Toddler seahorses are bumbling and adorable

Newborn seahorses look like their parents. They already have the power for beyond-fast strikes at prey. And their tails end with a miniature up-curl like a grown-up’s prehensile marvel. But they’re babies, and they bumble.

That’s the impression of evolutionary morphologist Dominique Adriaens, who has watched several Hippocampus species born in his lab at Ghent University in Belgium. In the seahorse world, it’s dad, of course, who’s pregnant. “A male gives birth in, let’s say, 10 seconds — tops,” he says. “You see small white dots squirting out. And five to 10 seconds later, you see hundreds of tiny seahorses floating at the surface of the water.”

His lab has found that the rice-grain-sized newborns strike at their prey fast, but not all that accurately. Like adults, they get recoil speeds by tensing muscles without moving and building up tension in their tendons. They suddenly let fly, and their snouts whip upward to suck in prey. If the rotation actually could continue at strike speed, the heads could spin around more than 80 times in just a second. “They start doing this from the time they are born, only they are very lousy at catching prey with it,” Adriaens says.

Nor do babies manage signature seahorse tail grasps even though they have the basics of the adult shape. As far as Adriaens knows, species in the seahorse family and some small chameleons grow the only prehensile tails that are square in cross section instead of round.

The peculiar seahorse tail shape comes from a flexible, squared-off cage of bony struts that form from skin layers instead of cartilage. The square may be a legacy of ancestral seahorse armor that just happens to work for grasping too, Adriaens and his colleagues propose in the July 3 Science (SN Online: 7/2/15). Crushing tests on 3-D printed models found that square tails were four times as strong as round versions.

Babies, however, don’t get a grip until they’re old enough to venture to the sea bottom. There, the fish go prehensile. Out of some 30,000 known kinds of fishes in the world, the seahorse family is the only one to have evolved any form of tail that grasps a perch.