Many of the world's most unfamiliar species are just sitting around on museum shelves collecting dust. That's according to a report in the Cell Press journal Current Biology showing that it takes more than 20 years on average before a species, newly collected, will be described.

It's a measure the researchers refer to as the species' "shelf life," and that long shelf life means that any conservation attempts for unknown, threatened species could come much too late.

The problem, the researchers say, is due to a lack of experts and of the funding and resources needed to do the job.

"Species new to science are almost never recognized as such in the field," says Benoit Fontaine of Museum National d'Histoire Naturelle in Paris.

"Our study explains why it often happens that we describe species which were collected alive decades ago and which can be extinct now-just as astronomers study the light of stars which do not exist anymore."

Part of the problem is that many species are rare and may be represented in collections by a single specimen. Taxonomists will usually wait until more specimens of any new species are available before they will describe it.

In that sense, increased effort to seek out new species and specimens in the field would help to move things along in the world's museums and herbaria, the researchers say.

Fontaine and his colleagues calculated shelf life based on a random sample of 600 species described in the year 2007. The data show that those species had a shelf life of 20.7 years on average, with a median of 12 years.

Shelf life did vary according to biological, social, and geopolitical biases, they report. In fact, amateurs as a group describe new species more rapidly today than professionals do.

The findings come as yet another reminder of how much there still is to do when it comes to understanding and protecting the diversity of species on Earth.

"Our knowledge of biodiversity is still very scarce," Fontaine says. "Describing new species is - or should be - part of the everyday work of taxonomists, and we need to hurry; new species are disappearing faster than we can describe them."

Fontaine et al.: "21 years of shelf life between discovery and description of new species"