Testing time for snails in river estuaries

River estuaries can be dangerous places, particularly if you are a snail. Some are brimming with parasites that first castrate their victims and then convert their testis into factories to make more parasites.

These parasites can be so plentiful their accumulated biomass can actually exceed that of their unfortunate hosts.

These gruesome findings formed only a part of an extensive ecological study of three river estuaries, two in Mexico and one in California. Scientists from the US, Mexico and Panama collaborated on a study of 600km of coastline and 912 hectares of estuary.

The object was to detail the total biomass of free-living species from birds to fish to snails and everything down to 1mm across, and then to gauge the biomass of the infectious species carried by these animals.

The exhaustive study measured the accumulated biological mass including soft and hard parts, of 199 free-living species including one plant variety and 138 associated infectious agents, one that affects plants. Full details of the research findings are published this morning in the journal Nature.

They sampled at 69 sites and measured the density and sizes of anything they found. They then measured the parasitic burden carried by these organisms to get figures for the biomass of host and parasite for each species identified.

The single most important finding is that parasites are much more plentiful that originally assumed. Parasites were thought to make up less than 1 per cent of the total biomass in an ecosystem, but Dr Armand Kuris of the University of California, Santa Barbara, and colleagues found that up to 3 per cent of the total biomass measured in estuaries were parasites.

The total parasitic biomass easily outweighed the biomass of the top predators found in this ecosystem. And a particularly plentiful variety of free-living trematode, a species that includes flukes, accounted for more biomass than all of the bird life found in the three estuaries studied.

The single most abundant parasite was the trematode species that infects and then castrates unfortunate snails. The biomass of these "parasitic castrators" occasionally exceeded that of their uninfected hosts.

These parasites enter their host, castrate them and then take over the testis which are used to produce more parasites that are released to repeat the cycle of infection.

It is the sheer volume of parasitic biomass that so surprised the research team. "This biomass and productivity of parasites implies a profound role for infectious processes in these estuaries," the authors write.

They acknowledge that these estuaries along the Pacific southwest, near Baja, only represent one type of ecosystem. Yet the parasitic content in these watercourses is an order of magnitude higher than previous estimates for parasitic biomass in ecosystems such as coral reefs.