Comparative aspects of circulating erythrocytes in the trophic and reproductive phases of European eel. Ultrastructure and cytochemistry

Different cytochemical and morphological parameters were used to compare the functional patterns of the erythrocytes in the transition from the trophic (yellow eel) to the reproductive (silver eel) phases of European eel (Anguilla anguilla L.). Data on the nucleus were obtained by microdensitometric (Feulgen reaction) and cytofluoremetric (propidium iodide and Hoechst 33342 fluorochromization) evaluations of DNA staining intensity. In the silver eel, chromatin condensation was more heterogeneous and chromatin also appeared to be looser. The fluorochromes provided information on the different degrees of organization of the chromatin filament, with regard to both primary and higher order DNA structure. In the silver eel, the fine structure showed a higher number of erythrocytes with large euchromatinic areas and the heterochromatinic component was occasionally present in scattered clumps. As for the cytoplasm, in the silver eel a positive correlation could be established between the presence of intact cytoplasmic organelles (mitochondria, Golgi complexes, ribosomes, etc.) and the high fluorescence intensity. This aspect is revealed through the detection of definite membrane components such as glycoconjugates and primary amino groups (PAS reaction and fluorescamine staining, respectively). All these findings allowed us to establish that the transition from the trophic to the premigratory reproductive phase is marked by a higher heterogeneity of the erythrocytes, due to different cell maturation levels. This fact can be ascribed to different metabolic requirements as well as to a more intense erythropoiesis, typical of the most active phase of the eel's life.     

Comparative aspects of cerebellar cytoarchitecture in the European eel life cycle

The major anatomical divisions of the cerebellum of the European eel, i.e., corpus cerebelli, lobus vestibulolateralis, and valvula, were studied morphologically and morphometrically. There were differences in cerebellar cytoarchitecture and gross morphology in two stages of the eel life cycle, the trophic stage (yellow eel), and the reproductive stage (silver eel), which are characterized by different degrees of swimming activity. The principal differences between silver and yellow eels in the cytoarchitecture of the corpus cerebelli and the lobus vestibulolateralis were in distribution of Purkinje or Purkinje-like cells in the molecular layer, which is wider in silver eels, in part because of a decreased thickness of the granular cell layer. In the silver eel, the scattering of Purkinje cells was more evident in the lobus vestibulolateralis where the molecular layer is also thicker. The data indicate the transition from the yellow eel to the silver eel is characterized by a migration of granule cells from the ganglionic cell layer to the internal granular layer and by a further development of molecular layer components, e.g., parallel fibers, Purkinje-cell dendrites, etc. In contrast, the thickness of the granular layer and of the Purkinje cell layer, limited to the lower part of the valvula, decreased. There is also a slight increase of cerebellar volume in the silver eel. The volume of the lobus vestibulolateralis was constant. Hypertrophy of the valvula and eminentiae granulares is observed and is due to the migration of cells from the granular layer of the corpus cerebelli whose volume slightly decreases. Perhaps the lobus vestibulolateralis also contributes to the increased volume of eminentiae granulares. Our findings suggest that the cerebellum continues to develop during the passage from the trophic to the reproductive stage of the eel. The appearance of new afferents from the lateral line which becomes more visible in the silver eel probably completes cerebellar ontogeny.     

Hormonal stimulation, in vivo, of the ovary of European eel in the yellow stage

The immature ovary of yellow eel was stimulated by a long-term treatment with a gonadotropin II rich extract of carp pituitary. Ovarian follicles of treated yellow eels showed an accumulation of lipidic vacuoles in the oocyte cytoplasm and a thickened follicular envelope (in this regard, they resembled the ovarian follicles of previously studied silver eels). The treatment also resulted in a significant but limited increase in the gonadosomatic ratio and mean follicle size (these values were much lower than those measured in silver eels). No change in pigmentation or general morphology was observed. Thus, under our experimental conditions, a gonadotropic stimulation of the yellow eel leads to some but not the totality of the ovarian modifications which occur physiologically during silvering.     

