Molluscs dump spores on chewed grass then eat crop.
Seaside snails grow their own fungus, new research reveals. They chew grass to prepare it for cultivation then defecate on the wounded blades to fertilize their crop.
The molluscs are the first non-insects found to use such farming methods.
The marsh periwinkle (Littoraria irrorata) lives on salt marshes along the east coast of North America. The 2.5-centimetre-long snail grazes on the grass Spartina but wastes away when fed bitter grass alone, find Brian Silliman of Brown University in Providence, Rhode Island, and his colleague Steven Newell.
Rather than eating the grass, it seems that the snails are preparing its tissues for colonization by their preferred food - fungus. "The wound reduces the plant's defensive capabilities," says Newell who is based at the University of Georgia Marine Institute on Sapelo Island. "The snails return to the wound and eat the fungus that has grown in it."
"We're always ignoring faecal pellets, but they play a vital role [in ecosystems]," says ecologist Roger Wotton of University College London. Limpets may pull a similar trick, he says, by defecating on the patches of rock they have just scraped clear of algae.
Fungal infection severely stunts the grass's growth. Removing the mould, but not the snails, increased plant growth by about 50%, found Silliman and Newell. The interaction between periwinkle, Spartina and fungus shapes about 3,200 kilometres of marshy coastline, the researchers conclude.
It's tempting to say that there's a huge amount of this going on Roger Wotton , University College London
Some beetles bore holes in wood, which are then colonized by fungi that the beetles subsequently eat. Termites and ants have evolved sophisticated ways to tend their fungus gardens, involving weeding and pesticides. Some ants and bees have even taken up ranching, protecting aphids for their honeydew.
The snails' farms are much less complex. But their simplicity suggests that fungus farming may be much more common than we have realized. "It's tempting to say that there's a huge amount of this going on, but we just haven't looked for it," says Wotton.
Silliman, B.R. & Newell, S.Y. Fungal farming by a snail. Proceedings of the National Academy of Sciences, USA, (2003).
University College London
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Whitfield, J. Snails farm fungus. Nature (2003). https://doi.org/10.1038/news031201-2