White River Stone with seahorse raised or embossed motif
Source from wikipedia.com
Seahorses are any species belonging to the genus Hippocampus, which, in turn, belongs to the family Syngnathidae, which also includes pipefish and leafy sea dragons. There are over 47 species of seahorse, mainly found in shallow tropical and temperate waters throughout the world. They prefer to live in sheltered areas such as sea grass beds, coral reefs, or mangroves. Colonies have been found in European waters such as the Thames Estuary. From North America down to South America there are approximately four species, ranging from very small in size (dwarf seahorses are only about 2.5 cm / 1″ long) to those much larger, found off the Pacific Coast of Central America (the foot-long Hippocampus ingens). Hippocampus erectus are larger seahorses found anywhere from Nova Scotia down to around Uruguay. Three different species of seahorse live in the Mediterranean Sea: Hippocampus hippocampus (long snout), Hippocampus brevirostris (short snout) and Hippocampus fuscus (immigrated from the Red Sea). These fish form territories, with males staying in about one square meter of their habitat while females range about one hundred times that area. They bob around in sea grass meadows, mangrove stands, and coral reefs where they are camouflaged by murky brown and grey patterns that blend into the sea grass backgrounds. During social moments or in unusual surroundings, seahorses turn bright colors.
Seahorses are so named for their equine profile. Although they are bony fish, they do not have scales, rather a thin skin stretched over a series of bony plates arranged in rings throughout their body. Each species has a distinct number of rings. Seahorses swim upright, another characteristic that is not shared by their close pipefish relatives, which swim horizontally. Seahorses have a coronet on their head, which is distinct to each seahorse, much like a human fingerprint. They swim very poorly by using a dorsal fin, which they rapidly flutter to propel them, and pectoral fins, located behind their eyes, which they use to steer. Seahorses have no caudal fin. As they are poor swimmers, they are most likely to be found resting in beds of sea grass or coral reefs, with their prehensile tails wound around a stationary object. They have long snouts, which they use to suck up food, and eyes that can move independently of each other, much like a chameleon. Seahorses eat small shrimp, tiny fish and plankton.
Evolution and fossil record
Anatomical evidence, supported by molecular and genetic evidence, demonstrates that seahorses are highly modified pipefish. The fossil record of seahorses, however, is very sparse. The best known and best studied fossils are from the Marecchia River Formation of Rimini Province, Italy, dating back to the Lower Pliocene, about 3 million years ago. The earliest known seahorse fossils are of a pipefish-like species from the “Coprolitic Horizon” of Tunjice Hills, a lower Miocene lagerstätte in Slovenia dating back about 13 million years.
Seahorses typically mate for life. The male seahorse is equipped with a brood pouch on the ventral, or front-facing, side. When mating, the female seahorse deposits the eggs in the male’s pouch, which the male then internally fertilizes. The male carries the eggs until they emerge, expelling fully-developed, miniature seahorses in the water.
When two parties discover a mutual interest at the beginning of breeding season, they court for several days, even while others try to interfere. During this time they have been known to change color, swim side by side holding tails or grip the same strand of sea grass with their tails and wheel around in unison in what is known as their “pre-dawn dance”. They eventually engage in their “true courtship dance” lasting about 8 hours, during which the male pumps water through the egg pouch on his trunk which expands and cleaves open to display an appealing emptiness. When the female’s eggs reach maturity, she and her mate let go of any anchors and snout-to-snout, drift upward out of the seagrass, often spiraling as they rise. The female inserts her ovipositor into the male’s brood pouch, where she deposits her eggs, which the male fertilizes. The fertilized eggs then embed in the pouch wall and become enveloped with tissue. New research indicates the male releases sperm into the surrounding sea water during fertilization, and not directly into the pouch as was previously thought. Most seahorse species’ pregnancies last two to four weeks.
As the female squirts anywhere from dozens to thousands of eggs from a chamber in her trunk into the male pouch, her body slims while his swells. Both seahorses then sink back to the bottom and she swims away. Scientists believe the courtship behaviour serves to synchronize the movements of the two animals so that the male can receive the eggs when the female is ready to deposit them. The eggs are then fertilized in the father’s pouch which is coursed with prolactin, the same hormone responsible for milk production in pregnant women. He doesn’t supply milk, but his pouch provides oxygen as well as a controlled environment incubator. The eggs then hatch in the pouch where the salinity of the water is regulated; this prepares the newborns for life in the sea. Throughout the male’s incubation, his mate visits him daily for “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.
The number of young released by the male seahorse averages 100-200 for most species, but may be a low as 5 for the smaller species, or as high as 1500, with pregnancy lasting from two to four weeks, depending on the species When the fry are ready to be born, the male undergoes muscular contractions to expel them from his pouch. He typically gives birth at night and is ready for the next batch of eggs by morning when his mate returns. Like almost all other fish species, seahorses do not care for their young once they are born. Infants are susceptible to death from predators or being swept into ocean currents, where they drift away from rich feeding grounds or into temperatures too extreme for their delicate bodies. Fewer than five infants of every 1,000 born survive to adulthood, helping to explain why litters are so large. The survival rates of these infants are actually fairly high compared to fish standards, because they are initially sheltered in their father’s pouch during the earliest stages of development, while the eggs of most other fish are abandoned immediately after fertilization. This makes the process worth the great cost to the father of incubating his offspring.
This entire process costs the male a great amount of energy. This brings into question why the sexual role reversal even takes place. In an environment where one partner incurs more energy costs than the other, one would expect the lesser of the two to be the aggressor. Within the seahorse species, males are shown to be the more aggressive sex and sometimes “fight” for female attention. According to Amanda Vincent of Project Seahorse, only males tail-wrestled and even snap their heads toward each other. This discovery prompted further study in finding out whether males actually are incurring more costs than their female counterparts. To estimate the female’s direct contribution, researcher Heather D. Masonjones of Amherst College performed a chemical analysis of the energy stored in each egg. Furthermore, to measure the toll that incubation takes on a male, Masonjones built a tiny respirator that records oxygen concentrations in water flowing into and out of a chamber. Before a male took on eggs, she checked his baseline need for oxygen. Then, she monitored the increase as the incubation progressed. The male’s body had to work hard by the end of incubation, consuming almost a third again as much oxygen as he did before mating. To correct for oxygen used by the growing brood, Masonjones managed to keep ¼ inch-high premature seahorses alive outside the pouch so she could measure their oxygen needs. Although they undergo weeks of incubation, males directly contribute only half as much energy for offspring as females do. Therefore, they do in fact fit into the widespread pattern of the less-invested sex being the “less-choosy.”