Structure and Innervation of Inner Ear Sensory Epithelia in the European Eel (Anguilla anguilla L.)

Hair cell orientations of all inner ear sensory epithelia in glass eel, yellow eel and silver eel are presented. The patterns of hair cell orientation do not change with age. All sensory epithelia increase in area during growth of the eel. Examination of the hair cell population in macula utriculi show constant hair cell densities and increased hair cell population during development. Further, regional differences in hair cell densities and hair cell types are observed. The hair cells/axons ratio increases 3-fold from glass eel to silver eel stadium. Nerve stainings in silver eel reveal complex innervation patterns with large stubby fibres confined to restricted regions. Histograms of nerve fiber diameters show marked differences from glass eel to silver eel. Growth of sensory epithelia is discussed.     

Gonad differentiation and body growth in Anguilla anguilla L.

Using histological sections, the gonads of samples of yellow and silver eels of two populations were examined. The populations were previously analysed for growth and sex ratio. The histological structures observed are similar to those described in previous publications for the European eel, Anguilla anguilla and to those indicated for the Pacific eel, A. japonica . Well differentiated gonads are present in the silver eels. In the yellow eels, ranging in age from 0 + to 2 + years and from a length of 20 cm to that at which they become silver, undif-ferentiated and both sex gonads are found. Histological evidence is presented which suggests that the ovary, found even in young and small eels, is completely differentiated at a very early stage. The testis-like gonad of the yellow eel is a more primitive, and possibly reversible, gonad which differentiates completely at the beginning of sexual maturation and the change to the silver phase.     

Maturation patterns in female European eel: age and size at the silver eel stage

The maturation pattern in the female European eel Anguilla anguilla was studied by investigating age and size patterns of silver eels in different aquatic environments in Sweden, covering limnic, brackish and marine waters. The results neither supported the hypothesis that there is a critical size or age when eels enter the silvery stage, nor that size and age at maturity are positively related. Age at maturity, however, was observed to be negatively related to growth rate in all localities, i.e. the female reproductive tactic apparently is to become sexually mature at the earliest possible opportunity. Furthermore, it was recognized that a significant amount of variation was due to habitat differences, since the female eel maturation pattern deviated systematically between sampling sites, as it did also when the effect of growth rate was eliminated. Thus, the ability of the female eel to adjust maturation to an optimal size and age can be questioned, because the panmictic nature of the eel means local adaptations are unlikely Growth rate dependent differences suggest that variations in maturation patterns between eel environments are linked more to the opportunity for nutrient accumulation than to other aspects of growth.     

Spatial and temporal trends in unexploited yellow eel stocks in two shallow lakes and associated streams

Mean yellow eel density and biomass in two adjacent shallow (mean depth c 1·5 m) lochs varied significantly between years. Temporal patterns of density, biomass and size were similar in both soft and rocky substrata in the lochs, although eels were consistently smaller in the latter habitat. In both substrata, average length and weight showed a non-significant inverse relationship with density, supporting the hypothesis of density-dependent regulation of the yellow eel population. Fyke net catches were size selective, catching no eels <30 cm long, and providing length-frequency information for silver eels. Fyke net catch per unit effort (CPUE) declined consistently each autumn but specific annual trends were different. When eel density increased, fyke net CPUE declined substantially.     

