The pelagic harpacticoid copepod Macrosetella gracilis uses the colonial cyanobacterium Trichodesmium not only as a physical substrate for juvenile development, but also as a food source. By associating itself with a buoyant colonial cyanobacterium, M.gracilis has developed a successful mode of life for existence in the plankton. Further evidence of M.gracills ' dependence on Trichodesmium as a physical substrate is demonstrated by previously undescribed microscopic observations of a gravid M.gracilis female attaching eggs to a Trichodesmium colony. Shipboard experiments investigating the ingestion and assimilation of Trichodesmium carbon (C) were conducted in September 1991 and January/February 1992 in waters of the Bahamas and the Caribbean, respectively. Macrosetella gracilis not only ingested, but rapidly incorporated, cyanobacterial organic matter into its own cellular material. Utilization of ingested Trichodesmium by M.gracilis was investigated by assessing the metabolic partitioning and incorporation of 14 C-labelled Trichodesmium into copepod lipids, proteins, polysaccharides and low-molecular-weight (LMW) compounds using sequential biochemical fractionation techniques. Despite variations in grazing rates between the two sites and times (September 1991,0.017 μg C * μg −1 C h −1 ; January 1992, 0.134 μg C * μg −1 C h −1 , the partitioning of incorporated C into the different biochemical fractions was relatively consistent. There was rapid assimilation of ingested C into the LMW (˜60%) and polysaccharide fractions (˜30%) in the first few hours, with a subsequent increase in the percent C incorporated into protein. On average, ˜21% of the Trichodesmium C ingested by M.gracilis was assimilated. Therefore, M.gracilis is an important secondary link in the food web of oligotrophic waters where Trichodesmium is abundant.
