Short Note on the Effects of Cadmium on the Gill of the Glass Eel (Anguilla anguilla)

Freshwater-adapted glass eels have been exposed to cadmium (5 mg/l) for 3 days and then transferred in clean water for 28 days. Perturbations occurred very early (after 2 hrs only of exposure to cadmium) and concerned mainly the respiratory epithelium: the epithelial cells were very swollen and they showed an important dilation of the endoplasmic reticulum and nuclear membrane. After the decontamination period, signs of recovery appear. Perturbations of osmo-regulatory and respiratory processes are suggested.     

Are eels (Anguilla anguilla L.) planktonic feeders? Evidence from parasite communities

The metazoan parasite communities of eels Anguilla anguilla were investigated in three riverine localities and one lake in Devon. Altogether 21 species of parasite, the majority of the British eel parasite fauna, were recorded. Parasite communities in the three riverine localities were more similar to each other than to that of the lake. The four parasite component communities were typical and representative of those from eels in other parts of the U.K. They were characterized by low species diversity and domination by a single species: Pseudodacytlogyrus anguillae in the rivers and Diplostomum spathaceum in the lake. Both the dominant species and the majority of parasite species, and hence individuals, in any locality infected eels directly by free swimming planktonic larval stages or indirectly by the ingestion of infected planktonic copepod intermediate hosts. Examination of other species of fish in each locality showed that only Raphidascaris acus utilized a fish species as an intermediate or paratenic host to infect eels. Increases in prevalence and intensity of infection with size of eel by helminths such as Camallanus lacustris and Paraquimperia tenerrima were observed in localities in which transmission of these parasites could not have involved another fish species. Over 63% of the specialist, specific parasites of eels throughout Europe employ planktonic transmission to their hosts, as do several species of the commoner generalists. A study of eel parasite communities thus leads to the conclusion that eels are widely and closely associated in their habits with plankton and that eels of all sizes feed regularly upon planktonic copepods: the widely accepted view of eels as being more or less exclusively benthic in habit and feeding behaviour requires re-appraisal.     

Modelling of unexploited and exploited populations of eels, Anguilla anguilla (L.), in the Thames Estuary

Eels were fyke-netted in different zones of the Thames Estuary at various times of year and subsamples sexed, aged and stomachcontentsexamined. Instantaneous mortality coefficients (Z) were calculated for each 3 cm length class for eels > 30 cm and used to construct a compartmental model to assess effects of natural and fishing mortality and migration on different life stages of males and females. Growth rates were good and fishing mortality effects low and masked by effects of emigration/immigration (and, possibly, local migrations). Density-dependent mortality appeared to be having minor effects. Males dominate freshwater sites, females the more saline zone and possible implications of food availability and density are discussed. It is concluded the stock is not being over-exploited but that elver recruitment is relatively low. Means of increasing exploitation are discussed, as are ways of increasing production of females to enhance elver recruitment to the Thames and other European rivers.     

Behavioural responses of European silver eels(Anguilla anguilla) to the geomagnetic field

Magnetic orientation of European silver eels(Anguilla anguilla) was tested in an octagonal tank. Orientation was determined from photo-registrations of eel positions in tests performed alternately in the natural magnetic field and a field with the horizontal component rotated 180°. Tests were performed in LD 11 : 13. At a daytime light intensity of 100 lux the fish were diurnally active, while at 0.10 lux crepuscular or nocturnal activity dominated. The eels probably differed in preferred orientation, largely depending on the clockwise or anti-clockwise swimming of some of the animals. Therefore there was no preferred direction common to all eels. The orientation of single eels differed, however, significantly between the two magnetic fields, suggesting that the eels responded to the geomagnetic field.     

The population biology of Camallanus lacustris (Zoega) in eels, Anguilla anguilla (Linnaeus), and their status as its host

The population biology of Camallanus lacustris (Zoega) and the status of the eel, Anguilla anguilla (Linnaeus), as its definitive host have been studied in a small Devon lake. No clear seasonal pattern in prevalence and abundance was observed, and recruitment of the new generation may occur in all seasons. However, reproduction exhibits some seasonality as adult nematodes were the commonest stage in the parasite population from early summer onwards and production of first-stage larvae appeared to occur primarily during this period. These larvae were viable, and shown experimentally to be infective to copepods. The occurrence of the nematode and its development to full maturity in eels, the absence of its typical definitive host ( Perca fluviatilis Linnaeus) from the lake and the absence of the nematode from other species of fish in the lake indicate that eels serve as the only, true definitive host of the nematode in this locality. It is suggested that account must be taken of this alternative host when the population of C. lacustris is investigated in the future.     

