Résumé
In situ spawning rate of the calanoid copepod Acartia clausi in a tropical lagoon (Ebrie, Cote d'Ivoire): diel variations and effects of environmental factors. Zoological Studies 43(2): 244-254. Diel changes in the spawning rate of Acartia clausi were studied during several 24 h time series performed between 1993 and 1997 at several sites in Ebrie Lagoon (Cote d'Ivoire) at which the morphology and hydrological structures differed. The daily egg production rate ranged from 8 to 60 eggs female(-1) d(-1) (5% to 45% of body carbon) and varied according to stations and periods. A correlation analysis, performed with the present results, revealed no significant effect of either environmental factors (salinity, temperature, and chlorophyll a biomass) or individual weight of females. However, the roles of salinity and food abundance (chlorophyll a) appeared when grouping the present data (1993-1997) with results from the same sites obtained previously (1981-1982) by Saint-Jean and Pagano (1984). This grouping showed the importance of inter-annual variations as compared to seasonal variations. The rhythm of egg laying was characterized by a night maximum which occurred earlier in the estuarine zone (22:00-01:00) than in the western oligohaline zone (04:00-06:00). These peaks of egg laying always occurred after the peak of gut fluorescence which was consistently observed at the beginning of the night (data from Kouassi et al. 2001). Our data suggest that the 2 rhythms are relatively independent, with different shifts between ingestion and spawning peaks being more or less fortuitous and resulting from regional variabilities in diel spawning rhythms. The modulations of these rhythms represent various adaptive responses to the environmental constraints (limited trophic levels in the estuarine zone and lower salinities in the western zone). Differences in behavior in the 2 studied zones and the confined situation of the western zone led to the hypothesis of a physiological and/or genetic differentiation of the populations.