September 29, 2009 12:56:00 PM
Probably you have never seen a seahorse in the wild. Even Dr. Helen Scales, who is a scuba diver and marine biologist, has only seen them a few times. The first one she saw, after many dives of looking, was "like glimpsing a unicorn trotting through my garden." But everyone knows what a seahorse looks like, a fantastic looking creature that sparks curiosity, and it is a hit at aquariums or in oceanic picture books. Scales has satisfied many facets of the curiosity about seahorses in her book "Poseidon''s Steed: The Story of Seahorses, from Myth to Reality" (Gotham Books).
The reason seahorses don''t get seen very often (and part of the reason for their attractiveness) is that they are placid; they do not zip away from predators, but quietly anchor themselves to grass or coral. There are, sadly, only a few black and white pictures in Scales''s book, but she includes vivid explanations about the biology and legends of these strange creatures, as well as a broader look at our use of ocean resources.
Seahorses were so puzzling that it took a while for them to be classified as fish. Even in the 19th century there was confusion. The tail of the seahorse looks a lot like a caterpillar (and the Greek word for caterpillar, campus, might be a contribution to the scientific name of seahorses, Hippocampus, with the "hippo-" coming from the word for horse).
There was a school of thought that these were oceanic insects. Some also argued that they were a sort of legless shrimp. At least no one thought, as medieval merchants did, that these were baby dragons. Seahorses are marine fish, most certainly. Like all such fish, they have gills, and they have a swim bladder that controls their buoyancy. A seahorse has a spine, as well as a hard outer casing of plates. Seahorses are most closely related to the pipefishes, which, since they don''t have the S-curve of seahorses, look a little more fishy.
Some of their relatives have a pattern of the male taking care of his brood of newborns, and some even have special organs on or in the male to carry the fertilized eggs and embryos. Seahorses, however, are the only male animals that have a full pouch which does everything a womb does in the females of other animals. This was another puzzle the biologists had to sort out, for after the embryos develop within the male''s pouch, the male expels them in contractions that can go on convulsively for days. The embryo specks then swim away, fully formed but the size of fleas, with their snouts disproportionately large for their bodies. Of course, the pregnant male seems to be doing what the female usually does, and so how did the researchers find out that the males were the ones who got pregnant?
The answer is simple: it doesn''t matter where the embryos develop, because what matters is who supplies the eggs and who supplies the sperm. The females of all species supply the eggs, and the female seahorse is no different. However, during courtship, she extrudes a short tube that goes into the belly of her mate, and through it she shoots an egg-laden serum. It would make sense that the sperm of the male would be injected into the pouch holding the eggs, but evolution didn''t make things so simple. The male still has sperm ejected into the water, as spawning fishes do, and the sperm have to be sucked into the embryo pouch.
Although seahorses are hard to find, they can be found all over the world, in almost every marine environment. One of the reasons they are hard to see is that they are masters of camouflage, some of them looking like blades of grass or of leafy seaweed. They can change colors. This can cause confusion in identification. Scales says that if you are a diver lucky enough to see a seahorse, get a picture. If you don''t have your camera, note such characteristics as size, smooth or spiny, fat or slim, and stubby- or slender-nosed.
"Tempting as it may be, try not to focus too much on what color it is. A ''yellowy-orange'' seahorse could be almost any species that was feeling in a yellowy-orange mood the day you spotted it."
There are something like 40 different species to be differentiated, with some only recently distinguished. Researchers who want to look at individuals within a species have difficulty, as the little creatures do look so similar, but they can be tagged with tiny dots of ink under their skin. Subsequent photos of the tagged seahorses are used to show how long they live (several years, perhaps up to 10) and how much they move around (not much).
You are much more likely to see a seahorse in an aquarium than in the wild, and Scales gives a quick history of aquariums, which sprang up in the time of the Victorian enthusiasm for natural history. The world''s first public aquarium opened in London in 1853 and was extremely popular. When four seahorses, brought from Portugal, were installed six years later, they were a sensation. Seahorses became popular sights in all the public aquariums in the world, and people clamored to have them in their own home aquariums until they realized how much work a marine aquarium is and the fad died down.
These days, there are seahorse farms to breed seahorses for such home or civic aquariums. The farms were originally an idea to cash in on the seahorse trade, not for aquarium displays but for medicinal use. Traditional Chinese medicine uses powdered seahorse in panaceas for virility, ulcers, and who knows what else.
Seahorses are often dredged up as byproducts of commercial trawling for shrimp, and 25 million of them a year go into pills. It proved impossible for seahorse farms to keep up with the trawlers, whose catch can turn into dried seahorses sold by the ton. Taking seahorses in this way isn''t the main threat to them, but they are not flourishing these days, because of loss of ecosystems due to many different reasons.
Scales, however, tries to be optimistic about efforts at recovery, citing especially the marine park movement:
"Over the past 20 or 30 years, there has been mounting recognition that the most effective, not to mention the simplest, way of healing the oceans is merely to leave parts of them alone." Seahorses, however, are not "keystone" species; if they were all to vanish, there are not dependent species that would vanish with them nor prey that would burgeon uncontrolled. Seahorses, though, are special, as Scales shows in her review of the legends and stories we have made up about them.
They are, she says, "so strange and yet so perfectly pleasing at the same time," and we should take great satisfaction in "knowing that the unlikely seahorses are merely a result of the unseen forces of natural selection at work." Seahorses may not be an essential species, and may be hard to find, but she reminds us that "the world is absolutely a better place just knowing there are seahorses swimming through the oceans." Her delightful and informative book, full of enthusiasm for her subject, richly fosters our appreciation for a unique creature.
Rob Hardy is a local psychiatrist who reviews books for a hobby. His e-mail address is firstname.lastname@example.org.