EEL SILVERING - METAMORPHOSIS, ANTICIPATION, ADAPTATION

Eel ''silvering'' is a metamorphosis which, during the life cycle of these fish, precedes the marine reproductive migration. A better knowledge of this phenomenon would be useful not only from a fundamental point of view in developmental biology but also from an applied point of view for the management of eel populations. First, we review a number of characters wich distinguish between yellow and silver stages with a special attention to the meaning, for the life cycle, of the novelties expressed during silvering; most of them appear to be adaptative characters which anticipate the conditions of the marine migration. Then, we take an interest in the dynamics and the neuroendocrine determination of silvering for which the existence of several successive steps is suggested, from presilvering to the catadromous freshwater journey (''avalaison'' in French). The neuroendocrine data on silvering are rare so that one is led to use informations from other events such as amphibian metamorphoses and salmon smoltification. It appears likely that several hormones are involved in the determinism of silvering successive steps. One of the precocious events could be the stimulation of hypothalamic neurons with secretion of hypophysiotropic neuropeptides. An especially interesting point raised by eel silvering, the coupling between silvering and puberty triggerings, is discussed. An evolutionary scenario is proposed for the appearance of the present complex eel life cycle from the purely marine life which was probably that of eel ancestors as it is still for most of their close relatives, the other elopomorphs. This scenario in particular involves the acquisition of euryhalinity and then the ''genetic assimilation'' of processus previously triggered during accomodation to sea water. That is to say that these processes should have been placed under the control of the developmental clock which initiates silvering; neuroendocrine hypotheses are suggested to explain this genetic assimilation. In conclusion, one wonders about the morphological and biochemical definition of the successive steps of silvering and on the recognition of these steps by easily detectable characters, more precise and reliable than colouring. Progresses in these scopes would allow for instance to know before ''avalaison'' the proportion of a given stock which is likely to leave fresh waters in the year.