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Just when you think it’s safe to go back in the garden, the natural world pulls another fast one on us. The picture above is a snail infected with a parasitic flatworm that goes under the (rather menacing) name the green-banded broodsac.  Found mostly in Central and Eastern Europe, its binomial scientific name is Leucochloridium paradoxum which translated means “strange little white-green creature”.  Isn’t that just the understatement of the decade?

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Some might be inclined to exchange paradoxum with nauseosum (nausea inducing) because although it looks strange in a photo, you should see this creature in… motion.  So, if you are even remotely squeamish by nature, don’t read on. And certainly, don’t play this video (definitely not narrated by David Attenborough). Don’t say you weren’t warned…

Well, you were warned. 

The snail is the intermediate host – which is the organism where the parasite lives and develops.  However, it does not reach sexual maturity with this host.  The snail is the staging ground – the green-banded broodsac has its sights on its definitive host (the name for the host where a parasite reproduces - in parasitology, primary host isn’t a formal category).  You may have already guessed the form the definitive host takes – birds.

The broodsac is a pulsating tube full of parasite larvae that invades and expands inside the snail’s tentacle, making it look like a juicy, twitching caterpillar. The rhythmic pulses attract birds (we may recoil at this grotesquery but birds find it irresistible).  The broodsac goes through the bird’s digestive system and ends up (alive and well) in its small intestine.

To do this, the flatworm develops into a free-swimming larval form called cercariae – which then encyst (and that means that they form protective outer walls so that they are not digested or absorbed into the bird’s body).   The latter form is called metacercariae and they lodge in the gut.

Once there, they develop into adult distomes from the Greek meaning “two mouths” – but in fact you could call them suckers.  These are anchored to the gut wall.  At this point the creature is about 1.5mm and is ready to reproduce.  As mating might be difficult, being attached to the gut wall, the adults are hermaphrodites, meaning each individual has both male and female reproductive organs.   Eggs are produced, fertilised and released, carried out of the bird by its faeces.

That’s the first stage of the green-banded bloodsac life-cycle.  Once the snails have ingested the bird faeces, the eggs hatch into miracidia – the second stage.  These tiny life-forms have only ten cells but are motile – that is, capable of movement.  It has a primitive, piercing mouthpiece which it uses to penetrate the snail’s intestinal wall.  From there it reaches the digestive gland and it’s time for the next stage (it has enough to keep Ellen Ripley busy on any day of the week).

That is called the sporocyst stage and consists of branches that spread through the snail’s circulatory system.  Some of these branches develop into tubes, ending in a distended broodsac that you can see in the pictures, protruding in the snail’s eye-stalks.   Two are usually produced – one for each eye stalk – but there are cases where a third appears.  The base of the sporocyst produces many tail-less larvae. These larvae turn into metacercariae inside the sporocyst and form a thick, slimy coating around themselves.  They accumulate in the broodsac and we’re back at the “here, birdy birdy” stage.

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Now, at this stage of the parasite’s life-cycle, the snail has not stopped moving or functioning.  However, the change in color and aspect (those eerie, rhythmic pulsations) of the snail greatly increases the probability of it being eaten by a bird.  The snail helps – it sits on higher vegetation than non-infected snails.  Even though the parasite adds another 20% to its weight, infected snails also seem to be more mobile and they do seem determined to make themselves available as prey.  And no, they are not blind.  They can use the eyes at the end of their tentacles despite their unwanted guests and have been seen, in studies, to survive at least another year (if they are not eaten first, of course).  So they are not quite the zombie snails that other sites have christened them!

They do, however, seem less interested in sex than “normal” snails – but who can seriously blame them?

Video Credit (no changes) and License

The appearance and behaviour of Leucochloridium paradoxum's branches (sporocysts) are a strange and unusual example of aggressive mimicry: the parasite imitates something the host would normally eat, tricking its way into the host’s body. In this case, the swollen, pulsating broodsacs resemble caterpillars or larvae, which attract birds. What makes this unusual is that, unlike most examples of aggressive mimicry (where the mimic does the eating) here, the mimic gets eaten. It’s a role reversal: the parasite disguises itself not to catch prey, but to become prey and complete its life cycle.

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So, unlike reminiscent but thankfully fictional alien species, the Green-Banded Broodsac isn’t out to destroy humanity - so we don’t really need to worry (unlike the gastropod population). Ripley can take the day off after all.

First Image Credit