Morphological and gonadal histological characteristics of the silver-phase male Indo-Pacific eel Anguilla marmorata.

Eight silver-phase male Indo-Pacific eels (giant mottled eels) Anguilla marmorata were collected from three small rivers on subtropical Amami-Oshima Island, Japan, and their morphological and gonadal-histological characteristics were examined. The total length, body mass and age of the silver eels were 543.8 ± 49.3 (496-659) mm, 376.0 ± 116.0 (282-660) g and 14.6 ± 2.3 (11-19) years old, respectively. The silvering-related characteristics (eye index, pectoral-fin index, gut-somatic index) and reproductive characteristics (gonadosomatic index, histological stage) of the silver eels were significantly advanced compared to those of yellow eels, which is similar to other anguillid species.

Developmental features of Japanese eels, Anguilla japonica, from the late leptocephalus to the yellow eel stages: an early metamorphosis to the eel-like form and a prolonged transition to the juvenile.

Organogenesis of Japanese eels (Anguilla japonica) was investigated histologically from the late leptocephalus to the yellow eel stages. Early organogenesis, such as the formation of inner ears and the appearance of round blood cells that might be larval erythrocytes, had already begun at the late leptocephalus stage. During the first developmental phase (M1-M3 stages) of metamorphosing into early glass eels (G1 stage), the formation of gills and lateral muscles progressed conspicuously with a drastic body shape change from leaf-like to eel-like. In contrast, obvious regression in oesophageal muscle and pancreas occurred during metamorphosis. Formation of lateral line canals advanced continuously until the yellow eel stage. When the second developmental phase was initiated at the G1 stage, cone photoreceptor cells appeared, and the formation of oesophageal, stomach and intestinal muscles was initiated. Differentiation of gastric glands began at 1 week after metamorphosis. Erythrocytes increased continuously in density in glass eels and elvers (G1-E2 stages), and the morphological features of cone cells and olfactory epidermal cells became clearer with stage progression. In early elvers (E1 stage), the swimbladder initiated inflation, the stomach fully expanded and the rectal longitudinal fold changed to a circle. Swimbladder gas glands appeared in late elvers (E2 stage). In the yellow eels (juvenile stage), almost all organ structures were formed. These observations indicate that the organogenesis of A. japonica is ongoing after metamorphosis into glass eels, and the M1-E2 stages are considered to be a homologous phase to first metamorphosis, which is a transformation from the larval to the juvenile stages in other teleosts. In comparison to conger eels, the completion of the body shape change to eel-like occurs at the G1 stage, when organogenesis is still in progress, being followed by a prolonged duration of the G1-E2 stages before reaching the yellow eel juvenile stage, which may be a unique characteristic that is related to the early migratory life history of A. japonica.

From yellow to silver: Transforming cranial morphology in European eel (Anguilla anguilla).

The European eel (Anguilla anguilla) has been extensively studied, especially because of its highly specialized migratory behaviour associated with substantial phenotypic transformations. During this migration, one of those transformations the eel undergoes is from yellow to silver eel, a process known as silvering. Although the cranial morphology during the earlier glass, elver and yellow eel stages are well studied, little is known about actual morphological changes during the transformation process from the yellow to the silver eel stage. Yet, literature suggests drastic changes in musculoskeletal anatomy. Here, we investigated the cranial musculoskeletal morphology of 11 male European eels at different stages during silvering, resulting both from natural and artificial maturation. Using 3D-reconstructed µCT data of the head, the skull and cranial muscles associated with jaw closing and respiration were studied. Eye size was used as a proxy for the silvering stage. Size-adjusted jaw muscle volumes increased during silvering, although insignificantly. Accordingly, a near-significant increase in bite force was observed. Respiratory muscles size did increase significantly during silvering, however. Considering the eel's long migration, which often includes deep and thus potentially oxygen-poor environments, having a better performing respiratory system may facilitate efficient migration. Both overall skull dimensions and specifically orbit size increased with eye index, suggesting they play a role in accommodating the enlarging eyes during silvering. Finally, artificially matured eels had a wider and taller skull, as well as larger jaw muscles than wild silver eels. This could be caused (a) by different conditions experienced during the yellow eel stage, which are maintained in the silver eel stage, (b) by side effects of hormonal injections or (c) be part of the maturation process as artificially induced silver eels had a higher eye index than the wild silver eels.

