Un cas type de migration verticale ‘retard’ chez l'amphipode Ampelisca typica

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Abstract
Detailed analysis of the nocturnal vertical migration of the tube-living Amphipoda Ampelisca typica was carried out in consideration of the results of plankton hauls regularly performed during several cycles of 24 h. The whole population of the species presents a typical ‘planlctonic’ nocturnal distribution with zero concentration in the uppermost layers. The behaviour of the juvenile animals can be considered as ‘classic’, the migration beginning not long after dusk and ending at dawn, whereas the adult behaviour appears very unusual for a species respresenting this type of distribution: migration, very delayed and short lived, starts and stops suddenly during the second part of the night without previous light stimulus. Nevertheless, this migration presents several original features in Peracarida that greatly increases its interests from a theoretical point of view: it is the most delayed migration so far observed; contrary to ‘delayed’ migrations of this type it affects a species which presents a planktonic and not a ‘hyponeustonic’ nocturnal distribution; females as well as males may show the characteristic behaviour. The interval between the migration and dusk changes regularly according to the seasons; it appears to be inversely related to the duration or to the intensity of the diurnal photophase. The discovery of such a behaviour should lead to a better understanding of the respective roles of the diverse factors controlling the delayed vertical nocturnal migrations: phototaxis, geotaxis, endogenous or exogenous variations of the swimming activity or of the number of emergences.

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