Thursday, April 14, 2005

Parasites can be disturbingly influential

Today, I read that the human tapeworm’s lifespan is 18 years. In that time, it would have laid (passed out of the human host’s system in the faeces) about 10 billion eggs. That’s 10,000,000,000.

My favourite parasite story was from Discover magazine way back in mid-2000. The parasite in question infects a certain species/genus of crabs. The following is derived from memory and by no means completely accurate.

When the parasite finds an uninfected crab, it crawls along her shell, looking for joints in the crab’s limbs. At the joint, where the shell is not continuous (to facilitate movement, of course) the parasite penetrates the soft covering and injects a tiny portion of itself into the crab’s interior.

To fully understanding this process, let’s consider a lobster’s growth. At certain stages in its life, it sheds its shell and climbs out of it, discarding the unwanted shell. In the case of our parasite, the unwanted portion is its entire body, and the remaining fraction of several cells is squirted into the depths of the crab.

In the crab, this small group of cells begin to multiply and spread. They start to form tendrils that penetrate into the blood supply of the crab, extracting processed nutrients from her circulation. Soon, she begins to develop sacs on the lower side of her abdomen. In normal crabs, these would be the egg sacs. However, in our infected crab, these sacs are populated with the eggs of the parasite. Nonetheless, she lovingly cares for the sacs, cleaning them periodically and generally keeping them well protected.

It must be noted that the infected crab is genetically dead- it can no longer reproduce. All its hunting and foraging goes to the parasite’s propagation. When the time is ripe for the parasite’s larvae to be released into the environment to continue with their journey, the crab climbs up to a rock or similarly elevated area and releases the eggs. In the ocean’s currents, she would bob up and down to facilitate the clouds of larvae issuing from her abdominal sacs, waving her pincers in the water above her to help the little parasites along their way down the road of life.

If the infected crab is a male, its naturally narrow abdomen will start to widen. This wider abdomen then starts to develop egg sacs.

Here’s a parasite site with some “Eeew!” factor: HSS [caution: material may be disturbing to certain individuals]