Two isoforms of the Na+/K+/2Cl- cotransporter are expressed in the European eel (Anguilla anguilla)

Two cDNA isoforms of the NKCC1 secretory cotransporter have been isolated from the European eel. The NKCC1a isoform exhibited mRNA expression in a wide range of tissues in a similar fashion to mammals, whereas NKCC1b was expressed primarily in the brain. The effect of freshwater (FW) to seawater (SW) transfer on NKCC1a expression was dependent on the developmental stage. In non-migratory yellow eels, NKCC1a mRNA expression in the gill was transiently up-regulated 4.3-fold after 2 days but also subsequently by 2.5-6-fold 3 weeks after SW transfer. Gill NKCC1a expression was localised mainly in branchial chloride cells of SW acclimated yellow eels. In contrast to yellow eels, NKCC1a mRNA abundance was not significantly different following SW acclimation in silver eel gill. NKCC1a mRNA abundance decreased in the kidney following SW acclimation and this may correlate with lower tubular ion/fluid secretion and urine flow rates in SW teleosts. Kidney NKCC1a mRNA expression in silver eels was also significantly lower than in yellow eels, suggesting some pre-acclimation of mRNA levels. NKCC1a mRNA was expressed at similar low levels in the middle intestine of FW- and SW-acclimated yellow or silver eels, suggesting the presence of an ion secretory mechanism in this gut segment.     

Localization of beta-hydroxysteroid dehydrogenase in Anguilla anguilla testis

The histochemical reaction for 3 beta-hydroxysteroid dehydrogenase was applied to glutaraldehyde fixed eel testis and successively processed for transmission electron microscopy. The reaction product was found only on the smooth endoplasmic reticulum and in the intermembrane space of mitochondria of the Leydig cells of hCG treated silver eels. The positive cytochemical reaction appears to be coupled with an enlargement of the Leydig cells and an increase of their smooth endoplasmic reticulum.     

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.     

Parasitological study of a population of Tiber River eels (Anguilla anguilla)

A study was conducted on the parasitofauna of Anguilla anguilla caught in the section of the Tiber river south of Rome (Italy). The sample examined consisted of 417 specimens that were classified by stage (elver, young yellow eel, yellow eel and silver eel) and season of sampling. The following species were identified: Trypanosoma granulosum, Eimeria anguillae, Myxidium giardi, Sphaerospora reichenowi, Myxobolus sp., Trichodina anguilli, Ichthyohodo sp., Gyrodactylus anguillae, Bothriocephalus claviceps, Proteocephalus macrocephalus, Raphidascaris acus, Acanthocephalus clavula. Each parasite species was considered in its prevalence according to stage and season. For some parasite the pathogenic effect was also examined in a histological study of serial sections. The results of the statistical analysis of the single parasitic infection as related to different seasons pointed to marked seasonal trends only in some species (Trypanosoma granulosum and Gyrodactylus anguillae). The intensity of infection assessed for acanthocephalan infection only was found to affect neither weight nor condition index (weight/length3). No difference in weight or condition index was found to be ascribable to the presence or absence of a species. An attempt to find a rule-of-thumb algorithm to gauge the overall effect to parasitic diseases failed to give sufficiently significant results.     

Immunolocalization of steroidogenic enzymes in gonads of European eel, Anguilla anguilla (L.)

The localization of cytochrome P450 cholesterol side-chain cleavage (P450scc), 3β-hydroxysteroid dehydrogenase (3β-HSD) and aromatase (P450arom) was investigated using polyclonal antibodies during gonad development in wild European eels, Anguilla anguilla (L.), from the River Po Delta (Ferrara, Italy). The first steroidogenic cells, observed in undifferentiated gonads of 14–16 cm yellow eels, showed no P450scc, 3β-HSD or P450arom activity, but positive regions appeared in head kidney insulae from this stage until the silver eel stage. In undifferentiated gonads of 16–20 cm yellow eels the steroidogenic cells were positive to all enzymes. Pre-Leydig steroidogenic cells, identified in Syrski organs of yellow eels of 22–26 cm evolving into testes, were positive to 3β-HSD and P450scc, but negative to P450arom. However, steroidogenic cells in Syrski organs evolving towards ovaries and in small but fully differentiated ovaries were positive to all enzymes. Immature testes of yellow and silver eels had Leydig cells positive to P450scc and 3β-HSD; the same reactions were also observed in some Sertoli cells of silver eel testes containing meiotic cells. Sex differentiation in A. anguilla apparently occurs through an initial female stage controlled by P450arom activity. Leydig and Sertoli cells appear involved in different steps of hormonal control of spermatogenesis: Leydig cells begin their steroidogenic activity before meiosis, while Sertoli cells begin their activity during meiosis.     

