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

Abstract

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.

Spatial and temporal distribution of Trichodesmium spp. in the stratified Gulf of Aqaba, Red Sea

“…Our results indicate that C22:2ω6 serves as a specific biomarker for Trichodesmium spp., since it is lacking in Synechococcus. We identified the C22:2ω6 marker in the known Trichodesmium spp.-feeder Macrosetella gracilis (Böttger-Schnack & Schnack 1989, O'Neil 1998 and in the salp Salpa maxima. However, it was not found in calanoid copepods which supposedly do not feed on Trichodesmium spp.…”

Toxic effect of the bloom‐forming Trichodesmium sp. (cyanophyta) to the copepod Acartia tonsa

“…On one hand, Trichodesmium blooms have been credited with causing rapid declines in zooplankton biomass and alterations of fish migration routes [Chen, 19821, while on the other, some oceanic harpacticoid copepods are known to feed on Trichodesmium colonies and 0 1994 Wiley-Las, Inc. such, is in the public domain in the United States of America. *This article is a US Government work and, as utilize them as substrate [Borstad and Borstad, 1977;Bottger-Schnack and Schnack, 1989;O'Neil and Roman, 1992;Roman, 19781. Trichodesmium thiebuutii, while non-toxic to harpacticoid copepods, causes significant mortality to calanoid and cyclopoid copepods and to brine shrimp.…”

Association of the copepod Macrosetella gracilis with the cyanobacterium Trichodesmium spp. in the North Pacific Gyre

“…A positive correlation between Trichodesmium spp. and M. gracilis abundances was also found in the Red Sea (Calef & Grice 1966, Böttger-Schnack & Schnack 1989, Böttger-Schnack 1991. However, no abundance data appears to be available for the North Pacific Gyre.…”

“…abundance encountered in our samples -with an average of 68.5 ± 33.4% of all trichomes in colonies -is highly favorable to M. gracilis reproduction. Previous studies encountered nauplii, copepodites, and gravid females of M. gracilis only in areas and at depths of high Trichodesmium abundance (Böttger-Schnack & Schnack 1989, Böttger-Schnack 1991. Females of M. gracilis carry between 5 and 12 eggs (an average of 6 eggs) in each of the 2 egg sacs, and nauplii hatch successively (Huys & Böttger-Schnack 1994).…”

“…is apparent, few data exist concerning the abundance, life-history, and feeding habits of M. gracilis in its natural habitat. The relative abundance of M. gracilis among tropical copepods is poorly known, although Böttger-Schnack & Schnack (1989) estimated that M. gracilis comprises 1 to 3% of adult tropical copepods in the Red Sea. Calef & Grice (1966) showed that M. gracilis abundance in the western tropical Atlantic closely tracked Trichodesmium spp.…”