LAUSR

Abstract The distribution and reproductive traits of copepods of the genus Clausocalanus were investigated during the Atlantic Meridional Transect cruise AMT-15, in September–October 2004 to estimate their ecological niches and secondary production in the epipelagic layer along a latitudinal cline (48°N–40°S). The distribution patterns of selected environmental parameters, i.e., temperature, salinity and chlorophyll a concentration, enabled eco-provinces to be identified as described by Longhurst (2006) . Clausocalanus represented on average 34% of total copepod abundance, with a large predominance of adult females and copepodites over males. Among the eleven Clausocalanus species found during the survey, eight species showed a wide distributional range, i.e.,C. paululus, C. pergens, C. furcatus, C. arcuicornis, C. jobei, C. parapergens, C. lividus, and C. mastigophorus, while C. ingens, C. brevipes, and C. laticeps were recorded only in the South Atlantic. The smallest C. furcatus, C. paululus, and C. pergens together accounted for 85% of total Clausocalanus adult abundance. The ecological niches were clearly separated among congeners of similar size and largely overlapped in congeners whose size differed. The small- and medium-sized species, which are egg-sac-spawners, had smaller clutch size and lower egg-production rate than the larger broadcaster congeners. Nevertheless, embryo viability was lower in broadcasters, which may explain their low abundance in terms of lower recruitment. A sex ratio largely skewed toward females in all Clausocalanus species and the observation of viable eggs in successive clutches from isolated females seem to indicate that re-mating is not necessary in this genus. Broadcast-spawners showed the highest weight-specific fecundity rates in the genus but similar secondary production to sac-spawners despite the fact that they never occurred at high abundance. In light of their abundant occurrence in oceanic waters and well-defined ecological niches, Clausocalanus species may be considered as good indicators of environmental conditions and monitored in relation to changes in ecosystem structure due to climate change.