长江口日本鳗鲡降海产卵洄游群体的银化指标研究

以长江口降海生殖洄游途中的日本鳗鲡为研究材料, 对繁殖群体银化过程中体色、年龄、全长、体重及银化指标等特征作了分析。结果显示, 依据“体侧黑色素沉积是否蔓延至胸鳍下缘”这一体色标准, 可将日本鳗鲡洄游群体分为银化前期和银化期二个发育期。日本鳗鲡银化过程中, 体型、胸鳍长、眼径、性腺指数呈递增趋势, 消化道指数则呈递减趋势。主成分分析亦表明, 银化期较银化前期的眼径和胸鳍增大, 还具有较高的性腺指数和较低的消化道饱满度。由于日本鳗鲡的产卵场位于3000 km之遥的马里亚纳海沟, 这些形态的修正可能是为了适应即将起始的长距离降海生殖洄游。根据银化指标所建立的判别方程, 对雌、雄群体银化前期和银化期的判别分析均取得了95%以上的成功率。研究认为, 长江口日本鳗鲡银化期的野外快速判别, 亦可采用“眼径指数大于5”作为识别依据。     

Long-term effects of hydropower installations and associated river regulation on River Shannon eel populations: mitigation and management

The Shannon, Ireland’s largest river, is used for hydroelectricity generation since 1929. Subsequently, the Electricity Supply Board assumed responsibility for management of its eel stocks, due to the impact of the hydro-dam on recruitment to the commercial fishery. In order to negate a decline in juvenile recruitment resulting from the installation of hydroelectric facilities, management was focused on stocking lakes with elvers and fingerling eels. These were trapped at the hydropower facilities and in estuarine tributaries during their up-stream migrations. Due to the decline of natural recruitment in more recent times, attempts have also been made to develop an estuarine glass eel fishery. Stock levels are then monitored through annual surveys of the population trends of juvenile (glass eel, elver), growing phase (yellow eel) and downstream migrating pre-spawners (silver eels). Survey results and fishery management programmes are reviewed in this article. In addition to the long-term effects the hydroelectric facilities have had on the stock levels, there is also an annual effect on the migratory patterns of downstream migratory silver eels. In the lower reaches of the river system flow rates are regulated by the hydroelectric stations. We review previous work that had highlighted the importance of flow in determining the timing of the silver eels migrations, and assess the relationship between flow and migration in more detail through the use of hydroacoustic and telemetric studies. Current research on seaward migrating silver eel populations, suggests that spawner escapement rates can most effectively be increased by trapping migrating eels at fishing weirs located up-stream of the power station and transporting them towards the estuary. Guest editors: R. L. Welcomme & G. Marmulla Hydropower, Flood Control and Water Abstraction: Implications for Fish and Fisheries     

Dopaminergic systems in the European eel: characterization,brain distribution,and potential role in migration and reproduction

In fish like in mammals, dopamine (DA) is a major catecholaminergic neurotransmitter that contributes to many functions of the nervous system like sensory perception, tuning of sensori-motor cues, and hypothalamic and pituitary functions. In the eel, DA inhibits gonadal development, and juvenile silver eels remain blocked at a prepubertal stage if their reproductive migration does not occur. From data in other teleosts and vertebrates, it is suggested that DA would be involved also in the last steps of eel reproduction (oocyte maturation, ovulation, and spermiation) as well as in eel reproductive migration (locomotion and olfaction). Investigating dopaminergic systems in the eel may help in understanding the mechanisms of its complex life cycle and provide new data for its conservation and reproduction. In this article we review the biosynthesis and catabolism of catecholamines and discuss available methods to investigate brain dopaminergic systems in vertebrates and their application to the eel. Immunocytochemistry, in situ hybridization, and different tracing methods are used to map dopaminergic neurons and projections in the brain and pituitary and infer their potential functions. Moreover, variations in dopaminergic activity may be approached by means of quantitative methods like quantitative real-time RT-PCR and HPLC. These tools are currently used to study dopaminergic systems in the eel brain, their anatomy, regulation, and potential roles with special emphasis on the regulation of reproduction and reproductive migration. Guest editors: S. Dufour, E. Prévost, E. Rochard & P. Williot Fish and diadromy in Europe (ecology, management, conservation)     

