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Vol. 43 No. 2, 2004
In Situ Spawning Rate of the Calanoid Copepod Acartia clausi in a Tropical Lagoon (Ebrié, Côte d'Ivoire): Diel Variations and Effects of Environmental Factors
Marc Pagano1,*, Ernest Kouassi2, Robert Arfi1, Marc Bouvy1 and Lucien Saint-Jean2
1Institut de Recherches pour le Développement (IRD), UR098, BP1386 Dakar, Sénégal
2Centre de Recherches Océanographiques, Boite Postale V18 Abidjan, Côte d,Ivoire
Marc Pagano, Ernest Kouassi , Robert Arfi, Marc Bouvy and Lucien Saint-Jean (2004) 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 Ebrié Lagoon (Côte 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.
Key words: Egg production, Diel rhythms, Acartia clausi, Brackish lagoon, Tropics.
*Correspondence: Tel: 221-8493306. Fax: 221-8321675. E-mail: pagano@dakar.ird.sn
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