Etude de la migration verticale quotidienne des larves de Mesochaetopterus sagittarius à Nosy-Bé (Madagascar)

The vertical distribution of the planktonic larvae of Mesochaetopterus sagittarius (Claparède, 1870) has been studied on the basis of samples taken with closing nets every 3 or 4 h, for periods of at least 24 h. The daily migration does not show the typical pattern. In the evening, the larvae begin to descend from the surface layer; they reach a maximum depth around midnight. The subsequent ascent ends at about noon, when the concentration again attains its maximum close to the surface. The biometric analysis of the samples shows that the migration range depends on the size of the larvae: the bigger they are, the higher is the respective amplitude of migration. Furthermore, there exists a size gradient independent of the time of capture which is most distinct during night time; at night, average size, as well as total number of larvae, decrease in the surface layer and, accordingly, increase in the greater depths. The techniques used allow a high degree of significance to be attached to the results obtained. The standard deviation calculated for each sampling series is lower when two parallel samples are taken at each depth, and in the same water mass, by using a parachute drogue set at 20 m depth and followed during the series.