Study Reveals Origins of Manta Ray’s Horn-Like ‘Cephalic Lobes’

Manta rays and their relatives of the family Myliobatidae have massive, flapping fins as well as a pair of fleshy projections called cephalic lobes. A new study, published in the journal Frontiers in Ecology and Evolution, shows that these two very different features have the same origin — a discovery that reflects an important lesson for understanding the diversity of life.

Manta rays. Image credit: Masako Uchida.

Manta rays. Image credit: Masako Uchida.

All types of fish have two sets of paired appendages, like fins. But somewhere in their evolutionary past, a group of rays appeared to acquire a third set.

These cephalic lobes are used for feeding, allowing some species to grapple with shellfish while helping species like manta rays more efficiently hoover up tiny plankton as they flap their way through the open ocean.

What wasn’t clear was just where these fleshy face funnels came from.

To investigate, San Francisco State University Professor Karen Crow and co-authors studied the embryos of cownose rays (Rhinoptera bonasus), the closest relatives of the massive mantas.

They took samples of genetic material at different stages of the rays’ growth to see which genes were active during fin development, akin to peeking at the growing ray’s assembly instructions.

The researchers examined hundreds of genes and paid special attention to several ‘Hox’ genes, which contain instructions for growth and development of fins and limbs. It’s a group of genes crucial to development in all animals, including humans.

The results showed that the ray’s horns aren’t a third set of appendages at all – they’re simply the foremost bit of fin, modified for a new purpose.

The team found that the same Hox genes that guide development of the rays’ cephalic lobes also play the same role in the fins of a closely related ray species, the little skate (Leucoraja erinacea), which doesn’t have cephalic lobes.

In fact, the way the horns develop is surprisingly simple. All it takes is a tiny notch that deepens and widens as the manta grows, separating each fin into two distinct parts: one for feeding and the remainder for swimming.

“The findings support a consensus that’s emerging among scientists who study evolution: strange, novel features in nature can often arise from tiny evolutionary tweaks,” the scientists said.

“Whatever genetic changes occurred, there were far fewer than what we expected,” Professor Crow said.

“A devil ray isn’t so different from its hornless cousins. And that lesson applies on a broader scale, too.”

“We share the same genetic toolkit with all the other animals — and we share many of our genes with all living things.”

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John D. Swenson et al. How the Devil Ray Got Its Horns: The Evolution and Development of Cephalic Lobes in Myliobatid Stingrays (Batoidea: Myliobatidae). Front. Ecol. Evol, published online November 13, 2018; doi: 10.3389/fevo.2018.00181

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