Morphological, ecological and physiological characteristics of downstream-migrating and non-migrating Pacific bicolor eels Anguilla bicolor pacifica.

This is the first study describing the morphological, ecological and physiological characteristics of two downstream-migrating and two non-migrating female Pacific bicolor eels, Anguilla bicolor pacifica. The total length and age of the downstream-migrating eels were 1005 mm and 10 years and 1110 mm and 11 years old, respectively, and those of the non-migrating eels were 892 mm and 8 years and 805 mm and 9 years, respectively. Silvering-related characteristics (silvering index, eye index, pectoral-fin index, gut-somatic index and swimbladder-somatic index) and reproductive physiological characteristics (gonado-somatic index, follicle diameter, oocyte stage, transcription of gonadotropins and concentration of sex steroids) of the migrating eels were more advanced than those of the non-migrating eels.

Reproductive physiological characteristics of tropical Celebes eels Anguilla celebesensis in relation to downstream migration and ovarian development.

Downstream-migrating (n = 64) and non-migrating (n = 21) female Celebes eels Anguilla celebesensis were captured from the Poso Lake-River system on Sulawesi Island, Indonesia, and their reproductive physiological characteristics were examined. A histological observation of the ovaries revealed that most non-migrating eels were at the perinucleolus (43%) or oil-droplet (48%) stage, whereas most migrating eels were at the early vitellogenic (36%) or midvitellogenic (61%) stage. Transcript levels of gonadotropin genes (fshb, lhb) in the pituitary gland and concentrations of sex steroids [11-ketotestosterone (11-KT), testosterone, 17ß-oestradiol (E2 )] in blood plasma of migrating eels were significantly higher than those of non-migrating eels. The fshb messenger (m)RNA levels were lower in perinucleolus and oil-droplet stages and then significantly increased in the early vitellogenic stage. The lhb mRNA levels in vitellogenic-stage eels were significantly higher than those in perinucleolus- and oil-droplet-stage eels. The 11-KT levels of eels at the oil-droplet and vitellogenic stages were significantly higher than those of eels at the perinucleolus stage. The E2 levels at the vitellogenic stage were significantly higher than those at the perinucleolus and oil-droplet stages. These dynamics of the reproductive hormones represented the physiological background of oogenesis in A. celebesensis that has remarkably well-developed oocytes just before downstream migration.

A digital dissection of two teleost fishes: comparative functional anatomy of the cranial musculoskeletal system in pike (Esox lucius) and eel (Anguilla anguilla).

Advances in X-ray computed tomography (CT) have led to a rise in the use of non-destructive imaging methods in comparative anatomy. Among these is contrast-enhanced CT scanning, which employs chemical stains to visualize soft tissues. Specimens may then be 'digitally dissected', producing detailed, three-dimensional digital reconstructions of the soft- and hard-tissue anatomy, allowing examination of anatomical structures in situ and making accurate measurements (lengths, volumes, etc.). Here, we apply this technique to two species of teleost fish, providing one of the first comprehensive three-dimensional (3D) descriptions of teleost cranial soft tissue and quantifying differences in muscle anatomy that may be related to differences in feeding ecology. Two species with different feeding ecologies were stained, scanned and imaged to create digital 3D musculoskeletal reconstructions: Esox lucius (Northern Pike), predominantly a suction feeder; and Anguilla anguilla (European eel), which captures prey predominantly by biting. Muscle cross-sectional areas were calculated and compared between taxa, focusing on muscles that serve important roles in feeding. The adductor mandibulae complex - used in biting - was larger in Esox than Anguilla relative to head size. However, the overall architecture of the adductor mandibulae was also very different between the two species, with that of Anguilla better optimized for delivering forceful bites. Levator arcus palatini and sternohyoideus - which are used in suction feeding - are larger in Esox, whereas the levator operculi is larger in Anguilla. Therefore, differences in the size of functionally important muscles do not necessarily correlate neatly with presumed differences in feeding mode.

