"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift
February 28, 2010
February 28 - Edwardsiella lineata
It seems that just about every group of organisms has evolved parasitism, but we haven't seen one of these yet - a parasitic sea anemone. And what do parasitic sea anemones feast on? (Don't be afraid...) Ctenophores - or comb jellies - are the hosts to these parasites. It's just the larvae of Edwardsiella lineata that are parasitic and the evidence is such that it's thought that this is a very recent strategy as all other members of the family have free-living larvae. Larval E. lineata embed themselves in their ctenophore hosts (they can do this from either direction - they can burrow in from the outside, or they can go through the gut if the ctenophore eats them first!). Once in position, they extend their mouths right into the ctenophore's gut, and ingest anything that the comb jelly has worked hard to eat. When it's had its fill, it simply removes itself and transforms into a regular looking sea anemone larva. Is it content to stop there? No, not always, as it turns out. Sometimes it is – and develops into a polyp (the anemone-looking thing). But sometimes it actually invades a second ctenophore and reverts back to the parasitic body form while it sucks away at more of the new host’s food. Greedy little buggers, aren't they?
Image and more info from this paper.
Click on the image to enlarge.