Food size selectivity of Artemia franciscana at three developmental stages

Food size selectivity was examined in Artemia franciscana metanauplii at th ree different developmental stages. Clearance rates were determined in shor t-term experiments either by measuring the decrease in concentration of liv e particles and plastic beads, or by measuring the radioactivity accumulate d in animals that grazed C-14-labelled live particles. The maximum clearanc e rate of A. franciscana metanauplii increased during development and was m easured at 50-63 mu l ind.(-1) h(-1), 254 mu l ind.(-1) h(-1) and 1.48-2.10 mi ind.(-1) h(-1) in 2-, 4- and 7-day-old metanauplii, respectively. A pre ference for particles with a diameter of 4-8 mu m was observed at all three developmental stages. The ability of A. franciscana metanauplii to graze b acterial particles was also demonstrated, although the efficiency in grazin g such small particles was low compared to microalgae (28, 20 and 9% of the maximum clearance rate in 2-, 4- and 7-day-old metanauplii, respectively). Electron microscopy showed that the inter-setular distance in antennae and thoracopods was 0.20 +/- 0.07, 0.16 +/- 0.05 and 0.18 +/- 0.04 mu m in 2-, 4- and 7-day-old metanauplii, respectively, and accordingly independent of stage.