A mechanistic model for studying the initiation of anguillid vitellogenesis by comparing the European eel (Anguilla anguilla) and the shortfinned eel (A. australis).

An inverse relation exists between the maturation stage at the start of the oceanic reproductive migration and the migration distance to the spawning grounds for the various eel species. The European eel Anguilla anguilla migrates up to 5-6000 km and leaves in a previtellogenic state. The shortfinned eel A. australis migrates 2-4000 km and leaves in an early vitellogenic state. In this study, we compared the early pubertal events in European silver eels with those in silver shortfinned eels to gain insights into the initiation of vitellogenesis. Immediately after being caught, yellow and silver eels of both species were measured and sampled for blood and tissues. Eye index (EI), gonadosomatic index (GSI) and hepatosomatic index (HSI) were calculated. Plasma 11-ketotestosterone (11-KT) and 17ß-estradiol (E2) levels were measured by radioimmunoassay. Pituitary, liver and ovaries were dissected for quantitative real-time PCR analyses (pituitary dopamine 2b receptor d2br, gonadotropin-releasing hormone receptors 1 and 2 gnrhr1 and gnrhr2, growth hormone gh and follicle-stimulating hormone-ß fshb; liver estrogen receptor 1 esr1; gonad follicle-stimulating hormone receptor fshr, androgen receptors α and ß ara and arb, vitellogenin receptor vtgr and P450 aromatase cyp19). Silver eels of both species showed a drop in pituitary gh expression, progressing gonadal development (GSI of ∼1.5 in European eels and ∼3.0 in shortfinned eels) and steroid level increases. In shortfinned eels, but not European eels, expression of fshb, gnrhr1 and gnrhr2, and d2br in the pituitary was up-regulated in the silver-stage as compared to yellow-stage females, as was expression of fshr, ara and arb in the ovaries. Expression of esr1 in European eels remained low while esr1 expression was up-regulated over 100-fold in silver shortfinned eels. The mechanistic model for anguillid vitellogenesis that we present suggests a first step that involves a drop in Gh and a second step that involves Fsh increase when switching in the life history trade-off from growth to reproduction. The drop in Gh is associated with gonadal development and plasma steroid increase but precedes brain-pituitary-gonad axis (BPG) activation. The Fsh increase marks BPG activation and increased sensitivity of the liver to estrogenic stimulation, but also an increase in D2br-mediated dopaminergic signaling to the pituitary.

Development, characterization and virus susceptibility of a continuous cell line from the caudal fin of marbled eel (Anguilla marmorata).

A continuous cell line consisting mostly of epithelioid cells was established from the caudal fin of marbled eels (Anguilla marmorata) and designated as marbled eel caudal fin (MECF)-1. The cells multiplied well in Leibovitz's L-15 medium containing 2% to 15% foetal bovine serum at temperatures of 20°C to 35°C and were subcultured for >90 passages during a 5-year period from 2012 to 2017. Transcripts of ictacalcin, keratin 13, cd146, nestin, ncam1 and myod1 were demonstrated in the cells using reverse transcription polymerase chain reaction. The results indicated that MECF-1 was composed of epidermal and mesenchyme stem and progenitor cells including myoblasts. MECF-1 was susceptible to Japanese eel herpesvirus HVA980811, marbled eel polyoma-like virus (MEPyV), aquabirnavirus MEIPNV1310 and aquareovirus CSV. By contrast, MECF-1 was noted refractory to megalocytiviruses RSIV-Ku and GSIV-K1 infection. Moreover, the cells were resistant to betanodavirus infection. In conclusion, MECF-1 derived from marbled eel is suitable for studies on anguillid viruses and interaction with host cells.

Reconsidering pectoral girdle and fin morphology in Anguillidae (Elopomorpha: Anguilliformes).

