Vertical distribution and population structure of Macrosetella gracilis (Copepoda: Harpacticoida) in the Red Sea in relation to the occurrence of Oscillatoria (Trichodesmium) spp. (Cyanobacteria)

Macrosetella gracilis is closely associated with the epipelagic filamentous cyanobacteria OsciUatoria (Trichodesmiurn) spp., which serve as food for adults and as a substrate for the non-pelagic juveniles. Plankton sampling in the Red Sea in 1980-81, using a multiple opening-closing net with 0.1 mm mesh, revealed M. gracilis populations from the surface to the deepest stratum sampled (1650 m). Their population biology was investigated in the central and northern Red Sea, and included analysis of ontogenetic composition and size distribubon, sex ratio and percentage of egg-bearing females. By relating population structure differences to variations in the abundance of Tr~chodesmium spp. at the surface, a hypothetical life cycle for the copepod could be proposed. (1) Reproduction takes place only in surface waters during blooms of Trichodesmjum spp. (2) Short-term, diurnal vertical migrations are not apparent. (3) At the surface and in the lower meso-and bathypelagic zone, the female population consists mainly of small specimens; in the upper part of the oxygen minimum zone, a t 300 to 450 m, long-lived large, non-reproductive females are congregated. The significance of these midwater populations in the life cycle of M. gracilis is discussed under 2 alternative assumptions: (a) the copepods s u ~v e periods of Trichodesmium spp. scarcity in a resting phase, or (b) the part of the population in deep waters is moribund. The unusually deep occurrence of M gracilis may have some significance for the nutrient cycle in these waters.