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How the ghost shark lost its stomach

Lack of digestive organ in fish and other animals linked to genes

LOOK MA, NO STOMACH  A ghost shark  manages just fine, thank you, without the highly acidic digestive zone known as a stomach. And now scientists have a better understanding, from a genetic perspective, why the fish and other animals lack the organ.

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Animals from lungfish and ghost sharks to platypuses have lost their acid-making stomachs over evolutionary time, and researchers have now traced the genetic changes behind these stomach upsets.

True stomachs with digestive glands that concentrate acid and release protein-cutting enzymes called pepsins evolved with vertebrates. The gastric glands arose some 450 million years ago but have dwindled away at least 15 separate times across the animal tree of life, explains Filipe Castro of the University of Porto in Portugal.

More than a quarter of known bony fish species digest food without a true acid stomach. Picking out what drove the evolutionary change is tricky, says Jonathan Wilson, also at Porto. For instance, pufferfishes now repurpose their organ to store food and bloat with water for menacing spines-out displays.

After scrutinizing genes of 14 vertebrates with and without stomachs, Castro and his colleagues determined that none of the stomach losers has high-functioning genes for maintaining a highly acidic zone in their digestive tracts.

The animals also lack or have low-functioning genes for secreting the peptic enzymes that slice and dice proteins under acidic conditions, the researchers report December 4 in the Proceedings of the Royal Society B

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