August 21, 2003

Sea creature confused for its lunch

Worm victim of mistaken identity reclassified.

by Helen R Pilcher
Nature News

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Imagine being called a bag of chips, just because that was your last supper. This unlikely fate befell an obscure marine worm, new research has revealed1.

The amorphous greenish-yellow sea creature was wrongly classified as a mollusc because specimens contained mollusc DNA and eggs. "Basically, the mollusc material was lunch," explains animal evolutionist Maximilian Telford of the University Museum of Zoology in Cambridge, UK.

Telford's team compared the DNA in fresh finds dredged up from soft mud 100 metres under a Swedish fjord. They conclude that the creature - called Xenoturbella bocki - is a worm.

It is the most primitive existing member of the group to which humans belong, called the deuterostomes, the authors argue. "It tells us what we evolved from 500 million years ago," says Telford. Deuterostomes include mammals, fish, starfish and worms.

The creature's rocky road to recognition began with its discovery more than 80 years ago. It's lack of recognizable features - it has no separate mouth or anus and no body cavity - led to its first labelling as a primitive flatworm.

Subsequent studies related it to acorn worms and starfish, because of similarities in its diffuse nerve network and outer epidermal layer. Then, in 1997, it received the lamentable mollusc moniker.

A preference for sludgy sea habitats makes Xenoturbella difficult to find, so researchers can only guess at their numbers. "Quite possibly, they're everywhere," says Telford, "but they're not the sort of thing you trip over on a beach."

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