The morphology of the pectoral girdle and fin of Anguillidae is reconsidered via the inclusion of skeletal components that have previously been unassessed. For example, the pectoral girdle and fin in Anguilla were erroneously reported to lack a scapular bone, a cartilaginous scapulocoracoid plate and a cartilaginous propterygium. The pectoral morphology of Anguilla is also compared with the closely related genus Nemichthys, including additional data on the anatomy of this eel family.

Broader head, stronger bite: In vivo bite forces in European eel Anguilla anguilla.

This work examined three different phenotypes of the yellow-eel stage of the European eel Anguilla anguilla, broad-heads, narrow-heads and eels with an intermediate head shape. The aim was to see whether broad-headed A. anguilla, which generally consume harder, larger prey, such as crustaceans and fish, exerted greater bite force than the narrow-headed variant, which mainly consume soft, small prey such as chironomid larvae. It was found that in 99 yellow A. anguilla, in vivo bite force of broad-heads are higher compared with narrow-heads and intermediates.

Age and growth of migrating tropical eels, Anguilla celebesensis and Anguilla marmorata.

The age and growth of migrating tropical eels, Anguilla celebesensis and Anguilla marmorata from central Sulawesi, Indonesia, were examined. Migrating eels (63 A. celebesensis and 38 A. marmorata) were obtained from weirs near the Poso Lake outlet and non-migrating eels (35 A. celebesensis and 119 A. marmorata) were captured by baited hooks, eel pots, scoop net and electro-fishing in the Poso River system, Laa River system, Baluga River, Tongku River and Padapu River from February 2009 to October 2010. In both species, the proportion of eels with opaque otolith edges showed a single peak in July, suggesting that one annulus (a pair of translucent and opaque zones) was formed each year in their otoliths. Mean ± s.d. and range of total length (LT ) and age was 785·2 ± 114·9 (585-1083) mm and 7·5 ± 1·6 (5-11) years in migrating female A. celebesensis and 1132·2 ± 173·7 (800-1630) mm and 11·6 ± 3·3 (7-23) years in A. marmorata. The age of migrating female eels was negatively correlated with annual growth rate, 100·7 ± 17·2 (68·1-145·0) mm year-1 in A. celebesensis and 97·9 ± 19·3 (66·6-131·6) mm year-1 in A. marmorata, but there was no significant correlation between the LT and annual growth rate in either species. The annual growth rates of these female tropical eels were typically higher than those of temperate anguillid species, suggesting a latitudinal cline in growth rate in the genus Anguilla reflecting the environmental conditions of their growth habitat.

Differential gene expression in narrow- and broad-headed European glass eels (Anguilla anguilla) points to a transcriptomic link of head shape dimorphism with growth rate and chemotaxis.

One of the major challenges in evolutionary biology is to understand the mechanisms underlying morphological dimorphism and plasticity, including the genomic basis of traits and links to ecology. At the yellow eel stage of the European eel (Anguilla anguilla), two morphotypes are found: broad- and narrow-heads. This dimorphism has been linked to dietary differences, with broad-heads feeding on harder, larger prey than narrow-heads. However, recent research showed that both morphotypes could be distinguished at the glass eel stage, the nonfeeding predecessor of the yellow eel stage, implying that nondietary factors play a role in the development of this head shape dimorphism. Here, we used transcriptome profiling (RNAseq) to identify differentially expressed genes between broad- and narrow-headed glass eels. We found 260 significantly differentially expressed genes between the morphotypes, of which most were related to defence and immune responses. Interestingly, two genes involved in growth (soma and igf2) were significantly upregulated in narrow-heads, while nine genes involved in chemotaxis showed significant differential expression. Thus, we found support for the observation that head shape is associated with somatic growth, with fast-growing eels developing a narrower head. Additionally, observations in the wild have shown that slow-growers prefer freshwater, while fast-growers prefer brackish water. The differential expression of genes involved in chemotaxis seems to indicate that glass eel growth rate and habitat choice are linked. We hypothesize that two levels of segregation could take place in the European eel: first according to habitat choice and second according to feeding preference.

