A ravenous flatworm is such a dangerous predator that it can even attack and kill a spider perched on its web, a first-of-its-kind observation shows.
In February 2014, – now at the Federal University of Uberlândia in Brazil – was a master’s student spending 15 days in Brazil’s Atlantic Forest conducting research on spiders. One afternoon along the Mirante River, Cardoso came across the web of a comb-footed spider (Helvibis longicauda) underneath a leaf. There he found a grisly scene – a striped worm coiled around the web’s eight-legged creator, which was still weakly trying to defend its egg sac.
Cardoso took the two combatants back to the lab, where he took photos and examined them more closely. The worm was a land-dwelling flatworm (in the genus Choeradoplana). Many flatworms, such as tapeworms and flukes, are aquatic or parasitic. But some species are terrestrial predators. These “land planarians” typically eat soft prey like slugs or earthworms.
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Predatory flatworms wrap around their catch and then eject their pharynx – a throat-like organ – out of their combination mouth and anus. The pharynx bathes the prey in a caustic cocktail of digestive juices that break it down so the worm can slurp it up. This is the fate that befell the spider.
After searching the scientific literature, Cardoso and his colleagues realised this was the first known case of a land planarian feeding on a web-building spider.
at the University of Cambridge notes that land planarian eyes don’t form images, so the worm probably found the web via a scent trail or through happenstance. Once near the web, Cardoso thinks the flatworm’s predatory instincts may have been triggered by the spider’s movements.
The flatworm wasn’t entangled in the web’s viscous silk, so Cardoso and his colleagues think its mucus must have allowed it to easily slither through the strands. The spider may have been further doomed by its maternal instincts. Female comb-footed spiders rarely leave their egg sac. If the spider stayed behind to defend its eggs instead of running away, that may explain how it was overpowered by the much slower flatworm. Together, all these factors may have led to an outcome that is extremely rare in the wild, says Cardoso.
“But even being such a rare interaction, it may demonstrate how interactions in nature can be context-dependent,” says Cardoso. “A worm, dominating a spider! It’s really amazing.”
Gerlach says he is surprised a land planarian could prey upon something as fast as a spider, but had a similar surprise finding New Guinea flatworms eating millipedes in French Polynesia.
“We know a lot about [the New Guinea flatworm’s] eating habits, almost all records being snails, and yet they were doing something quite unexpected,” says Gerlach. “It just goes to show how little we really know about most invertebrates.”
Neotropical Biology and Conservation
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