Changes in vision and olfaction during sexual maturation in the European eel Anguilla anguilla (L.)

Retinal structure was examined in sexually immature and artificially matured female Anguilla anguilla . Inner nuclear layer cell numbers decreased from 600 mm⁻¹ retinal cross-section to 300 mm^−I and ganglion cells from 60 to 30 mm⁻¹ cross-section, during sexual maturation. Most of the decrease occurred prior to the stage of maturation at which migration begins. Electroretinograms were recorded from the intact eyes of immature and maturing eels. There was no change in scotopic sensitivities to light of wavelengths 480 and 520 nm, with increasing sexual maturity. Olfactory organs were examined in female eels of a similar range of maturity states and were found to atrophy in artificially matured eels of advanced development. The density of mucous cells in olfactory lamellae decreased from a maximum of 443 mm⁻² in sexually immature eels to as low as 19 mm⁻² in sexually maturing eels. The changes in vision and olfaction were thought to indicate a change in the relative importance of the two sensory modalities with sexual maturation.     

Changes in body musculature with sexual maturation in the European eel, Anguilla anguilla (L.)

The structure and distribution of red and white muscles have been investigated in female Anguilla anguilla of varying sexual maturity. Red muscle volumes increased with sexual maturation from approximately 5% of total muscle volume in sexually immature eels to a maximum of 13.3% in sexually maturing animals. Volume increase was due to increasing fibre diameter rather than recruitment of new fibres. Intracellular volume fractions of lipid and mitochondria increased markedly in maturing fish. Artificially matured cels showed the largest increases in muscle lipid. The possible functional roles of red and white muscle during migration are discussed.     

Changes in the skin-scale complex with sexual maturation in the European eel, Anguilla anguilla (L.)

Structural features of the epidermis, dermis and scales were examined in adult female Anguilla anguilla of varying sexual maturity. Advanced sexual stages were obtained by hormone injection. The dermis increased in thickness with both size and sexual development. Epidermal thickness was unrelated to size or maturity but mucous cell densities decreased in more mature animals. Hormone treated eels showed a loss of mucous cells accompanied by degeneration of the epidermis. Scale areas increased from 50% in sexually immature adults to 145% in sexually maturing eels. Skin calcium values ranged from 0.27 to 0.83 mg cm⁻² and did not show any relation to either scale or sexual maturity. The possible relevance of structural changes to migration is discussed.     

Iron-rich particles in European eel (Anguilla anguilla L.)

Magnetic material in the European eel (Anguilla anguilla L.) was investigated by a combination of magnetic susceptibility measurements, energy dispersive X-ray fluorescence analysis and transmission electron microscopy. It was shown that the magnetic material is associated with iron. The main part of the iron is present in the form of iron-rich particles with irregular shapes about 100-3000 A large. The structures of magnetite (Fe3O4), hematite (alpha-Fe2O3) and alpha-iron (bcc structure) were identified. The particles are composed of more than one of these phases with magnetite being a minority phase when present. The iron-rich particles found in the eel are different from the materials reported for bacteria or bees.     

Induced oogenesis of the European eel (Anguilla anguilla L.) in freshwater condition

European eel is a catadromous fish species, which means that after living in freshwater premature individuals adapt to sea water, and migrate to the Sargasso Sea for spawning. Although male eel can be sexually matured even in freshwater, to date, it was believed that female eel can be matured only in seawater. Here we show that the process of sexual maturation may be induced in freshwater by treating female eels with carp pituitary (GSI = 9.87 ± 1.55%). It is thus proposed that seawater condition is not an obligatory environment for stimulating gametogenesis and for artificial maturation of the European eel in neither gender.     

Evidence for isolation by time in the European eel (Anguilla anguilla L.)

Life history traits of highly vagile marine species, such as adult reproductive success and larval dispersal, are strongly determined by oceanographic and climatic forces. Nevertheless, marine organisms may show restricted dispersal in time and space. Patterns of isolation by distance (IBD) have been repeatedly observed in marine species. If spawning time is a function of geographical location, temporal and spatial isolation, can easily be confounded or misinterpreted. In this study, we aimed at discriminating between various forces shaping the genetic composition of recruiting juveniles of the European eel (Anguilla anguilla L.). By controlling for geographical variation, we assessed temporal variation and tested for possible isolation by time (IBT) between recruitment waves within and between years. Using 12 polymorphic allozyme and six variable microsatellite loci, we show that genetic differentiation was low (F(ST) = 0.01-0.002) and significant between temporal samples. Regression analysis between genetic and temporal distance, was consistent with a subtle interannual pattern of IBT. Our data suggest that the population dynamics of the European eel may be governed by a double pattern of temporal variance in genetic composition: (i) a broad-scale IBT of spawning cohorts, possibly as a consequence of the large migration loop in anguillids and strong variance in annual adult reproductive contribution; and (ii) a smaller-scale variance in reproductive success (genetic patchiness) within cohorts among seasonally separated spawning groups, most likely originating from fluctuating oceanic and climatic forces. The consistency of both mechanisms remains to be verified with fine-scale analyses of both spawning/migrating aged adults and their offspring to confirm the stochastic/deterministic nature of the IBT pattern in eel.     