Comparative evaluation of dietary probiotics Bacillus subtilis WB60 and Lactobacillus plantarum KCTC3928 on the growth performance, immunological parameters, gut morphology and disease resistance in Japanese eel, Anguilla japonica.

The current experiment was conducted to evaluate and compare the efficacy of two different probiotics Bacillus subtilis WB60 and Lactobacillus plantarum KCTC3928 in diet of Japanese eel, Anguilla japonica. Seven experimental diets were formulated to contain no probiotics (CON), three graded levels of B. subtilis at 106 (BS1), 107 (BS2), 108 (BS3) and L. plantarum at 106 (LP1), 107 (LP2), 108 (LP3) CFU/g diet. Twenty fish averaging 8.29 ± 0.06 g were distributed in to 21 aquaria and were randomly assigned to one of the experimental diets in triplicate groups. Average weight gain (WG), feed efficiency (FE), and protein efficiency ratio (PER) of fish fed B. subtilis at 107 (BS2) and 108 (BS3) CFU/g diet were significantly higher than those of fish fed other experimental diets (P < 0.05). Nonspecific enzymatic activities including lysozyme, superoxide dismutase (SOD), myeloperoxidase (MPO) from fish fed B. subtilis at 107 (BS2) and 108 (BS3) CFU/g diet were detected to be significantly higher than that from fish fed CON diet (P < 0.05). Whereas, level of intestine glyceraldehyde-3-phosphate dehydrogenase (GAPDH), heat shock protein 70, 90 (HSP70, 90) and immunoglobulin (IgM) from fish fed B. subtilis at 107 and 108 CFU/g diet were recorded to be significantly higher than those of fish fed other experimental diets (P < 0.05). Parameters for intestinal morphology and histology suggested a healthier gut for the fish fed108 (BS3) CFU/g diet as compared to other treatment groups. Whereas, results from the disease challenge test with bacteria Vibrio angulillarum showed significantly lower survival rate for fish fed CON diet than those of fish fed other experimental diets. Therefore, these results indicated that oral supplement of B. subtilis at 108 (BS3) CFU/g diet could be a more effective source of probiotic compared to L. plantarum in Japanese eel.

Fusiform vateritic inclusions observed in European eel (Anguilla anguilla L.) sagittae.

Microscopic inclusions have been observed in 7 out of 106 European eel (Anguilla anguilla L.) sagittae using polarizing microscope and scanning electron microscope meanwhile the annual increments were studied to characterize the age structure of the population living in Lake Balaton. The presence of vaterite, a rare calcium carbonate polymorph was observed in these inclusions using Raman spectroscopy. Vateritic sagittae in wild fish are usually considered as symptom of physiological stress. The observed fusiform inclusions represent a new morphological type of vaterite inclusions in eel otolith. Two alternatives are hypothesized to explain their formation: 1) metabolic disorder, such as erroneous protein synthesis; 2) introduction of an alien protein into the eel's inner ear. The origin and physiological significance of this new morphological type of vateritic inclusions is still an open question. Same as whether it can be found in other species or specific only to eel otoliths.

How the European eel (Anguilla anguilla) loses its skeletal framework across lifetime.

European eels (Anguilla anguilla) undertake an impressive 5 000 km long migration from European fresh waters through the North Atlantic Ocean to the Sargasso Sea. Along with sexual maturation, the eel skeleton undergoes a remarkable morphological transformation during migration, where a hitherto completely obscure bone loss phenomenon occurs. To unravel mechanisms of the maturation-related decay of the skeleton, we performed a multiscale assessment of eels' bones at different life-cycle stages. Accordingly, the skeleton reflects extensive bone loss that is mediated via multinucleated bone-resorbing osteoclasts, while other resorption mechanisms such as osteocytic osteolysis or matrix demineralization were not observed. Preserving mechanical stability and releasing minerals for energy metabolism are two mutually exclusive functions of the skeleton that are orchestrated in eels through the presence of two spatially segregated hard tissues: cellular bone and acellular notochord. The cellular bone serves as a source of mineral release following osteoclastic resorption, whereas the mineralized notochord sheath, which is inaccessible for resorption processes due to an unmineralized cover layer, ensures sufficient mechanical stability as a part of the notochord sheath. Clearly, an eel's skeleton is structurally optimized to meet the metabolic challenge of fasting and simultaneous sexual development during an exhausting journey to spawning areas, while the function of the vertebral column is maintained to achieve this goal.

