Estimating migration speed of glass eels during their colonization of a Mediterranean lagoon.

Migration speed can have important evolutionary consequences as it can affect the timing of arrival, remaining energy reserves, and habitat choice. Environmental conditions and individual phenotypic traits can impact the migration speed of individuals. In this way, estimating migration speed is of particular importance, especially for species under strong management strategies and colonizing highly diversified habitats, as is the case for the European eel. However, estimating the migration speed of glass eels, which is the life stage when eels colonize continental habitats, presents challenges due to typically low re-capture probabilities and difficulties in tagging individuals. Using recruitment time series at two sites, one at the sea connection and another inland, we estimated the temporal lag between the two migration peaks to compute migration speed. Because we worked on the Mediterranean coasts and in a lagoon, the weak tidal amplitudes may inhibit individuals from efficiently performing the selective tidal stream transport. We obtained migration speed values coherent with the few values available in the literature for Atlantic estuaries. The values we obtained that are lower than those obtained for Atlantic estuaries are also coherent with the weak tides along the Mediterranean coasts and lead to necessary further studies to understand the migratory behavior of glass eels in such hydro-systems.

Metabolomics to study the sublethal effects of diazepam and irbesartan on glass eels (Anguilla anguilla).

Since glass eels are continuously exposed to contamination throughout their migratory journey in estuaries, to a certain extent the fall in the population of this endangered species might be attributed to this exposure, which is especially acute in estuaries under high urban pressure. In this work, metabolomics was used to address the main objective of this study, to evaluate the effects of two pharmaceuticals previously identified as potential concerning chemicals for fish (diazepam and irbesartan) on glass eels. An exposure experiment to diazepam, irbesartan and their mixture was carried out over 7 days followed by 7 days of depuration phase. After exposure, glass eels were individually sacrificed using a lethal bath of anesthesia, and then an unbiased sample extraction method was used to extract separately the polar metabolome and the lipidome. The polar metabolome was submitted to targeted and non-targeted analysis, whereas for the lipidome only the non-targeted analysis was carried out. A combined strategy using partial least squares discriminant analysis and univariate and multivariate statistical analysis (ANOVA, ASCA, t-test, and fold-change analysis) was used to identify the metabolites altered in the exposed groups with respect to the control group. The results of the polar metabolome analysis revealed that glass eels exposed to the diazepam-irbesartan mixture were the most impacted ones, with altered levels for 11 metabolites, some of them belonging to the energetic metabolism, which was confirmed to be sensitive to these contaminants. Additionally, the dysregulation of the levels of twelve lipids, most of them with energetic and structural functions, was also found after exposure to the mixture, which might be related to oxidative stress, inflammation, or alteration of the energetic metabolism.

Salinity, freshwater and agricultural water preferences of glass eels of the Japanese eel Anguilla japonica collected in southern Japan.

Water-choice trial experiments revealed that Anguilla japonica glass eels collected in southern Japan possess strong preferences for fresh water and agricultural water. Their locomotor activity and preference for fresh water were higher and stronger, respectively, in this study when compared to previous studies conducted at lower temperatures. These results suggest that their locomotor activity and preference for fresh water is influenced by water temperature. The attraction to agricultural water indicates their upstream migration and habitat selection could be influenced by agricultural water.

Warming enhances lanthanum accumulation and toxicity promoting cellular damage in glass eels (Anguilla anguilla).

Cumulative and continuing human emissions of greenhouse gases to the atmosphere are causing ocean warming. Rising temperature is a major threat to aquatic organisms and may affect physiological responses, such as acid-base balance, often compromising species fitness and survival. It is also expected that warming may influence the availability and toxicological effects of pollutants, including Rare Earth Elements. These are contaminants of environmental emerging concern with great economic interest. This group comprises yttrium, scandium and lanthanides, being Lanthanum (La) one of the most common. The European eel (Anguilla anguilla) is critically endangered and constitutes a delicacy in South East Asia and Europe, being subject to an increasing demand on a global scale. Considering the vulnerability of early life stages to contaminants, we exposed glass eels to 1.5 µg L-1 of La for five days, plus five days of depuration, under a present-day temperature and warming scenarios (△T = +4 °C). The aim of this study was to assess the bioaccumulation, elimination and specific biochemical enzymatic endpoints in glass eels (Anguilla anguilla) tissues, under warming and La. Overall, our results showed that the accumulation and toxicity of La were enhanced with increasing temperature. The accumulation was higher in the viscera, followed by the head, and ultimately the body. Elimination was less effective under warming. Exposure to La did not impact acetylcholinesterase activity. Moreover, lipid peroxidation peaked after five days under the combined exposure of La and warming. The expression of heat shock proteins was majorly suppressed in glass eels exposed to La, at both tested temperatures. This result suggests that, when exposed to La, glass eels were unable to efficiently prevent cellular damage, with a particularly dramatic setup in a near-future scenario. Further studies are needed towards a better understanding of the effects of lanthanum in a changing world.

