Saturday, January 19, 2013

Leucochloridium paradoxum

Brood Sacs of L. paradoxum
 Sometimes (in fact, more often than not) parasites aren't satisfied with living inside a host, they feel the need to actually manipulate that host.  This is often the case with intermediate hosts like ants and snails that only serve the parasite for a part of the parasite's life cycle.  Parasites need to make it to their final host in order to reproduce and live out their adult lives.  In many instances, this means that the intermediate host needs to be manipulated in some way that attracts the parasite's definitive host so that the parasite doesn't have to wait around and hope that the intermediate host gets eaten by the right host quickly. This parasite is one such manipulator, and how it manipulates its snail host is a thing of both beauty and amazement.

Leucochloridium paradoxum is also called the "green-banded broodsac" and is a type of flatworm belonging to phylum platyhelminthes.  It is a type of fluke, further classifying it as a member of class trematoda. It is a digeneid worm that falls into order strigeidida alongside the human parasite Schistosoma.  It belongs to family leucochloridiidae. It belongs in the genus leucochloridium, which houses many parasites of snails and other invertebrates.

Life Cycle
This parasite begins its life as an egg falling from the sky surrounded by a safety net of bird feces.  After landing in a splatter on the ground, a tree branch, or other area frequented by snails it waits for someone to come along that likes to eat bird droppings.  Enter an amber snail of the Succinea genus that shows up for a nice bird poop meal. The snail feasts upon the bird's feces and ingests our little parasite egg. 

The parasite makes its way out of the egg and takes the form of a miracidium. After living in the gut a short while, the miracidium begins to wander around the body.  Some of the miracidia will wind up near the head as the others overtake the snail's internal organs. The ones that reach the head change into their next developmental stage known as a sporocyst stage. In this stationary stage, the parasite begins to replicate itself until it produces a large "brood sac" that grows in size and begins to invade the snail's eye stalks. Here the sac begins to take on specific color patterns of yellow and green bands as it continues to grow. Within the sac, sporocysts are being produced, but so are the next life stage known as the cercaria stage.  Some cercariae stay in the sac as they are while others change into a metacercaria stage to await a bird gut paradise. The invasion renders the poor snail's vision in the infected eye stalk useless. (Most of the time only the left eye stalk is infected, but there plenty of instances were both eye stalks fall victim to these parasites.) 

Now the parasites override the snail's natural instincts to stay in dark areas safe from bird predators and forces the snail our into the light.  The light instigates a twitching movement of the brood sac.  This twitching becomes more rapid as the worms are exposed to more light.  The convulsing action of the eye stalk coupled with the coloring of the underlying brood sac looks like caterpillars to birds.

Along comes a bird looking for a tasty snack, and there goes the snail. In some cases, the birds only snag the eye stalk rather than eating the whole snail. When this happens, the snail is able to regrow its eye stalk, but the stalk is likely to be reinfected by some of the parasites already living in the snail's guts.

Now the cercariae in the bird's gut can make their way to the intestine and become adult worms. The metacercariae and sporocysts will change into cercariae and follow suit.  This bird's last meal has now created a new, large, vibrant parasite population with the bird's body.  The adult worms are monecious (having both sexes' reproductive parts within one individual) and will begin to cross-fertilize with other individuals or will self-fertilize to create eggs.  The eggs will be passed in the bird's feces, thus completing the parasite's circle of life.

That's for the Birds!
Despite the way that these parasites disfigure their snail hosts, they don't really cause any harm to their bird hosts. They live in the bird's intestines eating waste material and pumping out a LOT of eggs to ensure their species' survival.  After all, not all bird droppings get eaten right away...many dry out, which kills our little parasite eggs before they can be eaten by a snail.  Some sources say that shore birds are the preferred host for this parasite, and others say that birds like crows, sparrows, and finches are more likely to play host.  The only real criteria for a definitive host seems to be that the host be a bird living in temperate North American or European forests that house amber snails and like to eat green and yellow-banded caterpillars. The types of birds that do play host to the parasite are rarely eaten by humans, therefore human infection with this parasite is unlikely.

Peckhamian Mimicry and Extended Phenotypes
One of the coolest things about this parasite is how it can be used to teach people a variety of biological concepts.  First and foremost it displays an incredible ability to manipulate host behavior, a fundamental concept in studying parasites. It also brings up an important point about fecundity in organisms that have high egg mortality rates. If you know that many of your offspring are going to die, you can ensure your species' survival by producing an overabundance of eggs to compensate. These worms developed the ability to pump out lots of eggs in response to having a low egg survival rate.  That topic leads us into a discussion of evolution, which blows my mind with regard to how this species ever evolved such a complex and unique life cycle.

Continuing with the concept of evolution, one can bring up the idea of extended phenotypes.  An extended phenotype is the idea that a phenotype (outward expression of an internal combination of genes) does not have to be limited to internal biological processes like the creation of proteins that give rise to physical features. The idea is that one's phenotype can be extended to include the effects of a gene on the environment outside of an individual organism's body. This parasite is a good example of such effects.

It is a good example of a form of mimicry called aggressive mimicry or Peckhamian mimicry.  In this type of mimicry, an organism pretends to be a prey item or a member of the opposite sex to take advantage of a predator or potential mate.  We see this with Australian katydids that mimic the sounds of cicadas to attract and eat cicadas of the opposite sex.  We see this with many other animals that will mimic sounds, behaviors, pheromones, and even the way that a prey item/mate looks to be able to get what they want.  In this case, the worm pretends to be a caterpillar and gets eaten, but this is all part of an intricate plan to take over the bird's gut and convert it into a trematode egg factory and distribution center.

Moral of the Story
Although Leucochloridium paradoxum has no ecological impact on humans, is certainly interesting and important for teaching budding biologists about the wonders of nature.  Like most parasites, its life cycle is utterly fascinating. Yet despite all of the nasty things it does to the snail, it doesn't really hurt the bird at all.  And the snail doesn't necessarily die from all the abuse, but I'm sure there are some emotional issues one might develop from having an eye plucked out by a bird after being invaded by green-banded eye bandits if snails do in fact have emotions. Below is a link to a video of a pulsating infected eye stalk. You should definitely check it out! Gotta love some nature footage! ;)


  1. Imho its breathtaking... as fascinating as Fasciola hepatica and Dicrocoelium dendriticum :o


  2. How can the parasite manipulate the host stay in the light for the predators (birds) to eat them?