Pregnant—and Still Macho - seahorses

Science News, March 11, 2000 by Susan Milius

Male seahorses allow scientists to test extreme notions of sex-role reversal

One-half of the human race may find the idea more interesting than the other half does, but regardless, there's been plenty of speculation on what the world would be like if males were the ones who got pregnant.

The more elaborate hypotheses--about warp-speed progress in obstetrics or Tiger Woods endorsing maternity clothes--have yet to receive rigorous testing. However, some of the basic theories of sexual behavior and sexual selection are getting attention thanks to a burst of new studies in the topsy-turvy social world of the seahorse.

In these unusual fish, the female still produces the eggs, but she deposits them into a pouch in her mate's body. Then she swims away, returning only for 5 to 10 minutes once a day during his several weeks of pregnancy. Finally, his distended body contracts and labors as he expels tiny seahorses one by one. Males of some species give birth to a few dozen, but others bear more than 100, and one Caribbean male astonished researchers as his half-table-spoon pouch held more than 1,500 babies.

"I spend a lot time saying, `Yes, they're real,'" notes Heather Hall, a fish geneticist at the London Zoo and cofounder of Project Seahorse, an international project to study and conserve seahorses. Many of her visitors come to the zoo believing that seahorses are myths, and until the last decade, the paucity of scientific literature on the topic might have justified their beliefs.

"I find it incredible, still, that an animal that's so popular has been so little studied," says Hall. "I expected to be looking at research gaps and filling them in, not starting with a blank sheet of paper."

Seahorse study had been so neglected that until last year, taxonomy "was a complete mess, to put it politely," remarks Hall. About 150 Latin names appeared in print to describe roughly 35 species. One of the names mentioned most often in seahorse-trade records has been applied to a range of animals that Hall considers to include at least 10 distinct species. Finally, in 1999, creative fundraising from such sources as Guylian Chocolates of Belgium, which makes seahorse-shaped candy, allowed Project Seahorse to publish the first identification guide to the fish.

It lists 52 species in one genus, Hippocampus, within the oddball syngnathid fish family. The syngnathids generally share a hunting style, snorting up small crustaceans and other prey through strawlike snouts. Body styles of the 35 syngnathid genera diverge from the few-frills swimming cylinders of pipefish to the abundantly adorned frames of sea dragons, which resemble a cross between a seahorse and a wilting begonia.

Hall often has to convince zoo visitors that seahorses are really fish. They breathe with gills, and they swim with fins. The biggest, the Pacific seahorse, reaches about 11 inches in height, whereas an Australian species stays under an inch.

Seahorses stick to narrow zones along tropical and temperate coasts, bobbing in sea grass meadows, mangrove stands, and coral reefs. All seahorses are marvels of camouflage, cloaked in murky brown-and-gray patterns that vanish into sea grass backgrounds.

Yet the animals assume startling colors during social moments or in unusual surroundings. Mates can blush a creamy yellow when meeting each other in the morning, reports Project Seahorse cofounder Amanda C.J. Vincent of McGill University in Montreal. She encountered one Australian male that took on the orange of the high-visibility tape that she used to mark a grid in the study area.

Vincent's studies of seahorse family life, starting in the mid-1980s, led the new wave of research. Lurking for hours underwater in sea grass beds in Australia, she detailed the doings of a typical species, Hippocampus whitei.

Males stick to about a square meter of their habitat though females range over perhaps a hundred times that area, she found. When two parties discover a mutual interest at the beginning of the breeding season, they court for several days. In lab tests, interlopers of either sex tried to interfere.

The courting pair might change color, sidle by each other, swim side-by-side holding tails, or grip the same strand of sea grass with their tails and wheel around it in unison. A male pumps water through the egg pouch on his trunk, which balloons and gapes open to display an alluring emptiness.

When a female's eggs reach maturity, she and her mate abandon grips on any anchors and, snout-to-snout, drift upward out of the sea grass, often spiraling as they rise. The female squirts perhaps 200 eggs from a chamber in her trunk into his pouch. Her body slims as his bulges. Both seahorses sink back to the bottom, and she swims off.

A male seahorse's body courses with prolactin, the same hormone that governs milk production in pregnant women. He doesn't supply milk, but his pouch provides oxygen as well as a controlled-environment incubator for about 21 days.

Throughout the male's pregnancy, his mate visits him daily for what Vincent calls morning greetings. The female seahorse swims over for about 6 minutes of interaction reminiscent of courtship. They change color, wheel around sea grass fronds, and finally promenade, holding each other's tails. Then, the female swims away until the next morning, and the male goes back to vacuuming up food through his snout.