Use of colloidal iron as a tracer to measure the ingestion rates of free-living marine nematodes

Ingestion rates of free-living marine nematodes were measured using colloidal iron as a tracer. Detritus labeled with colloidal iron was fed to the dominant nematode species and cultured. The iron ingested by the animals was quantified using a scanning electron microscope equipped with differential X-ray energy analyzer (EDX). Average ingestion rates (μgC ind.⁻¹day⁻¹) measured were 0.28 for Symplocostoma sp., 0.29 for Polygastrophora sp., 0.73 for Mesacanthion sp. and 0.15 for Metachromadora sp. These values were higher than the values determined using radioactive organic materials as tracers, though the present method is considered to provide conservative values. This result thus strongly suggests that the ingestion rates of nematodes measured so far were underestimated, and the use of colloidal iron has advantages over the use of radioactive organic matter as a tracer.