Spatial and seasonal variations in size, body volume and body proportion (prosome:urosome ratio) of the copepod Acartia tonsa in a semi-closed ecosystem (Berre lagoon, western Mediterranean)

Variations in size (prosome), body volume and proportion (prosome:urosome ratio) of female Acartia tonsa copepods were studied during three different seasons (June, October and November) in a network of 13 stations distributed throughout the Berre Lagoon, near Marseille. Strong morphological differences were found between the populations collected through the different seasonal surveys, but also between the different stations or groups of stations. They were related to the variations of environmental parameters (temperature, salinity, chlorophyll, particulate seston) according to the season and to the location of the stations (submitted to the marine influence in the south and to the intake of fresh water in the north of the lagoon). Considering all seasonal data, the size and body volume were inversely related to temperature. Body volume also showed a negative correlation with chlorophyll and carbon and a positive one with the C:N ratio of particles. The body proportion was positively correlated with temperature, chlorophyll and carbon. For each seasonal survey, the relationship between morphological features and environmental factors did not reach the significant level except in October when body proportion and volume were positively correlated to chlorophyll. Nevertheless, for each season, significant spatial changes in size or body proportion appeared in parts of the population of Acartia tonsa, in relation with local specific conditions of environmental factors, especially chlorophyll. These biometric differences suggest that individuals must develop in situ for at least the final period of larval growth, despite the dispersion effect caused by hydrodynamic movements. This stability in horizontal distribution may result from the diurnal vertical migrations of copepods between the surface and the bottom, two layers displaying currents of opposite directions. These results justify the use of somatic features (size and body proportion) to discriminate sets of individuals belonging to the same population.