Diet-induced phenotypic plasticity in European eel (Anguilla anguilla).

Two phenotypes are present within the European eel population: broad-heads and narrow-heads. The expression of these phenotypes has been linked to several factors, such as diet and differential growth. The exact factors causing this dimorphism, however, are still unknown. In this study, we performed a feeding experiment on glass eels from the moment they start to feed. Eels were either fed a hard diet, which required biting and spinning behavior, or a soft diet, which required suction feeding. We found that the hard feeders develop a broader head and a larger adductor mandibulae region than eels that were fed a soft diet, implying that the hard feeders are capable of larger bite forces. Next to this, soft feeders develop a sharper and narrower head, which could reduce hydrodynamic drag, allowing more rapid strikes towards their prey. Both phenotypes were found in a control group, which were given a combination of both diets. These phenotypes were, however, not as extreme as the hard or the soft feeding group, indicating that some specimens are more likely to consume hard prey and others soft prey, but that they do not selectively eat one of both diets. In conclusion, we found that diet is a major factor influencing head shape in European eel and this ability to specialize in feeding on hard or soft prey could decrease intra-specific competition in European eel populations.

Population demographics of American eels Anguilla rostrata in two Arkansas, U.S.A., catchments that drain into the Gulf of Mexico.

The goal of this study was to compare American eel Anguilla rostrata life history in two inland river systems in Arkansas, U.S.A., that ultimately discharge into the Gulf of Mexico via the Mississippi River and the Red-Atchafalaya catchments. From 21 June 2011 to 24 April 2014, 238 yellow-phase A. rostrata were captured in the middle Ouachita River and tributaries using boat electrofishing and 39 in the lower White River using multiple sampling gears. Most of them were caught downstream of dams in both basins (61%). Medium-sized A. rostrata ranging from 225 to 350 mm total length (LT ) were the most abundant size group in the Ouachita River basin, but they were absent from the White River. Mean LT at age 4 years (i.e. youngest shared age) was 150 mm greater for the White River than the Ouachita River basin. Anguilla rostrata appeared to have a greater initial LT (i.e. minimum size upon arrival) in the White River that allowed them to reach a gonado-somatic index (IG ) of 1·5 up to 4 years earlier, and downstream migration appeared to occur 5 years earlier at 100 mm greater LT ; these differences may be related to increased river fragmentation by dams in the Ouachita River basin. Growth and maturation of A. rostrata in this study were more similar to southern populations along the Atlantic coast than other inland populations. Adult swimbladder nematodes Anguillicoloides crassus were not present in any of the 214 swimbladders inspected. Gulf of Mexico catchments may be valuable production areas for A. rostrata and data from these systems should be considered as range-wide protection and management plans are being developed.

Musculoskeletal anatomy and feeding performance of pre-feeding engyodontic larvae of the European eel (Anguilla anguilla).

Being part of the elopomorph group of fishes, Anguillidae species show a leptocephalus larval stage. However, due to largely unknown spawning locations and habitats of their earliest life stages, as well as their transparency, these Anguilla larvae are rarely encountered in nature. Therefore, information regarding the early life history of these larvae, including their exogenous feeding strategy and feeding performance, is rather scarce. To better understand the structural basis and functional performance of larval feeding in captivity, the functional morphology of the cranial musculoskeletal system in pre- and first-feeding engyodontic leptocephali of the European eel (Anguilla anguilla) was studied. A 3D reconstruction of the feeding apparatus (head of the leptocephali < 1 mm) was used to visualize and describe the musculoskeletal changes throughout these stages. To analyze the ontogenetic changes in the functionality of the feeding apparatus towards the active feeding phase, 3D data of joints, levers and muscles derived from the reconstructions were used to estimate bite and joint reaction forces (JRFs). Observing a maximum estimated bite force of about 65 µN (and corresponding JRFs of 260 µN), it can be hypothesized that leptocephalus larvae are functionally constrained to feed only on soft food particles. Additionally, potential prey items are size delimited, based on the theoretically estimated average gape of these larvae of about 100 µm. This hypothesis appears to be in line with recent observations of a diet consisting of small and/or gelatinous prey items (Hydrozoa, Thaliacea, Ctenophora, Polycystenia) found in the guts of euryodontic leptocephalus larvae.

