Tuesday, 14 November 2006

What a fluke!

You might think that your life is complicated but at least you're not a trematode parasite*. Trematodes have complex life cycles that involve anywhere between one and four hosts depending on the species. If they miss out on a host or mess up the sequence, they're in trouble. A fluke's existence is very precise.

Leucochloridium starts off life inside a snail and finishes it in a bird. That's rather straightforward for a fluke. It passes from one to the other when a bird eats an infected snail. But how many snails get snaffled by birds? Snails are cryptic by nature. Birds are visual hunters. That's not an auspicious combination for a trematode wanting to get on in life.

So, it changes the snail's behaviour. And then it hangs out a neon light to make sure the bird gets the message.

This is how it happens. The snail (usually an amber snail of the family Succineidae) picks up the fluke's eggs while feeding. The eggs hatch inside the snail and each larva (miracidium) undergoes a transformations into a sporocyst. This is an asexually-reproducing individual that generates multiple cercariae—the next stage in a fluke's life cycle. This all takes place inside the snail. But the cercariae can only develop into adults inside a bird. Somehow, the sporocyst (housing the cercariae) has to engineer the snail's demise.

First, sporocysts migrate to the snail's tentacles. Under the influence of the parasite, the normally light-shy snails start moving about by day. More than that, they climb up plants. Could they be more conspicuous?

Well, yes. The sporocysts are banded in white and shades of green. Not only are they brightly-coloured, they flash on and off. They'd sound a klaxon if they could. (View a Quicktime video of the display here.). It's bad news for the snail but it's another rung on the ladder of life for a Leucochloridium.

(I can't believe I just said that.)


*And if you are, you're doing a fine job of operating your host. I, for one, welcome our new trematode overlords.


David Nelson said...

That video is a must see!


Now I am wondering if those snails, mostly hygromiids, that often climb on plants during daylight (& for which behavior there is no satisfactory explanation) are all infected with trematodes.

Snail said...

Interesting. I know there are some trematode parasites that alter ant behaviour, making their hosts crawl up grass stems so they can be chomped by herbivores.

tapperboy said...

I have seen film footage similar to the video in your "here' link on the TV somewhere sometime. Those pulsating banded snailbits, how could I ever forget.