Fax +41 61 306 12 34 E-Mail karger@karger.ch www.karger.com Original Paper Neuroendocrinology 2005;82:221–232 DOI: 10.1159/000092642 Endocrine Evidence that Silvering, a Secondary Metamorphosis in the Eel, Is a Pubertal Rather than a Metamorphic Event Salima Aroua a Monika Schmitz b Sylvie Baloche a Bernadette Vidal a Karine Rousseau a Sylvie Dufour a a MNHN, Département des Milieux et Peuplements Aquatiques, USM 0401, UMR 5178 CNRS, Paris, France; b Department of Aquaculture, Swedish University of Agricultural Sciences, Umeå, Sweden in plasma thyroid hormones were observed, indicating no or moderate change of the thyrotropic axis during silvering. Similarly, the somatotropic axis was not acti- vated, as shown by pituitary growth hormone expres- sion (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 character- istics 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 con- sidered as an onset of puberty rather than a ‘genuine’ metamorphosis. Copyright © 2005 S. Karger AG, Basel Introduction Metamorphosis is a critical event in the biological cy- cle of many animals. The transition from larva to adult involves a number of morphological, physiological and behavioral modifications, which permit adaptation to a new environment and lead to a new lifestyle. In verte- brates, the most studied example of this transition has been the metamorphosis of amphibians, since the report Key Words Silvering  Puberty  Metamorphosis  Gonadotropins  Thyrotropin  Growth hormone  Gonadal steroids  Fish Abstract Silvering (transition from yellow to silver eel) has been traditionally considered as a metamorphosis in view of the numerous morphological, physiological and behav- ioral changes preparing the eel for the oceanic migra- tion. However, some changes, such as increases in go- nad 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 in- duction of silvering, we compared the profiles of pitu- itary 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 dur- ing the early stages of silvering, followed by a later in- crease in luteinizing hormone (protein and mRNA) levels. In addition, plasma levels of sexual steroids (estradiol, E2; testosterone, T, and 11-ketotestosterone) and of vitel- logenin significantly increased. In contrast, thyrotropin mRNA levels did not change and no or weak variations Received: December 19, 2005 Accepted after revision: February 20, 2006 Published online: April 10, 2006 Sylvie Dufour Muséum National d’Histoire Naturelle Département des Milieux et Peuplements Aquatiques, USM 0401 UMR 5178 CNRS 7, rue Cuvier, CP 32, FR–75231 Paris Cedex 05 (France) Tel. +33 1 40 79 36 12, Fax +33 1 40 79 36 18, E-Mail dufour@mnhn.fr © 2005 S. Karger AG, Basel 0028–3835/05/0824–0221$22.00/0 Accessible online at: www.karger.com/nen © Free Author Copy - for per- sonal use only ANY DISTRIBUTION OF THIS ARTICLE WITHOUT WRITTEN CONSENT FROM S. KARGER AG, BASEL IS A VIOLATION OF THE COPYRIGHT. Written permission to distri- bute the PDF will be granted against payment of a per- mission fee, which is based on the number of accesses required. Please contact permission@karger.ch