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Lokman, P. Mark; Vermeulen, G. J.; Lambert, J.G.D.; Young, Graham

doi:10.1023/a:1007719414295

Cited by 120 publications

(17 citation statements)

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“…We confirmed the increase in plasma levels of various sexual steroids between yellow and silver stages ( A. australis and A. dieffenbachii [25]; A. anguilla [26]; A. rostrata [27]; A. japonica [28]). The distinction between yellow, intermediate and silver stages allowed us to observe a first increase in E2 levels, at the early stages of the silvering process (in intermediate eels), then a further increase of E2, accompanied by an increase in 11-KT and T in silver eels.…”

Section: Discussionsupporting

confidence: 68%

“…All these morpho-physiological changes, as well as downstream migratory behavior, are similar to those observed during smoltification, which traditionally led to defining eel silvering as a metamorphosis [24, 18]. However, unlike smoltification, silvering also includes some changes related to the reproductive function, with significant increases in gonad weight as well as in serum levels of sexual steroids ( Anguilla australis and Anguilla dieffenbachii [25]; Anguilla anguilla [26]; Anguilla rostrata [27]; Anguilla japonica [28]). These changes indicating an onset of sexual maturation, led to the hypothesis that silvering could correspond to a pubertal event [29].…”

Section: Introductionmentioning

confidence: 91%

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Endocrine Evidence that Silvering, a Secondary Metamorphosis in the Eel, Is a Pubertal Rather than a Metamorphic Event

et al. 2005

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Silvering (transition from yellow to silver eel) has been traditionally considered as a metamorphosis in view of the numerous morphological, physiological and behavioral changes preparing the eel for the oceanic migration. However, some changes, such as increases in gonad weight and steroidogenesis, suggest that silvering could also be considered as a pubertal event. In order to assess which endocrine axis may be involved in the induction of silvering, we compared the profiles of pituitary and peripheral hormones during the transition from yellow to silver female eels. A strong activation of the gonadotropic axis was shown during silvering. Follicle-stimulating hormone (FSH) mRNA levels increased during the early stages of silvering, followed by a later increase in luteinizing hormone (protein and mRNA) levels. In addition, plasma levels of sexual steroids (estradiol, E2; testosterone, T, and 11-ketotestosterone) and of vitellogenin significantly increased. In contrast, thyrotropin mRNA levels did not change and no or weak variations in plasma thyroid hormones were observed, indicating no or moderate change of the thyrotropic axis during silvering. Similarly, the somatotropic axis was not activated, as shown by pituitary growth hormone expression (protein and mRNA) and plasma levels. In addition, we studied the effects of chronic treatments of female yellow eels with thyroid hormone (thyroxine, T4) and sex steroids (T and E2) on biometrical parameters characteristics of silvering. T induced an increase in eye size and a reduction of digestive tract, whereas T4 and E2 had no effect. These hormonal profiles and experimental data lead to the conclusion that eel silvering should be considered as an onset of puberty rather than a ‘genuine’ metamorphosis.

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Section: Discussionsupporting

confidence: 68%

Section: Introductionmentioning

confidence: 91%

Endocrine Evidence that Silvering, a Secondary Metamorphosis in the Eel, Is a Pubertal Rather than a Metamorphic Event

et al. 2005

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“…At the onset of the natural spawning migration, the gonadosomatic index (GSI) of A. anguilla females is close to 2% [A Palstra, unpublished data] and they are still in a previtellogenic state when they migrate to sea. However, females of A. australis have a higher GSI, of up to 4% [14], indicating that they are sexually more advanced than A. anguilla at the same stage The same holds true for A. japonica , which has a GSI of up to 4% at the commencement of its spawning migration [15]. Induction of vitellogenesis and final maturation in A. australis requires approximately six to eight weekly hormonal injections [[10], Kurwie, unpublished data] while 9-12 injections [4], or 6-15 weekly injections [11], are required for A. japonica and up to 12-25 weekly injections for A. anguilla [7-9].…”

