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 N 2 fixing cyanobacterium Trichodesmium spp. ) 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 cyanobacterial 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.

Fitness consequences of habitat variability, trophic position, and energy allocation across the depth distribution of a coral-reef fish

“…Trichodesmium sp. may be a low-quality food source that is secondarily ingested by fish while they target zooplankton (Eberl and Carpenter 2007) despite evidence of consumption of the diazotrophic cyanobacteria by other pomacentrids (Emery 1973). Lower densities of S. partitius in deeper environments (Goldstein et al 2016b) suggest that SS fish may also be influenced by competition that can affect energy allocation and condition (Samhouri 2009).…”

Carbon and nitrogen stable isotope ratios of pelagic zooplankton elucidate ecohydrographic features in the oligotrophic Red Sea

“…It was assumed that the isotopic composition (δ 13 C, δ 15 N) of the mucus net-feeding Thalia democratica (Tunicata; Mullin, 1983;Vargas and Madin, 2004) represented the isotopic baseline of small 'autotrophic phytoplankton' and bacteria fueled by deep water nitrate. As a proxy for the diazotrophic food web based on biological N 2 fixation we estimated the isotopic baseline by cyanobacteria (Trichodesmium) from the copepod Macrosetella gracilis (Harpacticoida), a species known to feed extensively but not exclusively on Trichodesmium (Harpacticoida; O'Neill, 1998;Eberl and Carpenter, 2007;Mompeán et al, 2013). As a third potential macronutrient end-member for zooplankton, POM was used as a proxy for fresh and detrital organic matter within the 0.7 to 50 µm size range that is available through the microbial food web.…”

Incorporation of nitrogen from N2 fixation into amino acids of zooplankton

“…Direct grazing of diazotrophs via zooplankton appears to be uncommon; however, some copepod species are known to graze on the filamentous cyanobacteria Trichodesmium and Nodularia (O'Neil et al 1996;Koski et al 2002). The factors regulating rates of diazotroph consumption are not well understood (O'Neil et al 1996;Eberl and Carpenter 2007), although a number of factors, including toxicity, large size, and poor nutritional quality, tend to deter direct grazing by zooplankton (O'Neil 1999;Koski et al 2002). In contrast to measurements of grazing rate, the nitrogen isotopic composition of zooplankton in regions with high diazotroph abundance implies substantial transfer of diazotroph nitrogen into the food web (Loick et al 2007;Landrum et al 2011).…”