Structure and colour of the integument of the European eel Anguilla anguilla (L.)

The structure of the integument of 26 wild and nine artificially matured female Anguilla anguilla , and 30 sexually immature eels maintained in controlled lighting, was investigated. Melanophore densities varied in a way unrelated to maturity while xanthophores were progressively lost from lower levels in the skin. Reflecting layers made up of the purines, guanine and hypoxanthine were associated with chromatophore layers. Purine concentrations (as a percentage of total skin dry weight) decreased with maturity but a reorganization of the reflecting elements occurred. Colour changes associated with maturation were due to the redistribution of purines. Spectral and angular reflectance measurements indicate that the integument has camouflage value and colour changes can be related to the change in habitat occurring with migration. The usefulness of colour as an index of sexual maturity is discussed.     

Muscle fibre differentiation and vascularisation in the juvenile European eel (Anguilla anguilla L.)

Summary The development of the pituitary gland and its blood vessels is described in rat embryos (gestational day 12 through day 21) injected with India-ink via the umbilical vessels. The vascularization of all components of the pituitary gland develops from the surface network covering the prosencephalic vesicle. However, vascular connections exist between the prospective median eminence and the anterior pituitary gland in the earliest stages examined (day 12) but are not augmented by vessels from the stomatodeal roof until day 13. Primary portal veins appear initially on day 13, the vascularization of the pars distalis is visible on day 15. The Mantelplexus covering the floor of the diencephalon is discernible on day 16. Large-caliber portal veins appear immediately before birth, but otherwise there is no significant change in the vascular pattern during the last five days of gestation. The pars intermedia and the median eminence-pituitary stalk region remain avascular throughout embryonic life.     

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.     

Olfactory sensitivity to amino acids in the juvenile stages of the European eel Anguilla anguilla (L.)

Scanning electron micrograph observations of the olfactory mucosa from both unpigmented glass eel(GE)andpigmentedelvers(EL)of the European eel, Anguilla anguilla (L.), revealed the presence of various cell types; amongst these, the ciliated and microvillous ones are likely to possess a chcmosensory function. Recording of underwater electro-olfactograms (EOGs) showed that various amino acids (glycine, L-alanine, L-valine, L-leucine, L-asparagine, L-glutamine and L-methionine) are effective stimulants for the olfactory mucosa. Dose response curves of stimulus concentrations v. EOG amplitudesfit regression linesat both GE and EL stages. Leucine was more stimulatory at the GE than at the EL stage. The stimulatory effect of the other six amino acids tested was similar at both developmental stages. The possible role of olfactory sensitivity in animal behaviour at different developmental stages is discussed.     

Clinal genetic variation and isolation by distance in the European eel Anguilla anguilla (L.)

The genetic variability and structure of the European eel ( Anguilla anguilla L.) in populations throughout Europe was reassessed using 15 allozymic loci, seven of which were polymorphic. Seven sites were sampled on a latitudinal gradient across the natural continental range, extending from southern France to southern Norway. Heterozygosity ( H _e = 0.05) and level of polymorphism (P = 0.43) were comparable to other marine fish. Populations were poorly differentiated ( G _ST = 0.014, F _ST = 0.002), which is not surprising considering the high dispersal capability of the European eel. However, a significant geographical cline was detected at two alleles ( IDH-1 * 100 and GPI-1 * 110 ), and genetic distances ( D _CE) were concordant with geographical coastal distances. Mantel tests, pairwise F _ST's and multidimensional scaling analyses identify three distinct groups: Northern Europe, Western Europe and the Mediterranean Sea. We propose that the clinal genetic structure in the European eel may be due to (1) isolation by distance (as recently detected with microsatellites), (2) temporal reproductive separation, (3) post-larval selective forces, (4) contact between formerly separated groups or (5) some combination thereof.  © 2002 The Linnean Society of London, Biological Journal of the Linnean Society , 2002, 77 , 509–521     

Olfactory sensitivity to conspecific bile fluid and skin mucus in the European eel Anguilla anguilla (L.)

The present study assessed the olfactory potency of conspecific bile fluid and skin mucus in the European eel Anguilla anguilla by the electro-olfactogram. Immature males showed high olfactory sensitivity to conspecific bile, giving large amplitude responses in a concentration-dependent manner with estimated thresholds of detection of <1:10⁷ ( n = 6). Mucus also proved to contain highly potent odorants with thresholds of detection of c . 1:10⁶ ( n = 6). Crude solid-phase extraction of bile fluid (C-18 and C-2/ENV+ cartridges) showed that the majority of olfactory activity in bile fluid was contained in the eluate of C-18 cartridges ( n = 6). There were quantitative differences, however, between the sexes; female bile fluid had a higher proportion of activity in this fraction. Similar solid-phase extraction of mucus showed that it contains a higher proportion of odorants in the C-18 filtrate than bile fluid. Mucus from mature eels, however, had a higher proportion of olfactory activity in the eluate than immature fish ( n = 6). Cross-adaptation experiments suggest that there are qualitative differences in the odorants contained in bile and mucus depending on both the sex and state of sexual maturation of the donor ( n = 6). These results are consistent with a role for chemical communication in the reproduction of the European eel and suggest that both bile and mucus are potential sources of the odorants involved.     