The parapineal and pineal organs of the elver (glass eel), Anguilla anguilla L.

Summary The parapineal organ of the glass eel (elver) consists of approximately 400 cells and is situated to the left of the connection of the pineal stalk to the third ventricle. A conspicuous nerve tract containing approximately 350 fibers arises from the parapineal organ and runs in spatial relationship to the habenular commissure toward the left habenular nucleus. The dominating cell type of the parapineal organ of the elver is a neuron (sensory neuron) of small diameter provided with atypical cilia (9×2+0, or rarely 8×2+0 types). Well-developed photoreceptor outer segments are lacking, and no interstitial cells of ependymal type have been observed with certainty in the parapineal organ. The axonal processes from the nerve cells form the tract leaving the parapineal organ.The pineal organ proper of the elver consists of photoreceptor cells with well-developed outer segments, interstitial cells of ependymal type, and ganglion cells. Axons from the latter form the pineal tract, which leaves the pineal organ and runs in close contact with the subcommissural organ toward the posterior commissure. The proximal part of the pineal stalk contains only a few photoreceptor cells the outer segments of which are less developed than those of the pineal body and the distal part of the pineal stalk.     

Head shape dimorphism in European glass eels (Anguilla anguilla)

The life cycle of the European eel (Anguilla anguilla) remained a mystery until the 20th century, when Schmidt discovered that the Sargasso Sea was its spawning area. However, many aspects of the eel's life cycle remain poorly understood. Among these is the bimodal distribution in head shape, with broad- and narrowheaded phenotypes reported in the yellow eel stage. Although this has been linked to dietary preferences of the yellow eels, very little is known about why, how and when this dimorphism arises during their ontogeny. To determine whether this dimorphism indeed appears in relation to trophic niche segregation, we examined head shape variation at an earlier ontogenetic stage, the glass eel stage, as at this stage eels are considered to be non-feeding. Head shape was studied in a large dataset, containing glass eels captured from the Yser river mouth, the Leopold Canal (Belgium) and from the rivers Severn, Trent and Parret (UK), by both taking measurements (head width/head length) and using an outline analysis. Our results show that there is already considerable variation in broadness and bluntness of the head at the glass eel stage. In most cases, equal support for a unimodal and bimodal head shape distribution is found, whereas some cases support head shape bimodality in glass eels, suggesting that glass eel head shape might be shifting from a unimodal to a bimodal distribution. This, in combination with the observation that variation in head width/head length ratios in non-feeding glass eels shows a similar range as in feeding yellow eels, indicates that head shape in European eel might be at least partially determined through other mechanisms than trophic segregation.     

Occurrence of Phyllodistomum folium (Olfers) (Digenea) in the eel, Anguilla anguilla (Linnaeus), in a small river

The occurrence of Phyllodistomum folium in a population of eels was studied throughout one year. Abundance exhibited a summer peak. Gravid adults were found in all months, indicating that eels can serve as a definitive host for this parasite.     

Stereoselective routes towards the synthesis of unusual bis(amino acids) and cyclic amino acids

Stereoselective syntheses are described of bridged bis(glycines) as conformationally constrained substitutes for cystine, and of cyclic -amino acids where the -carbon of the amino acid is part of a five-, six- or seven-membered ring which may hold a hydroxy group as a threonine analogue.     

Growth rate and age at migration of Anguilla anguilla

The length, age and growth rate were investigated for downstream migrating male and female eels in the unexploited Burrishoole system, western Ireland. Significant differences were found in the age and length at migration with the larger, older female eels also showing faster annual growth as early as the first year in fresh water. Female eels normally migrated at lengths from 40.5 cm, exceptionally to 92.9 cm, and male eels at lengths between 28.9 and 46'0 cm. Back-calculation showed an irregular pattern of fast and slow annual growth. Mean annual growth increments were almost always greater for females than for males.     

Occurrence and seasonal dynamics of Pseudodactylogyrus anguillae (Yin & Sproston) (Monogenea) in eel, Anguilla anguilla (L.), in England

Established populations of Pseudodacrylogyrus anguillae are reported from eels in England for the first time. Prevalence and abundance peak in late summer in all three localities and the parasite overwinters at low levels on eels.     

Isolation and characterization of two different 5S rDNA in Anguilla anguilla and in Anguilla rostrata: possible markers of evolutionary divergence

We cloned and sequenced the HaeIII 350‐bp 5S ribosomal DNA (rDNA) band of Anguilla rostrata and designed specific primers from this sequence. Polymerase chain reaction performed with these primers is able to distinguish DNA samples obtained from European (Anguilla anguilla) and American (Anguilla rostrata) eels. Two amplicons of 1200 bp and 600 bp were obtained, respectively, from A. rostrata and A. anguilla, and the whole 5S rDNA repeated unit from these eels was cloned and sequenced. Southern blot experiments, using four different restriction enzymes and the 5S nontranscribed spacers regions as probe, are able to point out specific diversity in these eels.     

