Seahorses first stood up sometime after 25 to 20 million years ago, when bodies of open water between Australia and Indonesia changed, leaving these horse-resembling bony fishes with shallow sea grass habitat, according to a new study.
The study, published in the latest Royal Society Biology Letters, explains that an upright stature, shared by humans but which is extremely rare throughout the entire animal kingdom, allowed seahorses to reside in what was essentially a lawn under water.
"Not only can an upright fish maneuver much better in such an environment than a horizontally-swimming one, but the upright sea grass blades would have improved their camouflage," explained Peter Teske, who co-authored the paper with Luciano Beheregaray.
"Camouflage is of great importance to seahorses, which are poor swimmers," added Teske, a postdoctoral researcher in Macquarie University's Molecular Ecology Lab. "It affords them both protection from predators and allows them to sneak up on their prey — including small crustaceans and tiny fish — without being noticed."
Teske and Beheregaray first took note of pygmy pipehorses, which look like "non-upright proto-seahorses." They wondered when these two bony fishes diverged, and how this fit into the bigger picture for the Syngnathidae, a family of fish that includes seahorses, pipefishes and both weedy and leafy sea dragons.
10 deep-sea secrets revealedTo answer these questions, they constructed a family tree using genetic information gathered from the genus Hippocampus, which includes all seahorses, and the genus Idiotropiscis, which contains the very seahorse-resembling southern little pipehorse.
Molecular dating indicates seahorses split from their pygmy pipehorse sister taxon during the Late Oligocene, right when tectonic events in the Indo-West Pacific resulted in the formation of new shallow water sea grass habitats. It appears that seahorses remained mostly isolated in these new regions, allowing them to evolve a number of distinctive features.
"Seahorses are often considered to be highly unusual bony fishes because of their upright posture, their hard exoskeleton, their long suction-snout, their prehensile tail and the fact that it's the males that give birth," Teske explained.
Virtually all male bony fishes are dutiful dads, but none compares to this fish super father.
"The most advanced form of male parental care is found in the seahorses, where a brood pouch has evolved that resembles a placenta, and male seahorses even go into labor!" he marveled.
The tail on both males and females, he added, permits seahorses to grab onto blades of sea grass and other aquatic vegetation. Many seahorses have evolved plant-like anatomical features, so swimmers by often miss seeing them, thinking they are just part of the marine flora.
Unfortunately, human collectors don't always miss these exotic, beautiful fish. Of the 34 known seahorse species, eight are now listed as threatened by the International Union for Conservation of Nature's "Red List."
"So little is known about seahorses that we are at risk of losing these animals before we even know them," said Ava Ferguson, senior developer for "The Secret Lives of Seahorses," an exhibit now underway at the Monterey Bay Aquarium. "Until recently we had no idea that seahorses were being caught and collected at a rate that threatens their survival."
She remains hopeful, however, that conservation measures may save both seahorses and their homes.
"When you save a seahorse," she said, "you also save some of Earth's most precious marine habitats."
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