Small, soft animals like tardigrades rarely have legs and hardly ever bother walking. But a new study finds that water bears push themselves through sediment and soil on eight stubby legs, in a way similar to that of insects 500,000 times their size.
Plump and heavy, tardigrades earned the nickname “water bears” when scientists first observed the awkward, characteristic gaits of 0.02-inch-long animals in the 18th century. Their slow pace, however, raises the question of why tardigrades evolved to walk.
For example, round worms of similar size and body type wiggle, crawling their pasty forms over unpredictable substrates. Yet the water bear, a micro-animal so distinct that scientists they were forced to assign it to their own phylum (Tardigrada, Spallanzani 1777), it uses eight stubby legs to propel itself improbably through marine and freshwater sediments, through desert dunes and under the ground.
Now, a new study in PNAS analyzes the gaits of tardigrades and discovers that water bears walk in a way very similar to that of much larger insects. The discovery implies the existence of a common ancestor or evolutionary advantage that explains why one of the smaller, softer creatures it evolved to walk just like larger insects and hard-bodied.
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“Tardigrades have a robust and clear way of moving, they’re certainly not like clumsy animals that stumble across the desert or litter,” says Jasmine Nirody, a colleague at Rockefeller’s Center for Studies in Physics and Biology. “The similarities between their locomotive strategy and that of much larger insects and arthropods open up several very interesting evolutionary questions.”