'Snail vs flatworm' is no sluggish predatory encounter (video)

When we think of high-drama predation, it's often bigger beasts doing it: lions wrestling buffalo, wolves tugging down elk, white sharks torpedoing sea lions. Pay attention to the micro-world at ground level, though, and you'll discover life-and-death contests just as intense playing out among those boneless critters.

Case in point: this recent video taken by Dr Konrad Mebert in the Michelin Ecological Reserve in eastern Brazil's biodiverse Atlantic Forest. The role of the quarry is played by a land snail, the role of the hunter by what looks like a banana peel come to vicious life, but which is actually a flatworm: a land planarian, to be exact.

"Snail vs flatworm" sounds like a subdued and rather "sluggish" predatory encounter. Nope. This is a genuinely vigorous (and genuinely slimy) struggle, the snail flailing in vain as the planarian whips and writhes and ultimately fully envelops its escargot.

Dr Ana Maria Leal-Zanchet, a biology professor at Brazil's Unisinos (Universidade do Vale do Rio dos Sinos) specialising in flatworms, noted that the exact species of land planarian in the video is impossible to determine from the footage alone. "Even at the genus level, we can't be sure about the identification without anatomical studies," she told us.

A graduate student of hers, Piter Keo, who studies the ecology and dietary habits of land planarians, said Mebert was lucky to see (let alone film) the snail attack. "It is very unusual to witness such a behaviour in the wild, especially because land planarians are nocturnal animals," he noted.

These stretchy predators employ the super-handy feeding method of inverting their pharynxes and thereby unleashing digestive fluids upon their victims, though Keo notes the exact method of ingestion depends on the size and type of their meal.

"Snails are not swallowed as a whole piece like some small slugs may be," he said, "but are ingested in smaller portions after being partially digested outside."

In the Atlantic Forest (where multiple species of land planarian coexist by targeting different prey) and many other corners of the world, flatworms are among the leading predators of gastropods (snails and slugs), the slime trails of which they'll actively track – even up into trees! If land snails dream, their nightmares may well be wriggling with land planarians – just read this account from a 1963 paper of en masse assaults by the flatworm Endeavouria septemlineata on snails in Hawaii (as quoted here):

[…] in the vicinity of these worms, a snail is soon seen with a number of worms crawling almost frantically in its wake. The directive motion of the worms permits them soon to overtake the snail with its hesitant, probing locomotion. […] The sensitivity of the snail grows more acute with more worms moving into position; and the victim withdraws into its shell dragging the worms in with it and embracing them in the folds of the invaginated head and tentacles […] The harassed snail opens the pneumostome [breathing pore] in a desperate effort to get more air; and some of the worms crawl into the lung cavity. Soon the irritation and congestion in the lung, as evidenced by the copious amount of mucus produced in the lung and the bubbling from the pneumostome, causes the pneumostome to remain open, only to permit still more worms to enter until a veritable webbing of black worms can be seen.

Sheesh, makes a lion pride's takedown of a wildebeest seem borderline tender by comparison, huh?

Snails, incidentally, will sometimes attempt to thwart their coiling attackers by spewing out a defensive froth, a sort of mucus-brew that may or may not end up saving their necks.

Their efficiency as snail-hunters makes certain land planarians vital managers of mollusc populations in their native ecosystems, but also potentially devastating marauders where they've been accidentally introduced. The New Guinea flatworm, for example, has been implicated in the decline and even extinction of native land and tree snails on some Pacific islands where it's an exotic invasive; it’s also been deliberately released in an effort to control a notorious invasive gastropod, the giant African land snail.