By Tom Pelton
Published in the Fall 2012 issue of Save The Bay magazine
Seahorses are among the most peculiar denizens of the Chesapeake Bay and the world's oceans. But scientists warn their continued existence may be threatened by pollution and destructive fishing techniques.
Perhaps the most remarkable aspect of the seahorse lifecycle is how they reproduce. In the Chesapeake Bay, breeding occurs in the warm-water months of May to October. Males and females pair off and dance, often wrapping their tails around each other. They change colors as they swirl around, making clicking sounds until they click in unison.
When seahorses mate, it is the male that gets pregnant. The female extends a tube from her ovary to inject eggs into the male's inflated brood pouch at the base of his abdomen. The male seals his pouch, fertilizes the eggs in it, nourishes and carries the embryos, and after a 20-21 day gestation period, gives birth to hundreds of seahorse fry.
In many species, the couples repeat their ritualistic dances every morning to reinforce their marriage-like bonds, according to Dr. Amanda Vincent, an expert on seahorses with the International Union for the Conservation of Nature (IUCN) and Director of Project Seahorse at the University of British Columbia.
"There is a real commitment to the partner in these pair bonded animals, probably partly because these animals live at such low densities and move so little that looking for a new partner would be jolly hard work and possibly quite risky," Dr. Vincent said.
Populations of these romantic creatures are in trouble around the world, partly because of the disappearance of the seagrasses they depend on for shelter, Dr. Vincent said. This loss is caused both by pollution and by bottom trawling by fishing boats that clear-cut the bottom and kill everything in its path, she said.
"We are losing seahorse populations because of accidental capture in non-selective fishing gear that just grabs everything, and because of the loss and damage to their wild habitats," said Dr. Vincent. "Seahorses live in the world's most important and most threatened marine habitats, like seagrasses, mangroves, coral reefs, and estuaries. Those are all in trouble, and seahorses with them."
About 48 species of seahorses—from sea dragons to pot-bellied seahorses—inhabit the world's oceans and bays, and 20 percent of these species have been identified by the International Union for the Conservation of Nature as endangered or vulnerable.
The Chesapeake Bay is home to a single species, the lined seahorse, which is classified as threatened by the International Union for the Conservation of Nature. Lined seahorses get their names from their zebra-like stripes. They range from Nova Scotia to Uruguay, grow to about eight inches in length, live about four years, and are masters of changing colors to match their backgrounds.
The fins of seahorses are tiny, and so they are easily buffeted by currents. To survive, seahorses cling to underwater grasses with their prehensile tails and hide using chameleon-like skin. They ambush small shrimp and other prey, which they vacuum up with their long snouts.
Because they prefer saltier waters, lined seahorses tend to remain in the southern half of the Bay, and even there, they are relatively rare. The Virginia Institute of Marine Science surveys the Bay every year by dragging a net to sample for fish. In 1,224 trawls last year, researchers only found 48 lined seahorses, according to the Virginia Institute of Marine Science.
Dr. John Musick, Emeritus Professor at the Virginia Institute of Marine Science, said not enough data have been collected over time to conclusively determine whether lined seahorses are in decline in the Bay. But he said it is logical to assume that seahorse numbers have probably fallen over the decades as the amount of underwater eelgrass covering the Bay's bottom has shrunk.
"As the grasses decline, those animals that depend on the grasses for their cover and their habitat have to decline too," Dr. Musick said. "If there is no cover, there is no place for the seahorses to live."
More than half of the Chesapeake Bay's eelgrass has died since the early 1970s. Nitrogen and phosphorus pollution from sewage plants, farms, lawns, and other sources has stimulated algal blooms that darken the waters, blocking sunlight that eelgrass needs to grow.
Climate change also could be playing a role in the eelgrass decline, by creating spikes in summer water temperatures that cook the fragile submerged plant, which is sensitive to heat and at the northern edge of its range in the Chesapeake, according to research by Dr. Robert Orth, Professor of Marine Science at the Virginia Institute of Marine Science.
Morgan Denney, an aquarist who takes care of seahorses at the National Aquarium in Baltimore, said pollution limits for the Bay created by the U.S. Environmental Protection Agency in December 2010 will likely help seahorses, if regional states put these limits into action by building improved pollution-control systems.
"A reduction in nutrient pollution would help the eelgrass, and the seahorse population would be expected to bounce back as well," Denney said, as he fed brine shrimp to a lined seahorse in a tank.
Globally, other factors are also causing declines in seahorses, including the catching of millions of them off Southeast Asia and Africa to be ground up and sold in Chinese traditional medicines, according to Dr. Vincent.
Seahorses are also often netted for sale as pets in fish tanks, or dried and sold in tourist shops. Dr. Vincent said the most significant cause of seahorse deaths, however, is the shrimp industry, because it employs both bottom trawling (which catches seahorses and rips up their seagrass homes) and the removal of mangrove forests (another seahorse habitat) for shrimp farms.
For this reason, the simplest thing people can do to save seahorses is to stop eating shrimp, Dr. Vincent said. Investments to reduce pollution and restore eelgrass in the Chesapeake Bay would also help, she said.
Seahorses are worthy of the public's attention, Dr. Vincent said, not only because they are beautiful and fragile, but also because their reproductive systems are so rare in the animal kingdom, with the males acting as the "Mr. Moms" by carrying the developing embryos and giving birth.
"It would be a terrible tragedy," Dr. Vincent said, "if our lack of attention to our oceans and bays resulted in us losing this most marvelous of innovations, the male pregnancy."