A silvering index for the Japanese eel <Emphasis Type="Italic">Anguilla japonica</Emphasis>

To establish a simple and reliable index for determining silvering stages of the Japanese eel, Anguilla japonica, we observed the colorations of various body parts and biological characteristics of the eels collected in a coastal area of Japan (Mikawa Bay). The four silvering stages are characterized by the colorations of pectoral fins and ventral skin as follows: (1) Y1, yellow eel without a metallic hue at the base of pectoral fins, (2) Y2, late yellow eel with a metallic hue at the base of the pectoral fins but without melanization at the tip of pectoral fins, (3) S1, silver eel with complete melanization at the tip of pectoral fins but without full pigmented belly in black or dark brown, and (4) S2, late silver eel with black or dark brown belly. The body size, eye diameter and sexual maturity of each stage increased in the order of Y1, Y2, S1 and S2 stages, whereas the digestive tract degenerated in the same order, suggesting a sequential development of these ontogenetic stages identified in the study. The Y1, Y2 and S1 stages could be also distinguished by canonical discriminant function analysis using three internal (gonad-somatic index, GSI; hepato-somatic index, HIS; and gut index) and two morphometric (condition factor and eye index) parameters, supporting the significance of these stages. This method of staging for the silvering process of the Japanese eel appeared to be applicable to all specimens of this species, since this index used only simple external characteristics that would be easy to observe during field surveys.     

Ovarian morphology of the Japanese eel in Mikawa Bay

Annual changes in gonadal maturation of female Japanese eel Anguilla japonica in sea water were investigated histologically over 5 years in the Mikawa Bay, Japan, where they occurred throughout the year except in March. Almost all immature Japanese eels (yellow eels) occurred mainly from April to September, and they were rare after November. In contrast, maturing Japanese eels (silver eels) occurred from October to February. The gonado‐somatic index ( I _G) and oocyte diameters of yellow eels were <1·0 and 150 μm, respectively, and oocytes were at the peri‐nucleolus or the oil droplet stages. The I _G and oocyte diameters of silver eels were greater than those of yellow eels and most oocytes developed to the primary yolk globule stage. The numbers of silver eels lacking oocytes at the primary yolk globule stage increased after January in Mikawa Bay, although I _G and oocyte diameters remained unchanged. In contrast, silver eels caught at the mouth of the bay in January possessed oocytes that had advanced to the secondary yolk globule stage. These observations indicate that oocyte development changes seasonally, especially after winter in Mikawa Bay.     

Effects of high-pressure acclimatization on silver eel (Anguilla anguilla, L.) slow muscle contraction

As the spawning migration of the eel is supposed to correspond to a long swimming activity at depth, patterns of slow red muscle contraction have been investigated in European silver eel (Anguilla anguilla L.) exposed for 3 weeks to 10.1 MPa hydrostatic pressure. The results show that pressure-acclimated eels (male and female) show a three-fold decrease in maximum isometric stress of twitch and tetanic contractions while time to peak force, time from peak force to 90% relaxation and ratio of twitch tension to tetanic tension remain unchanged. The observed modifications in slow red muscle mechanical properties do not impede the spawning migration of the eel and are possibly partially compensated by an improvement in the efficiency of oxidative phosphorylation. Effects of changes in membrane fluidity are also discussed.