Scientists in New Zealand have shown for the first time that sedentary coastal creatures can travel long distances at sea by hitching a ride on seaweed.
Researchers have long thought that animals such as molluscs and sea stars, which cannot travel far under their own steam, must 'raft' from one coast to another. But so far evidence for rafting has been circumstantial.
Now a study published today in the journal Proceedings of the Royal Society B by researchers from the University of Otago in New Zealand have confirmed such a trip.
Dr Ceridwen Fraser, a zoologist at the university's Allan Wilson Centre for Molecular Ecology and Evolution, and colleagues analysed bull kelp, a monster-sized seaweed often found floating in the Antarctic Circumpolar Current of the Southern Ocean.
Genetic tests on specimens of bull kelp washed up on a beach in the country's South Island showed that the kelp must have come hundreds of kilometres from either the sub-Antarctic Auckland or Snare Islands.
Bull kelp is usually anchored to coastal rock by a strong root-like structure called a 'holdfast'. Animals can burrow into these dinner plate-sized structures, creating diverse communities, which remain when the kelp breaks off.
Tests on a type of crustacean found in the holdfasts showed that it too had come from the same islands. Along with the crustacean were molluscs, and a type of limpet, sea spider, snail and seastar.
"We can really see that this raft has come hundreds of kilometres across the ocean carrying animals and that they've survived the journey," says Fraser.
"I'm sure that this is happening all around the world. There are other types of seaweed that float in the Northern Hemisphere - it could be a major transport system for a lot of coastal animals," she adds.
Co-researcher Dr Jonathan Waters says the next step is to look for the phenomenon on a global scale.
"We'd really like to go and find that smoking gun of something that's gone from one side of the world to the other - from New Zealand to Chile, for example."
Bull kelp doesn't grow in Australia but it has been found on beaches so it could easily be bringing species to Australia, says Waters.
Dr Cynthia Riginos, an evolutionary biologist at the University of Queensland, describes the research as "very interesting".
"Most marine species have swimming planktonic larvae that can move across large stretches of open water - for them, long distance movements are expected."
"For organisms [such as these] where there is no swimming stage, this helps to understand how one species can be found among geographically widespread locations."