The differences between a tiger and a lion are easy to spot. But even to the trained eye, two species of earthworms can be tough to tell apart. Indeed, what was previously thought to be one species of common garden worm may in fact be four, according to research published online 8 October in Molecular Ecology. The surprising findings, say the ecologists who authored the report, may have implications for the use of pesticides in agriculture.
Earthworms play a vital role in food production. They help decompose organic matter, and their burrows keep soil drained and ventilated. Without earthworms, farmland would not be as productive, even using the most powerful fertilizers.
The new study focuses on Allolobophora chlorotica, one of the dominant worm species wriggling through European agricultural land. Scientists have previously recognized two types within this species, dubbed green and pink morphs because of color differences. The greens inhabit areas covered with grass and relatively wet soils, but the pink morphs prefer gardens, drier areas, and woodlands. Although the two morphs have distinct preferences for soil moisture, mixed populations are not uncommon. Yet crosses between the green and pink types produce sterile hybrids. "People did suspect that they were two distinct species, but apart from the color differences, the two morphs were indistinguishable," says lead author of the paper, ecologist Andrew King of the University of Cardiff in the United Kingdom.
King and his colleagues didn't set out to confirm that suspicion. They were originally studying what farm beetles eat, and earthworms are one of their favorite meals. But when genetic characterization of the contents of beetle guts gave some unexpected results, the researchers decided to take a closer look at earthworm DNA. They sequenced the DNA of 270 A. chlorotica worms collected from 24 British sites and five mainland Europe locations. The two green and pink morphs were confirmed as distinct species, but King's group found the pink morph can be further subdivided into three species. Genetically, the three pink species are further apart than even humans and orangutans, King says. He suspects that further sampling in Europe may uncover yet more cryptic types of worms, species that look identical but are indeed separate on a genetic level.
The identification of the new species could have major implications for the study of pesticides, which have to be tested on worms and other invertebrates to prevent collateral damage to these beneficial organisms. "It is possible that the different cryptic species will have different tolerances to these chemicals," says King.
The findings are "exciting and challenging," says Rob DeSalle of the Sackler Institute for Comparative Genomics at the American Museum of Natural History in New York City. "The study is an example of the problems that modern biologists face when trying to delineate species boundaries." Future research, he says, should look at the behavior and ecology of these worms to understand what caused the new species to arise.
Cardiff University video describing the research. (Video posted by BBSRC on YouTube.)