This groovy-looking, mind-controlling, gender-bending parasite is potentially the only creature alive that can make this author feel sorry for snails. The green-banded broodsac (Leucochloridium paradoxum) begins its life as a microscopic but incredibly hardy egg, spawned by the adult broodsac, which lives relatively harmlessly within the digestive tract of a shore bird. Like a tiny, dirty paratrooper, the egg is eventually excreted by the bird, and this is the stage at which the “grimbiosis” really begins.
If this egg is “lucky”, a very unfortunate snail will discover the birds’ faeces and in (disturbingly) the least disgusting part of the cycle, consume it. The tough eggs are indigestible by the snail, and so can easily spread around the snails’ digestive tract. The eggs then hatch into the larval stage, the miricidia, which thrive in the gut- but these mobile horrors aren’t content to merely thrive- and so they begin to spread around the snail, with especially successful larvae finding their way to the snails eye-stalks. They then metamorphose into immobile sporocysts, and begin to rapidly reproduce, forming the undeniably alien looking “broodsac”. For some unknown (but presumably malicious) reason, the broodsac seems to prefer the left eye stalk, though it is not uncommon to find an infected snail with a broodsac in each eye stalk. Once the broodsac is sufficiently, horrifically, large, the many sporocysts will cease reproducing, and instead metamorphose into cercaria (the first form in which they are able to infect a bird), and encyst themselves (become dormant within a hard “shell”) in order to prepare for their long journey.
This is where the uniquely malevolent nature of the green-banded broodsac comes into play. Rather than waiting patiently and hoping that the snail is eaten by a bird, this nasty parasite instead infects the snails miniscule brain, and alters its behaviour- causing it to seek out light areas, therefore exposing it to a greater number of predatory birds. On top of this, the now large and brightly coloured broodsac begins to pulsate when stimulated by light, making it almost indistinguishable from a large grub or caterpillar- an enticing meal for any passing birds. This concept is known as “aggressive mimicry”, and is surprisingly rare among parasites.
The flamboyant, light seeking snail is then usually, and unsurprisingly, eaten by a bird- and so the horrific cycle can begin again; and, to top it all off, in classic Grimbiosis style, the creature is monecious- meaning it has no gender and is free to reproduce with any of its peers. This seemingly unstoppable parasite is truly horrific and, admittedly, far more disgusting than the average snail.