Section: Introductionmentioning

confidence: 99%

First artificial hybrid of the eel species Anguilla australis and Anguilla anguilla

et al. 2011

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BackgroundStudies on artificial hybridization of different Anguilla species were conducted recently, i.e. female A. australis with male A. dieffenbachii, and female A. japonica with male A. anguilla. The existence of these artificial hybrids was however not demonstrated by independent genetic methods. Two species - A. anguilla and A. australis - that are phylogenetically close but have different sexual maturation times (12-25 weeks and 6-8 weeks, respectively), were expected to produce favourable hybrids for reproduction studies.ResultsA modification of the protocol for the reproduction of Anguilla japonica was used to produce eight-day Anguilla australis larvae, with a success rate of 71.4%. Thus ten out of 14 females produced eggs that could be fertilized, and three batches resulted in mass hatching. Hybrid larvae from female A. australis x male A. Anguilla survived for up to seven days post fertilization (dpf). The early development of the hybrid showed typical characteristics of A. anguilla tail pigmentation at 50 hours post fertilization (hpf), indicating expression of genes derived from the father.ConclusionsIn this paper we describe the first production of hybrid larvae from male A. anguilla and female A. australis and their survival for up to 7 dpf. A species-specific nucleotide difference in the 18 S rDNA gene confirmed that genes from both A. australis and A. anguilla were present in the hybrids. The developmental stages of the hybrid eel embryos and larvae are described using high resolution images. Video footage also indicated a heart beat in 5-dpf larva.

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“…Liver sensitivity may occur only upon seawater entry after downstream migration as Ca and vitellogenin levels in freshwater eels are not detectable (Versonnen et al 2004; Palstra and van den Thillart 2010), but in the brackish estuaries, some silver eels show increased plasma vitellogenin levels (Palstra and van den Thillart 2010). The absence of vitellogenesis in European eels in freshwater is in contrast to Anguilla species that need to migrate lesser distance to the spawning grounds like wild New Zealand freshwater eels Anguilla dieffenbachii and A. australis (Lokman et al 1998). One of the silvering phenomena is known to be a regression of the digestive tract (reviewed by Lokman et al 2003; Durif et al 2009) which explains the lower DTSI with increasing migration stage that will prevent the eels from feeding.…”

Section: Discussionmentioning

confidence: 99%

Temporal progression in migratory status and sexual maturation in European silver eels during downstream migration

Palstra

Guerrero

Laak

et al. 2011

Fish Physiol Biochem

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The onset of downstream migration of European eels is accompanied by a cessation of feeding and the start of sexual maturation which stresses the link between metabolism and sexual maturation, also suggesting an important role for exercise. Exercise has been tested with eels in swim tunnels and was found to stimulate the onset of sexual maturation. In this study, we have investigated the interplay between migration and maturation in the field during the downstream migration of female silver eels. Temporal changes in migratory status and sexual maturation among silver eels of the upstream Rhine River system over 3 months of the migration season (August, September and October) were determined in biometrical parameters, plasma 17β-estradiol and calcium levels, oocyte histology and gonadal fat levels. Furthermore, the ecological relevant parameters age as determined by otolithometry and health aspects indicated by haematocrit, haemoglobin and swim-bladder parasite load were measured. Silver eels were estimated to be 14 years old. A strong temporal progression in migratory stage was shown over the months of downstream migration. Catches probably represented a mix of reproductive migrants and feeding migrants of which the ratio increased over time. Furthermore, this study confirmed our hypothesis linking the migratory stage to early maturation as indicated by enlargement of the eyes, oocyte growth and fat deposition in the oocytes, exactly the same changes as found induced by exercise but not ruling out environmental influences. Migrants show extensive fat uptake by the oocytes, probably stimulated by the swimming exercise. In addition, at least 83% of the silver eels in this spawning run may have suffered from negative effects of swim-bladder parasites on their swimming performance.

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Cited by 120 publications

References 0 publications

Endocrine Evidence that Silvering, a Secondary Metamorphosis in the Eel, Is a Pubertal Rather than a Metamorphic Event

Endocrine Evidence that Silvering, a Secondary Metamorphosis in the Eel, Is a Pubertal Rather than a Metamorphic Event

First artificial hybrid of the eel species Anguilla australis and Anguilla anguilla

Temporal progression in migratory status and sexual maturation in European silver eels during downstream migration

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