Bone-eating 'zombie' worms drill with acid

Osedax worms living on a whale bone (c) Greg Rouse Bone-eating marine worms use acid to feed on the skeletons of fish and whales

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Deep sea worms use acid to eat the bones of seabed skeletons, according to US scientists.

The so-called "zombie worms" of the Osedax family are known to bore into bones and remove nutrients.

Fresh analysis of the root-like tissues the worms use to attach to bones has identified acid-secreting enzymes.

Until now scientists did not understand how the tiny creatures fed on bone, as they lack the body parts needed to "drill" physically.

Dr Sigrid Katz from the Scripps Institution of Oceanography, University of California, San Diego will present the team's research at the Society for Experimental Biology's annual conference.

Worms' world:

Polychaete worm marine habitat
  • Osedax are types of segmented worms (like earthworms and ragworms) in the Polychaeta group.
  • Polychaete worms are mainly marine-living and are more commonly known as bristle worms.
  • They can range in size from 1mm up to an astonishing 3m.
  • Some of the most recognisable of these worms are the Christmas tree worms, named for their conical, spiky resemblance.

Found at the bottom of the sea living on the fallen skeletons of whales and fish, the unusual group of worms have caused fascination since their "accidental" discovery in 2002 by the Monterey Bay Aquarium Research Institute (MBARI).

The MBARI team provided the whale bones and specimens used by Dr Katz and her colleagues.

Scientists were perplexed when they only discovered females but further investigation revealed that the males remained in their microscopic larval stage, living inside the female worms.

The unusual group's name Osedax is Latin for "bone devourer", and the worms have no mouth, gut or anus yet are still able to remove nutrients from bones.

Previous studies have revealed that symbiotic bacteria inside the worms digest the fats and oils extracted, but the question of how the worms physically bore into the bones had been a mystery.

Close analysis of the worms failed to find any abrasive structures the worms could use to mechanically "drill" into bone.

This prompted Dr Katz and colleagues to investigate whether the worms had a chemical strategy for penetrating the bones.

By analysing the worms' tissues, the team found that acid-secreting enzymes were abundant in the root-like parts that attach to bones.

"The acid is secreted through the skin of the roots region," said Dr Katz.

Osedax marine worm green "root" structure (c) Greg Rouse The acid released from the green-coloured "root" demineralises the bone

"The skin cells in this region are very long cells and the upper end has lots of [microscopic protrusions, which] enlarge the surface multiple times, so lots of acid can be secreted," she explained.

Seventeen species of the worms have been recorded worldwide, including in the North Atlantic ocean off the coast of Sweden, and the Pacific Ocean off the coasts of Japan and California.

Last year BBC Nature reported that traces of the worm have also been discovered in a fossil from the Mediterranean Sea.

Dr Katz said she hoped her discovery would help scientists to unravel the "mysterious lifestyle" of the worms.

"In the past 10 years we have learned a lot more about them, but there are also still a lot of questions to answer on their physiology and especially about nutrition, nutrient uptake and host-symbiont interactions," she told BBC Nature.

"The fact that we now know how they penetrate the bone is one step in understanding the functioning of this symbiosis."

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