Isolation and characterization of 12 dinucleotide microsatellites in the European eel, Anguilla anguilla L., and tests of amplification in other species of eels

Twelve polymorphic dinucelotide microsatellites in the freshwater eel Anguilla anguilla L. were isolated and characterized. Genetic diversity was assessed in eels from Lake Constance, Germany. Allele numbers ranged from five to 26 per locus with observed heterozygosities between 0.125 and 0.875. A portion of locus AangCT77 aligns with a transcribed region of the zebrafish gene crystallin beta B2. Cross-species amplification of most markers was possible for nine other Anguilla eel species. The newly developed primer pairs will facilitate population and conservation genetic studies in order to refine the understanding of the subtle population genetic structure typical of eels, and to identify interspecies admixture due to global trade.     

Factors determining length distribution and abundance of the European eel, Anguilla anguilla, in the River Mondego (Portugal)

1. The distribution and abundance of the European eel, Anguilla anguilla, were investigated in the Mondego River, a strongly modified river in central Portugal. Nine freshwater sites, located below the first impassable obstacle to eel migration, were surveyed for 2 years. The study was conducted monthly during the first year and seasonally during the second year. Fish were caught by electric fishing and, physical and biotic variables potentially influencing eel length distribution and abundance were determined at each sampling session. 2. Canonical correspondence analysis (CCA) was used to explain the relationships between abiotic and biotic habitat variables, and eel length distribution. Spatial variation was strongly associated with distance from the sea, number of obstacles, river width and percentage of instream cover. Depth, river flow, water temperature and cannibalism had weaker effects on the longitudinal distribution, but contributed as explanatory variables for the model. The influence of spatial variables outweighed the importance of temporal variables as predictors in this model. 3. Length distribution changed in space from a dominance of small eels (<100 mm) closer to the sea to larger eels (≥250 mm) in the upper reaches of the river. The smallest length class (eels <100 mm) was strongly associated with wider stretches where instream cover, mainly composed of aquatic submerged macrophytes, was abundant (>75% area coverage). On the contrary, larger eels (≥250 mm) preferred deeper stretches with stronger river flow and less instream cover. 4. The effect of biotic interactions with conspecifics as well as with the non‐native red swamp crayfish Procambarus clarkii, in sites with poor instream cover, modified this general pattern of distribution leading to fewer eels <100 mm at some sites closer to the sea. Therefore, despite the ubiquity and plasticity shown by this species, there were clear ontogenetic variations in habitat use such that the first two canonical axes of a CCA accounted for 58.4% of the spatial variability in size structure.     

The crystalline ultrastructure of otoliths of the eel (A. anguilla L. 1758)

The ultrastructure of the organic and the mineral components of eel ( Anguilla anguilla L. 1758) otoliths was studied using sections through the nucleus and observed by scanning electron microscopy. The otoliths were composed of crystalline material (aragonite) running radially from the centre to the edge. Structural discontinuities were described: seasonal rings, checks and daily growth increments.     

Population biology of Proteocephalus macrocephalus (Creplin) in the European eel, Anguilla anguilla (Linnaeus), in two small rivers

Seasonal changes in populations of Proteocephalus macrocephalus (Creplin) were investigated for 17 months in the European eel, Anguilla anguilla (Linnaeus), in two rivers in Devon, SW England, but no clear seasonal patterns in prevalence and abundance were apparent. Population levels of the cestode are low in both localities, and it is suggested that natural population levels of P. macrocephalus may generally be low. However, growth and maturation of the cestode show marked seasonality with both occurring mainly in early summer.     

Rod visual pigment changes in the elver of the eel Anguilla anguilla L. measured by microspectrophotometry

The visual pigments in the retinal rods of elvers ( Anguilla anguilla L.) were examined by microspectrophotometry and high performance liquid chromatography. The glass elver stage had a mixture of rhodopsin P501₁ and porphyropsin P523₂ with the former predominating (P501₁:P523₂= c. 60:40). More mature pigmented elvers had a predominance of the porphyropsin P523₂ (P501₁:P523₂= c. 10:90). The shift between the proportions of each pigment was monitored over a 7-month period and was shown to occur in individual retinal rods. This shift is a reverse of the pigment shift that occurs in adult eels during their downstream migration.