Roy L. Caldwell, Department of Integrative Biology, University of California, Berkeley, California.

Stalk and strike: Mantis shrimp hunt in a similar fashion as praying mantises, capturing prey with a specialized pair of legs.

Tiny brain structures called "mushroom bodies" play a key role in how insects learn and remember. But their arthropod cousins, crustaceans, appear not to have them.

Now, a new study in the journal eLife refutes that view, saying crustaceans have mushroom bodies, they've merely been modified.

"Consider it like basically a set of motherboards which really haven't changed very much. What's changed have been the peripherals and the intermediate computational devices," said lead author Nicholas Strausfeld of the University of Arizona Department of Neuroscience.

Nature is thrifty; it keeps what works and reinvents only when necessary — though it's not always easy to tell which has occurred.

"There's always been this debate amongst biologists, whether a structure in Species A that looks like a structure in Species B is actually the same structure but slightly modified, or has it or has it arisen independently?" said Strausfeld.

Cousins like insects and crustaceans retain more structures in common than, say, insects and humans, even if insect mushroom bodies and the hippocampuses of the human brain have similar functions.

Gabriella Hannah Wolff, et al., eLife Sep 26, 2017.

Mantis shrimp mushroom bodies reveal striking similarities to those found in insects. Shown here are the characteristic orthogonal networks of nerve fibers from the mushroom body of a mantis shrimp (top) and a cockroach (bottom).

Yet, strangely, researchers have long held that crustaceans and insects evolved different structures for learning and memory.

As Strausfeld explained, the notion that the "hemiellipsoid bodies" found in crustaceans were essentially different than the mushroom bodies found in insects might have begun with a misinterpretation.

In his 1882 study of the brains of a certain species of mantis shrimp (Squilla mantis), Itlian scientist Giuseppe Bellonci referred to two structures, the "corpo emielissoidale" ("hemiellipsoid body") and the "corpo allungato" ("elongated body") that together made up the cap and the columnar lobe of the crustacean mushroom body.

Years later, in 1924, Swedish scientist Bertil Hanström continued Bellonci's work, noting that all crustaceans except the above-mentioned mantis shrimp appeared to have lost their lobes. But what remained was still a mushroom body, he said.

"And nobody seems to have read that properly, including myself — some years ago — because the term 'hemiellipsoid body' was then used to suggest a completely independently evolved center in a crustacean," said Strausfeld.

Strausfeld has since written a paper detailing how terminology "led everyone up the garden path until very recently." Indeed, the notion that crustaceans don't have mushroom bodies persisted in the scientific literature for years.

But when Strausfeld and his colleagues examined a number of crustaceans, they found their so-called "hemiellipsoid bodies" possessed the 4-5 traits common to mushroom bodies in insects — along with nine more.

For further proof, researchers turned to a molecular marker related to mushroom bodies in the common fruit fly (Drosophila melanogaster). When they applied an antibody against that marker to crustacean brains, their "hemiellipsoid bodies" reacted in just the way a mushroom body would have.

"If you use that antibody against the brain of any crustacean — or actually almost any arthropod — it will light up paired centers in exactly the right position in the brain that corresponds to mushroom bodies in insects," said Strausfeld.

Gabriella Hannah Wolff, et al., eLife Sep 26, 2017.

When researchers stained mantis shrimp brain tissue for learning and memory proteins and studied them under the microscope, they "lit up very intensively" (left: mantis shrimp; right: cockroach; center: 3-D reconstructed mushroom body of a mantis shrimp)

That could mean mushroom bodies occur not just across crustaceans, but throughout arthropods in general, or even beyond. After all, the same antibody also lights up hippocampuses in vertebrates, from lampreys to mammals.

"So it's not just an exploration of the brain of a crustacean, but it's part of a broader exploration of brains across the phyla—across vertebrates and invertebrates of various kinds," said Strausfeld.

Why mushroom bodies in crustaceans have evolved such a variety of forms while those in insects have generally remained the same remains unknown, but Strausfeld hypothesizes it might have something to do with the need to exploit three-dimensional environments.

That would explain why reef hunters like mantis shrimp have retained their lobes, while crustaceans that dwell in more two-dimensional zones, like the sea floor, have lost theirs.