What Is It?

A closer look at a two-millimeter pond invertebrate

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Smaller fleas: What appears as a mere speck to the human eye has plenty of character when observed under a microscope. The nearly invisible Daphnia, a water flea, came to life with tufts of hair, big eyes and red “lips” when magnified 50 times. Kevin Mackenzie, manager of the University of Aberdeen’s Microscopy and Imaging Facility in Scotland, photographed this two-millimeter pond invertebrate whose wispy hair is actually a pair of antennae. The beauty mark below its compound eye (black) is a light-sensing organ called an ocellus. The flea’s transparent body also reveals its last meal: algae (green).

In February scientists reported sequencing the genome of a Daphnia species, D. pulex, for the first time. The sequence will help researchers study how the environment influences the functions of genes, says project leader John Colbourne of Indiana University. Municipalities have long monitored Daphnia population sizes for signs of water pollution, because the critter is extremely sensitive to it. Looking at alterations in gene behavior, he notes, can also provide new clues to how chemicals might affect human health.

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