SEX-RATIO OF CALANOID COPEPODS IN RELATION TO POPULATION COMPOSITION IN THE NORTHWESTERN MEDITERRANEAN

The population composition of the most abundant pelagic calanoids: Paracalanus parvus (Claus, 1863), Clausocalanus spp., Centropages typicus Kroyer, 1849, Temora stylifera (Dana, 1849), Acartia clausi Giesbrecht, 1889, Pleuromamma gracilis Claus, 1863, and Calanus helgolandicus (Claus, 1863), was studied from the upper 50 m in the northwestern Mediterranean (Golfe du Lion). The adult sex ratios were studied for these species and for three additional, less abundant, migrating species: Euchaeta marina Prestandrea, 1833, E. acuta Giesbrecht, 1892, and Candacia armata Boeck, 1872, from two different, partly overlapping water layers: the upper 50 m and the upper 200 m. In general the samples were composed of late copepodid stages, females and males in decreasing percentages. Maximum population densities were observed during different sampling periods for the different species. In most species the male-to-female ratio was highest during the periods of lowest female abundance. A pronounced dominance of females over males (>80%) was found throughout the year in the potentially herbivorous species Clausocalanus spp., P. parvus, A. clausi and C. helgolandicus. In the potentially and obligate carnivorous copepods P. gracilis, E. acuta, E. marina and C. armata female dominance was less striking, but seasonal variability was high. In the mixed feeders T. stylifera and C typicus sexes were about equally represented throughout the year. Some species showed deeper male distributions during different periods. In the potentially herbivorous species the low male-to-female ratio is suggested to be related to swarming behaviour, which increases the encounter probability, facilitates the mating process, and serves as an adaptation to a limited food supply (leaving females a greater share). The migrating carnivorous species, which are not known to perform swarming behaviour, showed higher male proportions.