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.     

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.     

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.     

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.     

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.     

A morphometric analysis of regional differences in myotomal muscle ultrastructure in the juvenile eel (Anguilla anguilla L.)

Summary Subpopulations of fast and slow fibres within the trunk musculature of elvers were examined using morphometric analysis of electron micrographs. Fibre regions were characterised by their histochemical staining characteristics, and individual fibres located using a coordinate mapping system utilising morphological features as reference points. Percentages of fibre volume occupied by mitochondria, myofibrils, sarcoplasmic reticulum (S.R.), and T-system were determined in each of the fibre groups, along a transect from the skin to the vertebral column (fibres 1–14, respectively).The fine structure of slow (red) fibres (1–2 fibres deep) is relatively homogeneous throughout its range, giving mean values for mitochondria, 21.4%; myofibrils, 61.0%; S.R., 2.10%; T-system, 0.31%. The fibres are relatively small (204 m²) and the mitochondrial cristae poorly developed.In contrast, there is a marked heterogeneity in the ultrastructure of fast (white) fibres, dependent on both position and size. The moderately small (333 m²) superficial fast fibres (3–4 fibres deep) have a significantly higher mitochondrial content (7.6%) than the larger deep fibres (1.2%) (6–12 fibres deep, 775 m²). The mean fractional volumes occupied by myofibrils, S.R., and T-system in the deep fibres are: 80.4%, 5.95%, and 0.38%, respectively. Fibres < 100 m² constitute up to 5% of the fast muscle and have a significantly higher mitochondrial volume (4.3%), more glycogen granules, and a slightly lower volume of S.R. (5.57%) than larger fibres.It is suggested that metabolic subpopulations of fast fibres correspond to different stages of fibre growth. The relatively poorly developed S.R. of eel fast muscle is thought to be correlated with the low frequency, high amplitude nature of the propagated waveform found in anguilliform locomotion.     

Relation of visual changes to the onset of sexual maturation in the European eel Anguilla anguilla (L.)

Eye size was correlated with body length and gonad development in 112 untreated and 33 hormone injected female Anguilla anguilla . A length-related measure of eye size was developed as an index of sexual maturity. Eels of eye index of 6.5 or less were classed as sexually immature adults , those of < 6.5 as sexually Maturing adults . Structural changes in the eye during maturation were examined. Total numbers of rods increased markedly while density of photoreceptors remained approximately constant. Cone density decreased with degenerative changes occurring. Changes in the eye were found to be essentially complete early in the maturation process.     

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.     

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     

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.     

Meeting energy budgets by modulation of behaviour and physiology in the eel (Anguilla anguilla L.)

Availability of energy for feeding, and the scope to accommodate the associated increase in oxygen demand (SDA: specific dynamic action) can, to a large degree, regulate the future feeding and energy availability of an animal. There is a fundamental conflict between locomotion and SDA within the physiological capacity of a mobile organism to respire sufficiently in order to simultaneously meet both requirements. This paper is a first attempt to integrate the costs of behaviour and physiology and produce a testable model of energy allocation in the eel. Total oxygen consumption (metabolic rate MO2) of the eel (Anguilla anguilla L.) was 109 micromol O2 x g(-1) x day(-1) with a cost of measured protein synthesis representing 49% of this value, and measured routine swimming (locomotor) activity representing approximately 34%. By allocating periods of reduced activity, the eel is able to develop a strategy to prudently meet the costs of feeding and temporally balance energy budgets (in terms of oxygen) by modulation of the behaviour and demands of physiology.     

Influence of stocking density of European eel (Anguilla anguilla, L.) elvers on sex differentiation and zootechnical performances

Survival rate, sex ratio and zootechnical performances were evaluated on 168 kg of Anguilla anguilla elvers (0.45 g) weaned into three groups (A1, A2, A3) at initial densities of 800, 1600 and 3200 g m⁻³, respectively. In order not to modify the sex ratio, animals weaned at the different densities were maintained separately during the trials, and no size grading was performed at the fattening phase.
Final mean weight achieved by females belonging to the different groups showed no statistically significant differences; weight of males was higher (P < 0.05) in A1 than in A3 where a higher percentage of males was observed. Sex ratio of eels was different among the groups, with a higher percentage of males in A3 (96%) than in A2 (78%) or in A1 (69%). This finding testifies to a sex differentiation strongly affected by pre-fattening stocking density of elvers. Final load showed an increase in males corresponding to a significantly reduced biomass between A1 and A2 (10.97 and 10.24 kg m⁻³) and A3 (8.44 kg m⁻³). Final survival rate ranged from 87% (A1) to 90% (A3). As to food conversion, a better rate was found in A1 (1.9:1) compared to A2 (2.1:1) and A3 (2.3:1) eels.