Japanese scientists have finally placed some strange-looking creatures known as arrow worms on the evolutionary tree, and the findings may be more significant than appearances suggest.
A research team from the Okinawa Institute of Science and Technology Graduate University has reported that the predatory marine worms, also known as chaetognaths (which means “bristle-jaw”), are not, as had been suspected, part of a large group, or clade, which includes molluscs, segmented worms and flatworms.
Instead, they belong to a sister clade – and that, the researchers suggest in a paper published in the journal Current Biology, challenges the classical view that complex organisms evolve from simple ancestors by gaining new traits over time.
“Arrow worms are predators, they have nervous systems, they have developed sensory organs; but the other organisms they’re grouped with are much simpler,” says molecular geneticist and first author Ferdinand Marlétaz.
“If you place arrow worms here, it means there was probably a lot of independent simplification, rather than the independent emergence of complexity.”
There are about 200 species or arrow worm, ranging in size from a millimetre to 12 centimetres, and they share many morphological and developmental traits with other organisms, which has made their history difficult to trace.
However, when Marlétaz and colleagues gathered data from 10 arrow worm species collected from different areas of the Atlantic Ocean and compared it to publicly available data from other animals, they noticed that “bristle jaws” share a unique jaw structure with a collection of microscopic organisms called rotifers, gnathostomulids and micrognathozoans.
“I was a little bit surprised,” admits Marlétaz. “We still don’t fully understand this association with rotifers and the others. That will be the focus of future research.”
Chaetognaths have been around since the Cambrian Period, which ended almost 500 million years ago, and despite their low profile play an important part in marine food chains around the world.