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If European silver eels are prevented from reproductive migration, they remain in …

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- Male silver eels mature by swimming

Males that were either stimulated by three months SW-swimming or by GnRHa-injection showed a two- to three-fold higher LHβ expression level than the male starters and resters (Fig. 1a). Both treatments also caused a three- to five-fold increase in GSI (Fig. 1b) and an induced spermatogenesis (>80% presence of spermatogonia late type b; Fig. 2). One male swimmer even showed the formation of spermatocytes (Fig. 2). In contrast, females were not stimulated by SW-swimming nor by GnRHa, and even showed regression of maturation over time as demonstrated by lower LHβ expression (Fig. 1c), GSI (Fig. 1d) and oocyte diameters in all groups after 3 months (Fig. 2). The expression of FSHβ did not significantly change under the different treatments in both males and females.

Figure 1. Expression (Q-RT-PCR) of luteinising hormone subunit (LHβ) in the pituitary and the gonadosomatic index (GSI) in male and female eels. Eels were sampled at the start, after three months rest, after three months of SW-swimming and three months after a single GnRHa injection (Gonazon For Fish, Intervet). In females, regression occurs during the experimental period, an effect which is more pronounced in the resting group than those stimulated by SW-swimming and GnRHa. In males however, SW-swimming and GnRHa activate maturation (student t-tests with * = P < 0.05; ** = P < 0.01; 3 m rest vs. start, 3 m swimming or 3 m GnRHa vs. 3 m rest).

Figure 2. Gonad development parameters in males and females. a) oocyte diameters at the start, after 3 months of rest, after 3 months of SW-swimming and 3 months after GnRHa injection; b) male testis stage with frequency distribution of spermatogonia type a, spermatogonia early type b, spermatogonia late type b and spermatocytes; c) testis in starter containing mainly spermatogonia type a (encircled), d) GnRH-stimulated testis containing spermatogonia late type b (encircled), e) testis of swimmer containing spermatocytes (encircled). The scale bar represents 100 μm. Asterisks indicate significant differences (student t-test with ** = P < 0.01; 3 m rest, 3 m swimming or 3 m GnRHa vs. start).

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