To a science-fiction filmmaker, the concept of being controlled by unseen forces is creative gold, but for the rest of us, it's a fairly unsettling prospect. But like it or not, it's clear that parasites - creatures that live off (and often control) the bodies of others - are an integral part of the world we live in and carry an influence that far exceeds their small size.
Now, a painstaking survey of the residents of river estuaries shows that parasites do indeed punch above their weight, and they aren't slouches in that department either. Despite their tiny size, their combined mass eclipsed that of the top predators in the area and their influence extended wider still. It's a parasite's world and we're just living in it.
Over five years, Armand Kuris and Ryan Hechinger from the University of California, Santa Barbara led an exhaustive census of life in three Californian estuaries. At 69 different sites, they assessed almost 200 species of free-living animals, from high-flying birds to burrowing shrimps, as well as the 138 species of parasites hitching a lifestyle on their bodies.
Using a wonderfully old-school combo of nets, binoculars, meshes, sieves and scales, the team collected samples of the estuaries' inhabitants and dissected over 17,000 individual animals. Their bodies were dissected and their parasites removed, classified and weighed.
Snails, crabs and bivalves (hinged shellfish) made up the bulk of the local fauna but every single animal group living in the estuaries was infected with parasites of some description. On average, these freeloaders made up about 2% of the weight of their hosts. That may not seem like much, but it's about ten times more than previous estimates; it was always assumed that parasites, being very small, would weigh next-to-nothing despite their weighty influence. Now, we know that isn't true.
Tapeworms and parasitic crustaceans were very common, but lording over all of them were the trematodes or flukes. This group of parasitic worms alone weighed as much as the local fish and outweighed the birds - the local top predators - by three to nine times. Pound for pound, the estuaries were home to more parasite tissue than bird tissue.
Kuris and Hechinger also found that some parasites use particularly successful strategies. The dominant group castrated their hosts by infecting and shutting down their reproductive systems. Castrators are the ultimate body-snatchers for their neutered hosts have zero chance of passing their genes down to the next generation. In effect, their usurped bodies become living shells for the parasites' own genes - a superb example of Richard Dawkin's "extended phenotypes".
Of the many strategies used by local parasites, this was the most successful, so much so that the combined weight of castrated bivalves, snails, shrimps and crabs was almost the equal of their fertile peers. Remember that these were the most common animal groups around, and take a second to reflect on that sobering statistic. It means that a massive proportion of the flesh in the estuaries is under parasite control.
Take the California horn snail, Cerithidea californica. It's the most abundant single species in area that Kuris and Hechinger studied and it's infected by 18 species of trematode. Almost all of the largest snails were castrated and on average, 22% of an infected snail's body was made up of parasites. The snails had been converted into trematode factories. In the pictures above, you can see that the snail's orange testes (left) have become corrupted by trematodes (right).
Every day, the infected snails released a swarm of cercariae - a part of the trematode's life cycle that swims with a tadpole-like tail. Each of these lives for just a day, but the snails discharge so many into the water that the combined weight of the cercariae alone also dwarfs the mass of local birds by three to ten times. The snail's tale illustrates that parasites have influence far beyond their already considerable weight. They may have the substantial combined mass of larger animals, but because they don't need to move or forage, they also enjoy the high productivity typically enjoyed by microscopic organisms.
Kuris and Hechinger admit that they only looked at one specific ecosystem, but they believe that their estimates will apply elsewhere and that they are, if anything, too conservative. If they are right, no consideration of food webs or ecology would be complete without taking the power of the local parasites into account.
Reference: Kuris, A.M., Hechinger, R.F., Shaw, J.C., Whitney, K.L., Aguirre-Macedo, L., Boch, C.A., Dobson, A.P., Dunham, E.J., Fredensborg, B.L., Huspeni, T.C., Lorda, J., Mababa, L., Mancini, F.T., Mora, A.B., Pickering, M., Talhouk, N.L., Torchin, M.E., Lafferty, K.D. (2008). Ecosystem energetic implications of parasite and free-living biomass in three estuaries. Nature, 454(7203), 515-518. DOI: 10.1038/nature06970
Photos by Todd Huspeni and Kevin Lafferty