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

Abstract

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.

Transcriptome analysis of the Japanese eel (Anguilla japonica) during larval metamorphosis

“…Elvers then migrate upstream during spring to summer and resume feeding. During this time period, food uptake eventually intensifies and then they settled as yellow eels [ 71 ]. Our results on the timing of gene expression are consistent with these observation of eels in the natural environment.…”

Low microplastic loads in riverine European eel (Anguilla anguilla) from southwest England during their marine–freshwater transition

“…The microplastics recovered from within the eel samples likely originate from either pelagic feeding by the leptocephalus stage in or around the Sargasso Sea, known to contain floating plastics (Carpenter & Smith, 1972), or freshwater feeding as glass eel and elvers. Nonetheless, a small proportion of the intermediate glass eel stage does feed within estuarine environments, providing opportunities for plastic ingestion there (Bardonnet & Riera, 2005; Van Wichelen et al ., 2022). Notwithstanding, as the Frome eels were collected some distance from the tidal limit and were considered to have been in the river for some time, it is likely their plastic items were all of freshwater origin.…”

Metabolomics to Study the Sublethal Effects of Diazepam and Irbesartan on Glass Eels (Anguilla Anguilla)

“…This means that the experiments should be carefully designed taking into account the sources of variation and minimizing the effect of all the non-controlled ones (e.g., the effect of methanol as carrier). As previously mentioned, two of the main sources of variation in this kind of studies, sex and feeding, are not influential since the sex of glass eels is still not defined and they fast until the juvenile stage (Van Wichelen et al, 2022).…”