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, 鈥� now at the Federal University of Uberl芒ndia in Brazil 鈥� was a master鈥檚 student spending 15 days in Brazil鈥檚 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 鈥� a striped worm coiled around the web鈥檚 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 鈥渓and planarians鈥� typically eat soft prey like slugs or earthworms.
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Predatory flatworms wrap around their catch and then eject their pharynx 鈥� a throat-like organ 鈥� 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鈥檛 form images, so the worm probably found the web via a scent trail or through happenstance. Once near the web, Cardoso thinks the flatworm鈥檚 predatory instincts may have been triggered by the spider鈥檚 movements.
The flatworm wasn鈥檛 entangled in the web鈥檚 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.
鈥淏ut even being such a rare interaction, it may demonstrate how interactions in nature can be context-dependent,鈥� says Cardoso. 鈥淎 worm, dominating a spider! It鈥檚 really amazing.鈥�
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.
鈥淲e know a lot about [the New Guinea flatworm鈥檚] eating habits, almost all records being snails, and yet they were doing something quite unexpected,鈥� says Gerlach. 鈥淚t just goes to show how little we really know about most invertebrates.鈥�
Neotropical Biology and Conservation
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