The platypus is an anthology of weirdness. It has a leathery duck-like bill, a flattened tail and webbed feet. The males have a venomous claw on their hind feet, and the females lay eggs. And if you look inside a platypus, you’ll find another weird feature: its gullet connects directly to its intestines. There’s no sac in the middle that secrete powerful acids and digestive enzymes.
In other words, the platypus has no stomach.
[Green spotted pufferfish, by Starseed; stuffed platypus, by Daniel Baker; Horatio the lungfish, by Jonathon Wilson]
The stomach, defined as an acid-producing part of the gut, first evolved around 450 million years ago, and it’s unique to back-boned animals (vertebrates).
It allowed our ancestors to digest bigger proteins, since acidic environments deform these large molecules and boost the actions of enzymes that break them apart.
But over the last 200 years, scientists have shown that many vertebrates have lost their stomachs. The platypus doesn’t have one, nor do its closest relatives, the spiny echidnas. Lungfish, a group of slender freshwater fish that can breathe in air, don’t have stomachs; nor do the chimeras, bizarre-looking relatives of sharks and rays.
And the teleosts—the group that includes most living fishes—have taken stomach loss to extremes.
Of the almost 30,000 species, it seems that around a quarter have abandoned their stomachs, including groups like wrasse, carp, cowfish, pufferfish, zebrafish and more. (It’s commonly said that pufferfish puff by expanding their stomachs, but while they have a sac in the right place, it’s not a glandular, acid-secreting one, so it doesn’t really count.)
On at least 18 separate occasions, vertebrates have abandoned their stomachs. And we now know that several of these losses were accompanied by disappearing genes.
Xose Puente from the University of Oviedo first discovered that the platypus has lost its main stomach genes, back in 2008. Now, Filipe Castro and Jonathan Wilson from the University of Porto have found the same pattern in other stomach-less vertebrates, like the zebrafish, pufferfish, medaka, platyfish, and Australian ghostshark.
They scoured the full genomes of these species and showed that they’re all missing the genes for the gastric proton pump—the enzyme that acidifies the stomach. They’ve also lost many of the genes for pepsinogens—digestive enzymes that break down proteins. The pufferfish was the sole exception—like the platypus, it has kept a single pepsinogen gene, which it uses for non-digestive purposes. “It’s a clear-cut pattern of gene loss and stomach loss across all of these species,” says Wilson.