ingredients, which indicated that further improvement is needed in diet formulation for this stage of development.

Draft genome of the American Eel (Anguilla rostrata)

Freshwater eels (Anguilla sp.) have large economic, cultural, ecological and aesthetic importance worldwide, but they suffered more than 90% decline in global stocks over the past few decades. Proper genetic resources, such as sequenced, assembled and annotated genomes, are essential to help plan sustainable recoveries by identifying physiological, biochemical and genetic mechanisms that caused the declines or that may lead to recoveries. Here, we present the first sequenced genome of the American eel. This genome contained 305 043 contigs (N50 = 7397) and 79 209 scaffolds (N50 = 86 641) for a total size of 1.41 Gb, which is in the middle of the range of previous estimations for this species. In addition, protein-coding regions, including introns and flanking regions, are very well represented in the genome, as 95.2% of the 458 core eukaryotic genes and 98.8% of the 248 ultra-conserved subset were represented in the assembly and a total of 26 564 genes were annotated for future functional genomics studies. We performed a candidate gene analysis to compare three genes among all three freshwater eel species and, congruent with the phylogenetic relationships, Japanese eel (A. japanica) exhibited the most divergence. Overall, the sequenced genome presented in this study is a crucial addition to the presently available genetic tools to help guide future conservation efforts of freshwater eels.