Changes in the gene expression profiles of the brains of male European eels (Anguilla anguilla) during sexual maturation.

BACKGROUND: The vertebrate brain plays a critical role in the regulation of sexual maturation and reproduction by integrating environmental information with developmental and endocrine status. The European eel Anguilla anguilla is an important species in which to better understand the neuroendocrine factors that control reproduction because it is an endangered species, has a complex life cycle that includes two extreme long distance migrations with both freshwater and seawater stages and because it occupies a key position within the teleost phylogeny. At present, mature eels have never been caught in the wild and little is known about most aspects of reproduction in A. anguilla. The goal of this study was to identify genes that may be involved in sexual maturation in experimentally matured eels. For this, we used microarrays to compare the gene expression profiles of sexually mature to immature males. RESULTS: Using a false discovery rate of 0.05, a total of 1,497 differentially expressed genes were identified. Of this set, 991 were expressed at higher levels in brains (forebrain and midbrain) of mature males while 506 were expressed at lower levels relative to brains of immature males. The set of up-regulated genes includes genes involved in neuroendocrine processes, cell-cell signaling, neurogenesis and development. Interestingly, while genes involved in immune system function were down-regulated in the brains of mature males, changes in the expression levels of several receptors and channels were observed suggesting that some rewiring is occurring in the brain at sexual maturity. CONCLUSIONS: This study shows that the brains of eels undergo major changes at the molecular level at sexual maturity that may include re-organization at the cellular level. Here, we have defined a set of genes that help to understand the molecular mechanisms controlling reproduction in eels. Some of these genes have previously described functions while many others have roles that have yet to be characterized in a reproductive context. Since most of the genes examined here have orthologs in other vertebrates, the results of this study will contribute to the body of knowledge concerning reproduction in vertebrates as well as to an improved understanding of eel biology.

Enteric neuroplasticity in seawater-adapted European eel (Anguilla anguilla).

European eels live most of their lives in freshwater until spawning migration to the Sargasso Sea. During seawater adaptation, eels modify their physiology, and their digestive system adapts to the new environment, drinking salt water to compensate for the continuous water loss. In that period, eels stop feeding until spawning. Thus, the eel represents a unique model to understand the adaptive changes of the enteric nervous system (ENS) to modified salinity and starvation. To this purpose, we assessed and compared the enteric neuronal density in the cranial portion of the intestine of freshwater eels (control), lagoon eels captured in brackish water before their migration to the Sargasso Sea (T0), and starved seawater eels hormonally induced to sexual maturity (T18; 18 weeks of starvation and treatment with standardized carp pituitary extract). Furthermore, we analyzed the modification of intestinal neuronal density of hormonally untreated eels during prolonged starvation (10 weeks) in seawater and freshwater. The density of myenteric (MP) and submucosal plexus (SMP) HuC/D-immunoreactive (Hu-IR) neurons was assessed in wholemount preparations and cryosections. The number of MP and SMP HuC/D-IR neurons progressively increased from the freshwater to the salty water habitat (control > T0 > T18; P < 0.05). Compared with freshwater eels, the number of MP and SMP HuC/D-IR neurons significantly increased (P < 0.05) in the intestine of starved untreated salt water eels. In conclusion, high salinity evokes enteric neuroplasticity as indicated by the increasing number of HuC/D-IR MP and SMP neurons, a mechanism likely contributing to maintaining the body homeostasis of this fish in extreme conditions.