Is This the World's Smallest Seahorse?

John Pickrell
for National Geographic News
July 31, 2003
If you were a male seahorse, you'd certainly have had reason to kick back and relax on father's day. Ranging in size from smaller than human fingernail to more than 30 centimeters (about a foot) in length, seahorse dads have had the tables turned and—unlike other vertebrates—bear the burden of pregnancy.

And that burden is typically heavier than any human mum might have to endure: wild seahorse dads can carry anywhere between just ten to more than one thousand babies. What's more, they spend nearly all of their (on average) two- to three-year lives pregnant.

"It's a hard life for a male," said Sara Lourie, seahorse biologist with the Project Seahorse marine conservation team, based at McGill University in Montreal. "In the species that have been studied, males always get pregnant again within a day or two after giving birth."

Lourie and tropical marine biologist John Randall of the Bishop Museum in Honolulu, Hawaii have discovered the smallest of all seahorse fathers. As detailed in a recent issue of the journal Zoological Studies, the scientists have described a new species of seahorse smaller than some human fingernails.

The species, Hippocampus denise, is no longer than 16 millimeters (about six-tenths of an inch) from its snout to the tip of its tail, though some pregnant males as small as 13 millimeters in length have been recorded.

In fact, it was so small that seahorse experts had previously believed it to be the young of the next smallest species, Hippocampus bargibanti, adults of which are at least 50 percent bigger.

The tiny orange-colored animal is found in coral reefs surrounding idyllic eastern Indonesian islands. Not only is it the smallest seahorse yet described, but also falls amongst the ranks of the world's most miniature vertebrates.

Britain's mammal society argues that the smallest mammal is the Savi's Pigmy Shrew, Suncus etruscus, which measures just 36 to 53 millimeters (1.5 to 2 inches) in length. The smallest fish—a type of goby, Trimmaton nanus—is only slightly smaller than H.denise at 8 to 10 millimeters in length, said Lourie. "It may not be physically possible to get much smaller than this…especially in seahorses, as having young developing in the body could be constraining," she said.

Pygmy seahorses, which are typically ten times smaller than non-pygmy relatives, were first discovered in the late 1960s and this is the third to be described. The animals live on soft coral in Asia and the Pacific.

"Due to the physical structure of the host [coral], it is an advantage to be small," said independent seahorse expert and biologist Rudie Kuiter, in Seaford, Australia. "To get away from predators it's easy to move [through the coral] from one side to the other in one simple move," he said.

Lourie, who was compiling a seahorse identification book in the late 1990s, first became aware that a new species might exist when diver and photographer Denise Tackett sent her images of seahorses she didn't recognize. Her curiosity piqued, Lourie—who's now studying the relatedness of South East Asian seahorse populations for her doctorate—decided to see if she could find the animal herself. During an Indonesian research trip in spring 2001, Lourie got the opportunity to join the crew of a dive boat for a week in the Flores Sea, and decided to take a chance.

It was on one of these dives, examining the kinds of delicate, deep-water coral the animal was seen associated with in Tackett's photographs, that Lourie spotted a pair. "It was pretty unbelievable," she said, "I was stunned and amazed…as I'd never really believed we would find such a small creature." One specimen collected on that trip, and two subsequently collected near the Pacific islands of Palau and Vanuatu, had young seahorses inside them, proving that they were not the young of another species. Lourie and Randall decided to name the animal H.denise after Denise Tackett, whose 1997 photographs were the key to the find.

Though it's impossible to predict the conservation status of the new species without estimating population sizes and rates of decline, said Lourie, it is known that 46 percent of the reefs around Indonesia are classified as highly threatened. Habitat destruction, and overfishing for use in traditional Chinese medicine, are the biggest threats to seahorses, she said. Project Seahorse most recently estimates that total global consumption of seahorses is 20 million or more annually, she added.

"Many time people identify a new species and six months later it's reclassified as something else," said Juan Romero Director of Animal Husbandry at the National Marine Aquarium in Plymouth, England. "This absolutely appears to be a new species, which hasn't been seen before, because it's so small." However, Kuiter has some nagging doubts that these specimens are H.bargibanti juveniles. Juveniles often look very different to adults, and seahorses can reproduce before fully grown, said Kuiter. Analysis of DNA samples is needed to confirm the classification, he said.

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