Scientists Find Genetic Clues As to How Male Seahorses Get Pregnant

A new analysis of seahorse DNA could reveal how it became one of the strangest beasts in the animal kingdom.

Dec 18 2016, 2:00pm

Slender seahorse, or Hippocampus reidi. Image: Flickr/Andreas März

There's so much to love about seahorses. These prehistoric-looking creatures are swimming contradictions—from the way they mate for life (unlike most fishes), to the fact that there's nothing equine about them at all.

But the strangest, and most endearing, quality to seahorses is the way they reproduce. Unlike any other animal on Earth, only male seahorses exhibit a true pregnancy, releasing as many as 2,000 babies, or "fry," from his pouch at a time. Male pregnancies last anywhere between 10 to 25 days, and scientists still aren't sure what evolutionary advantages this provides the fish.

However, a study recently published to Nature offers new insight into the genetic mechanisms that allow this gestational role-reversal. According to the paper's findings, a series of genetic mutations throughout the seahorse's history are likely responsible for the development of its curious characteristics, including male pregnancy.

An international team of evolutionary biologists analyzed the genome of the tiger tail seahorse(Hippocampus comes), which is a species found throughout southeast Asia. Tiger tail seahorses, according to the study, possess molecular evolutionary rates higher than any other member of the teleost infraclass, or group of ray-finned fish to which seahorses belong. Why, exactly, this is the case, remains unknown.

But by analyzing the species' DNA, the team identified changes to genes that help moderate pregnancy in other female fishes. In seahorses, six of those genes had been duplicated to exhibit new expression patterns. Five of the genes, like one that determines egg-hatching, were found in the brood pouches of male seahorses, "suggesting that they may be involved in male pregnancy, possibly through rewiring of their regulatory network," said the study.

A pregnant male seahorse. Image: Flickr/osattack

Other genetic mutations were directly related to seahorses' bizarre morphology. One of these, which regulates the creation of a protein that's found in tooth enamel, might explain why seahorses have such equine snouts. Behaviorally, their fused, pipette mouths allow them to suck up food like tiny crustaceans or larvae. But evolutionarily, the researchers now believe that a "loss of mineralized teeth" is responsible with their unique jaw shape.

An even stranger characteristic—a lack of pelvic fins—is also believed to be the result of a genetic deviation. Since seahorses are missing the gene for pelvic fins, the team decided to test this anomaly on another species: the zebrafish. By cultivating zebrafish who lacked the genetic determinant for pelvic fins, the researchers observed similar losses in the mutant line.

Still, other biologists warn that seahorse evolution warrants further investigation. "Most of what they report is a correlation; you can't say for sure that's what causes it," Tony Wilson, a professor at Brooklyn College who was not involved in the study, told the Los Angeles Times.

Indeed, to better understand how to protect seahorses—the International Union for Conservation of Nature considers many species threatened by extinction—it's important to learn more about their evolutionary past.