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

Abstract

The harpacticoid copepod Macrosetella gracilis is found in pelagic habitats in tropical and subtropical oceans associated with colonies of the N2 fixing cyanobacterium Trichodesmium spp. In the central North Pacific near Hawaii, M. gracilis was abundant (1.8 ± 1.4 (SD) M. gracilis adults m -3 and 4.7 ± 3.9 M. gracilis copepodites m -3 ) and constituted an average of 10.8% of the total copepod population. However, we observed no statistically discernable correlation between M. gracilis and Trichodesmium spp. abundances, suggesting that availability of Trichodesmium spp. did not limit the abundance of M. gracilis during our study. In previous laboratory studies M. gracilis had been shown to have the ability to ingest Trichodesmium spp. trichomes and appeared immune to cyano- bacterial toxins harmful to other species of copepods. Natural abundance of stable isotopes (δ 15 N, δ 13 C) in copepod tissue from field samples suggested that the diet of M. gracilis was not predominately composed of Trichodesmium spp. as proposed by previous research. Natural abundance of δ 15 N was similar for M. gracilis (3.06 ± 2.29), Miracia efferata (1.83 ± 0.88), and calanoid copepods (2.7 ± 1.95). No Trichodesmium spp. were observed in M. gracilis gut contents. Trichodesmium spp. was not a predominant food in the diet of this copepod, but colonies of the toxic cyanobacterium could provide shelter from predation and be used as a floating substrate for adult and juveniles of M. gracilis.