It’s a tough world out there, even for a parasite. Or perhaps, especially for a parasite. I know, I sound a little crazy, but bear with me.
Consider the life of a few of the different animals on our planet:
Humans, Homo sapiens sapiens: The average human is born to a parent(s) who cares for you for close to 2 decades, providing food, shelter, and education. Humans can eat a broad range of food items from vegetables and grains to meat. To pass on your genetic material (your DNA and everything in encodes), you need to find only one partner. [I am focusing on the biology of Homo sapiens, and oversimplifying. Certainly our lives are socially much more complex and where we live also shapes our lives.]
Snow leopard, Panthera uncia:
The snow leopard is a solitary animal, until a female gives birth to cubs. Then she raises her cubs for almost 2 years, providing shelter, protection, and food. The snow leopard is at the top of the food chain, but that doesn’t mean it’s easy to find food. It has to hunt for every meal, and food isn’t common or easy to catch (especially during the harsh winter months), so a leopard will often go days without having a meal. Finding food presents an even greater challenge for a female with cubs. If she doesn’t find enough food for her cubs, they will die.
Birds of paradise: These birds are born from eggs and receive parental care (parents feeding and protecting the hatchlings) until the young birds are capable of flying and hunting for food on their own. To pass on their genetic information, the males must groom his beautiful feathers and perform a song and dance for any female that might be nearby. Only if his performance is of high quality (or at least better than any other in the area) will he be able to mate with the female and pass on his genes. This is a skill that takes sometimes years to perfect.
Periodical cicada: These insects live as young cicada, or nymphs, underground at depths of up to 8ft for 13 or 17 years. After their long stay underground, en masse, they dig up to the surface and metamorphose into adults with wings. They live as adult for only a few days during which time they must mate, lay their eggs, and then die. When their offspring hatch, they dig down into the earth, and wait for 13 or 17 years.
As we can see, the life of an animal, depending on the critter can be pretty complicated. So you might be thinking, how could a parasite possibly have it tough? It has food and shelter and doesn’t have to work for it because it’s inside another organism doing all the work. Right?
The life of a parasite isn’t quite that simple. It’s true that internal parasites have shelter inside their host and also steal nourishment, but there’s more to the story. To illustrate, let’s focus on one group of internal parasites: trematode flatworms or flukes (blood flukes, liver flukes, etc.)
Before a trematode can reap the benefits of being a parasite, it must find its host. For a trematode parasite, this is no small order. Trematodes usually require 3 different hosts in order to complete their life cycle, and not just any animal will do. These parasites are very specific in which animals they can infect at specific phases of their life cycle (so there’s no cheating and skipping ahead). Let’s break down a typical trematode life cycle:
1.) A typical trematode is released into the water column as an egg and out hatches larva #1 (miracidium). This first larva infects host #1, usually a snail which incidentally ingests the miracidium while grazing (and it has to be the right species of snail for the species of trematode).
2.) Once inside the snail (and surviving the digestive system and immune system attack), the miracidium larva metamorphoses into larva #2, the sporocyst. The sporocyst is essentially a clone factory.
3.) In keeping with its job as a clone factory, the first sporocyst makes more sporocysts* (all genetic clones). These daughter sporocysts make larva #3, the cercaria (also all genetic clones, but different in body form).
4.) The cercariae must now leave their snail host to find host #2. Thousands of cercariae will exit the snail simultaneously (meanwhile the sporocyst clone factories keep making more) and find themselves swimming in open water. Cercariae survive for only a few hours and aren’t very good swimmers, so they can’t move far from where they exit the snail host. If a cercaria is lucky, it encounters host #2. Host #2 can be anything from a worm to a fish.
5.) Once inside host #2 (and again assuming they managed to enter the host and survive the immune system attack), the cercaria metamorphoses into larva #4 metacercaria. The metacercaria “balls up” and form a protective cyst around themselves. At this point it’s a waiting game. What are they waiting for? Host #3, AKA the predator of host #2.
6.) Host #2 is consumed by host #3 (and not just any predator will do, it has to be the right species, and is almost always a vertebrate). During the digestion of host #2, the metacercaria escapes and enters host #3 tissues (by now, I think you know what this assumes). The metacercaria metamorphoses, for the final time in the trematode’s life, into an adult.
7.) The adult trematode will live out its life inside this final host. But there’s still one more step. The adult trematode needs to mate with another trematode in order to reproduce and release eggs (via the host’s feces) into the world.
And repeat the cycle all over again.
So let’s recap: The trematode parasite has to metamorphose through 4 larval phases, locate 3 different hosts in the right order, and survive the repeated onslaught of digestive and immune system attacks. There’s a little bit of serendipity in the life of an adult parasite. It was one of only millions of its siblings (and one of millions of clones) to survive the whole way.
So like I said, it’s not so easy being a parasite.
References and Resources:
Snow leopards: http://www.snowleopard.org/
Birds of paradise: http://www.birdsofparadiseproject.org/content.php?page=87