The Green-Banded Broodsac is name that is equally horrifying and mysterious. The fact that they were given the descriptive title of 'green-banded' suggests that we live in a world so full of organisms befitting the name of a broodsac to need clarifiers. This alone should be enough to haunt your dreams for weeks to come. But it does leave one with the simple question of what it could possibly be, other then horrifying.
The Green-Banded Broodsac (Leucochloridium pardoxum) is a trematode, a class of flatworms (phylum Platyhelminthes) known for their exclusively parasitic nature. Trematodes, more commonly known as flukes--and in the case of the Broodsac, a Digenean--go through a series of developmental stages that take them through multiple host species. In L. pardoxum, the lucky contenders are the Amber Snails (genus Succinea) and various species of shore birds.
The Broodsac's primary hosts are the shore birds. Here they live in the digestive tract like many other Trematodes, feeding off of allthe extra nutrients. Other than depriving their host of some nutrients, they don’t really hurt the birds, which is good for the Broodsac as if the bird dies, so do they.
The adults live out their lives passing their eggs out through the digestive tract of the infected birds. They are monecious, meaning they possess both sexes and can either self-fertilize or cross fertilize when multiple individuals are close enough together, so there is always a 'healthy' supply of eggs.
The eggs lie in wait for a hungry snail to come along to feed on the bird droppings that they reside in. Once consumed, the eggs hatch into the initial larval stage, the miricidia. These miricidia start out by thriving in the gut but soon begin to spread through the body of the snail as their numbers grow, with the luckiest finding themselves near the head. It is here that they metamorphose into their next stage, the sporocyst.
While the miricidia are capable of movement, the sporocyst is not. At first they begin to rapidly reproduce themselves until they have developed into a large ‘brood sacs’. It is from this stage that they get their shiver inducing name. As the brood sac grows in size, it begins to invade the eye stalks of the snail, preferring the left one for some utterly unknown and possibly terrifying reason, though it is not uncommon to see snails with both eye stalks infested.
The sporocysts within the brood sac can either divide into more sporocysts or they can produce for yet another stage, known as the cercaria. The brood sac quickly becomes filled with hundreds of cercariae which are the final larval stage of these horrors, and the first stage that can actively infect their primary hosts. Many of the cercaria will encyst themselves, becoming metacercaria, to preserve themselves for when they enter a more suitable habitat, namely the gut of a bird.
But how can such tiny creatures hope to go from their comfy, bloated sack of a home inside the head of a snail to the promised land that is the intestines of a bird? Once again the titular broodsac comes into play. There is a reason why the broodsac forms specifically in the eye stalk and head. As it grows in size, it begins to gain a very specific coloring, hence the green-banded portion of L. paradoxum's common name. The bands of green and yellow very closely resemble the markings on certain species of caterpillars. Particularly, species of caterpillars that are commonly fed upon by birds.
But the broodsac doesn't stop there. It has already reduced the vision of the snail enough to hinder its ability to hide but evolution was not content with merely impairing the depth perception of the hapless snail. Instead it begins to alter the behavior of the snail. Snails naturally prefer the dark; this is because there is more rotting detritus for them to feast upon and fewer predators to eat them. But L. paradoxum wants desperately to get back into its primary host. So it over rides the snails preference for the dark and causes it to seek out the light where it will be more noticeable to birds.
Once in the light, the brood sac begins to twitch. The more light it is exposed to, the more rapidly it will convulse in a way that, to a bird, resembles a snack that cannot be turned down. This behavior, known as aggressive mimicry, is usually reserved for predators so that they can sneak up on their prey, but here, the broodsac has used the technique to attract its future primary host.
Once the snail is eaten--or just the infected eye-stalk which can potentially grow back and become re-infected by the miracidiae still thriving inside the snails gut--the cecariae collect inside the intestines and develop into the adult form. Any metacecariae will come out of their stasis as the surrounding tissue they have encysted themselves within is digested away and the remaining sporocytes will develop into cecariae as well, which in turn also develop into the adult form; thus creating a large and stable population.
The adult worms, then go about their happy lives doing little more than reproducing and feeding off the various nutrients they are constantly bathed in. Spewing out a steady stream of eggs that, with any luck, will go on to infect and deform other helpless slime crawlers.