Effects of various hormones on the sexual maturity of European eel (Anguilla anguilla L.) females from farm and lakes

Previously described and alternative methods of the induction of sexual maturation in the European eel were investigated. Weekly administrations of a gonadoliberin agonist (nnRH-A=D-Phe6-GnRH-Ea) did not induce statistically significant effect on the gonads of treated eels in none of the dosages used (0.1 microg and 10 microg/fish). Carp pituitary extract and carp pituitary extract together with a dopamine antagonist caused considerable external changes (increase in eye size) and significant gonadal development in two treatment groups: wild and cultivated stocks. The induction of the ovulation by double amount of CP and gonadoliberin agonist with dopamine antagonist mixture was not successful in a wild stock. Fertilisation of stripped eggs of farm eel was attempted unsuccessfully in, due to low egg quality. An advanced phase of the sexual maturation process could be induced in specimen infected by Anguillicola crassus indicating, that nematode infection is not a limiting factor in the artificial propagation of the European eel.     

Swimming performance of European eel (Anguilla anguilla (L.)) elvers

To determine the relation between swimming endurance time and burst swimming speed, elvers of the European eel, Anguilla anguilla (L.), were made to swim at speeds from 3.6 to 7.2 L (body lengths) s⁻¹ in both fresh and sea water. Swimming endurance time of elvers averaging 7.2 cm total length decreased logarithmically with increased swimming speed from 3.0 min at 3.5 L s⁻¹ to 0.7 min at 5.0 L s⁻¹, and again logarithmically but with a lesser slope to 0.27 min at 7.5 L s⁻¹. No differences were found between fresh and sea water elvers. In still water, elvers could swim at high speeds for about 10–45m before exhaustion, depending upon speed. Elvers would be able to make virtually no progress against water currents >50 cm s⁻¹. Drift in coastal water currents and selective tidal transport probably involve swimming speeds below those tested in this study. Migration into freshwater streams undoubtedly involves avoidance of free stream speeds and a combination of burst and sustained swimming.     

Response of prolactin cells to environmental calcium in the eel (Anguilla anguilla L.)

Prolactin (PRL) cell activity was investigated in eels kept in fresh water (FW), deionized water (DW) supplemented or not with Ca (2 mM), in Ca-enriched FW (10 mM), in normal (Ca 3.4 mM) or Ca-free 1/3 sea water (SW), and in SW (Ca 10.2 mM) or Ca-free SW (Ca 0.15 mM). Light-microscopic studies, including measurement of the nuclear area and cell height, showed that PRL cell activity, reduced in DW, is not affected by Ca supplementation. Activity is reduced in Ca-enriched FW, in 1/3 SW and in SW, conditions inducing an increase in the plasma sodium level. The lack of calcium in saline environments partly suppresses the nuclear atrophy occurring in SW. There is no significant correlation between external or total plasma calcium concentration and PRL cell activity. In artificial Ca-free SW, eels show a rapid increase in plasma osmolarity and sodium levels; there is a significant negative correlation between these two plasma values and the nuclear area or cell height of PRL cells. As in some other teleosts, plasma osmolarity and plasma sodium seem to play a more important role than external or internal calcium in controlling PRL secretion. This correlation is not apparent in eels kept in SW, having unstimulated PRL cells but active calcium-sensitive (Ca-s) cells in the pars intermedia.     

In vivo tracking of maturation in male European eel, Anguilla anguilla (L.), by computed tomography

The present study aimed in vivo tracking of maturation of male eel by computed tomography (CT). Additionally, individually monitored testes sizes were correlated with the conventionally used external maturity indicators (i.e. eye and nose indexes) in order to test and improve their usefulness at individual level. Testes could be clearly identified with the CT from the end of the third week of hCG administration routinely used to induce maturation in fish. The volume of testes increased exponentially during hormone treatment, and by the end of the sixth week of maturation procedure all males produced motilable spermatozoa. Present results prove that testes size can noninvasively be monitored with CT from maturity level where testes size rich 3000 mm3 volume. Eye and nose indexes are in close correlation with testes volume and thus can also be effectively used to monitor maturity level of male eel, but preferably only at stock level. However, due to their high individual variability, these indexes can be applied only with caution at individual level and should be supplemented with other noninvasive techniques such as CT.     

Structural characteristics of the epidermal mucosa in yellow and silver European eel, Anguilla anguilla (L.)

Histological observations were made on the distribution and concentration of the superficial mucous cells and on the thickness of the epidermis in immature yellow and in male and female silver eels. In all groups there was a decrease in the concentration of mucous cells from the anterior regions to the posterior regions, while the epidermis was thickest in the median parts of the body. Besides these common characteristics, a sexual dimorphism was shown: in silver eels the epidermis is thicker than in immature yellow eels; female silver eels have both the thickest epidermis and the greatest concentration of superficial mucous cells.
Results are discussed in the light of knowledge on the influence of environmental and endocrine factors on the structure and function of epidermal mucosa in teleosts.