Accumulation, elimination and neuro-oxidative damage under lanthanum exposure in glass eels (Anguilla anguilla).

Rare earth elements (REEs) comprise elements from lanthanum to lutetium that together with yttrium and scandium are emergent contaminants of critical importance for numerous groundbreaking environmental technologies. Transfer to aquatic ecosystems is expected to increase, however, little information is known about their potential impacts in marine biota. Considering the endangered conservation status of the European eel (Anguilla anguilla) and the vulnerability of early fish life stages to contaminants, we exposed glass eels, through water, to an environmentally relevant concentration (120 ng.L-1) of lanthanum (La) for 7 days (plus 7 days of depuration). The aim was to study the accumulation and elimination of La in eel's body and subsequent quantification of acetylcholinesterase (AchE), lipid peroxidation and antioxidant enzymatic machinery. Accumulation peaked after 72 h-exposure to La, decreasing afterwards, even in continuous exposure. Accumulation was higher in the viscera, followed by the skinless body and ultimately in the head, possibly as a protective mechanism to cope with La neurotoxicity. A significant increase in AChE activity was observed in La-exposed glass eels, suggesting that La3+ may inhibit the binding of acetylcholine. A depression in lipid peroxidation was registered under La exposure, possibly indicating that La3+ may play physiological activities and functions as a free radical scavenger. Catalase activity was significantly inhibited in La-exposed glass eels after 72 h, indicating that the availability of La may induce physiological impairment. The quantification of Glutathione S-Transferase activity revealed no differences between control and La-exposed organisms. Further investigation is needed towards understanding the biological effects of REEs.

Migratory behavior, metabolism, oxidative stress and mercury concentrations in marine and estuarine European glass eels (Anguilla anguilla).

The relationships between the migratory behavior, methylmercury (MeHg) concentrations, oxidative stress response and detoxification processes were investigated in glass eels collected in marine (Molliets) and estuarine (Urt) waters (Adour estuary, South West France) at the end of the fishing season (April). Glass eel migratory behavior was investigated in an experimental flume according to their response to dusk. Fish responding to the decrease in light intensity by ascending in the water column and moving with or against the flow were considered as having a high propensity to migrate (migrant). Glass eels still sheltering at the end of the 24h catching period were considered as having a low propensity to migrate and were called non-migrant. Our results provide some evidence that estuarine glass eels were bigger, presented a higher propensity to migrate and a lower oxidative stress response than marine glass eels. This might reflect a selection process, some marine glass eels progressively settling or dying before reaching Urt and/or a change in feeding behavior. In April, glass eels restart feeding in the Adour estuary which might decrease the oxidative stress possibly related to starvation, and enhance migration. MeHg concentrations was significantly higher in non-migrant than in migrant glass eels and it is suggested that non-migrant glass eels might present a higher vulnerability to stress (at least contamination and/or starvation), although the underlying mechanisms remain to be elucidated.

Regional variation in energy storage strategies in American glass eels from Eastern Canada.

Energy status was analyzed in glass eels captured during two early waves of arrival at the mouths of the Mersey River, Nova Scotia, Canada (MR), and Grande-Rivière-Blanche, Québec, Canada (GRB), and according to their salinity preference (freshwater, brackish, or saltwater). Glass eels captured in the GRB estuary were larger, more pigmented, and exhibited higher whole-body glycogen, phospholipid, and sterol and wax ester contents. Those from MR had a higher condition index and a higher whole-body triacylglycerol content, suggesting different patterns of storage and/or use of energy reserves. Within a river, a delay of two weeks in estuarine arrival was characterized by significantly lower energy reserves. No differences in energy storage were observed according to salinity preference. Thus, the results revealed the occurrence of different energy storage strategies according to glass eel migration distance and duration, but not according to salinity preference.

How does salinity influence habitat selection and growth in juvenile American eels Anguilla rostrata?

The influence of salinity on habitat selection and growth in juvenile American eels Anguilla rostrata captured in four rivers across eastern Canada was assessed in controlled experiments in 2011 and 2012. Glass eels were first categorized according to their salinity preferences towards fresh (FW), salt (SW) or brackish water (BW) and the growth rate of each group of elvers was subsequently monitored in controlled FW and BW environments for 7 months. Most glass eels (78-89%) did not make a choice, i.e. they remained in BW. Salinity preferences were not influenced by body condition, although a possible role of pigmentation could not be ruled out. Glass eels that did make a choice displayed a similar preference for FW (60-75%) regardless of their geographic origin but glass eels from the St Lawrence Estuary displayed a significantly higher locomotor activity than those from other regions. Neither the salinity preferences showed by glass eels in the first experiment nor the rearing salinities appeared to have much influence on growth during the experiments. Elvers from Nova Scotia, however, reached a significantly higher mass than those from the St Lawrence Estuary thus supporting the hypothesis of genetically (or epigenetically) based differences for growth between A. rostrata from different origins. These results provide important ecological knowledge for the sustained exploitation and conservation of this threatened species.