The species of the genus Hippocampus are the seahorses, a distinct group of fishes within the family Syngnathidae, which also includes the pipefishes. There are over 50 species of seahorse, mainly found in shallow tropical and temperate waters throughout the world. They prefer to live in sheltered areas such as seagrass 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.
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Seahorses are so named for their equine profile. Although they are bony fish, they do not have scales, but 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, physical, 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 two pipefish-like species, Hippocampus sarmaticus and H. slovenicus from the coprolitic horizon of Tunjice Hills, a middle Miocene lagerstätte in Slovenia dating back about 13 million years. Molecular dating finds that pipefish and seahorses separated during the Late Oligocene. This has led to speculation that seahorses evolved in response to the creation at this time of large areas of shallow-water as the result of tectonic events. These new areas of shallow water allowed the expansion of seagrass and so habitats that selected for the camouflage offered by the seahorses’ upright posture. These tectonic changes occured in the Western Pacific Ocean suggesting an origin there with molecular data suggesting two later and separate invasions of the Atlantic Ocean.
Seahorses are often thought of as being monogamous, though recent research shows this may not be true. 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 deposits anywhere from dozens to thousands of eggs from a chamber in her ovipositor 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 mammals. As seahorses are not mammals his pouch instead 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 as 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.
Questions surrounding reproductive roles
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 (this is known as Bateman's principle). 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. The study concludes that the female's energy expenditure while generating eggs is twice that of males during the incubation period; therefore, they fit the widespread pattern of the less-invested sex being the "less-choosy."
Why the male seahorse (and other members of Syngnathidae) carries the offspring through gestation is unknown, though some researchers believe it allows for shorter birthing intervals, in turn resulting in more offspring. When looking at each gender having the ability to produce more young if they had an unlimited number of ready and willing partners, males have the potential to produce 17 percent more in a breeding season. Also, females have “time-outs” from the reproductive cycle that are 1.2 times longer than those of males. This does not seem to be based on physiology, rather mate choice. When the female’s eggs are ready, she must lay them in a few hours or else she has to eject them onto the sea floor which is a huge cost to her physically, as her eggs amount to about a third of her body weight. To protect against unwillingly losing a clutch, the female demands a long courtship period. Furthermore, the daily greetings help to cement the bond between the pair.
A study conducted by Amanda Vincent of Project Seahorse demonstates the importance of the daily greetings ritual in establishing monogamous relationships between seahorses. Vincent kept a female seahorse in a tank with two males; after the female filled one male’s pouch with eggs he was then taken away and she was left with the unimpregnated male. During the weeks of her mate’s pregnancy, the female and her tankmate greeted each other daily, clinging to the same bit of grass and changing color, but did not display signs of serious courtship. After the original mate gave birth he was returned to the tank, and the female was again given the choice between the two males to carry her eggs. While both males expressed enthusiasm for her attention, even tail wrestling with each other, in all six tests the female rejected her original mate and presented the next clutch of eggs to the tankmate she had greeted each day.
Although monogamy within species is not common, it does appear to exist for some. In this case, the mate-guarding hypothesis may be an explanation. This hypothesis states that “males remain with a single female because of ecological factors that make male parental care and protection of offspring especially advantageous.”  Because the rates of survival for newborn seahorses are so low, incubation is essential at the beginning stages of life. Though not proven, males could have taken on this role because of the time period in which it takes females to produce their eggs. If the males carry the offspring while the females gather the nutrients needed to produce new eggs (which is again, 1/3 of their body weight), then they can continually reproduce batch after batch together, depending on one another for efficiency in spreading both of their genes.
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While many aquarium hobbyists will keep seahorses as pets, seahorses collected from the wild tend to fare poorly in a home aquarium. Many will eat only live foods such as ghost shrimp and are prone to stress in an aquarium, which lowers the efficiency of their immune systems and makes them susceptible to disease.
In recent years, however, captive breeding of seahorses has become increasingly widespread. These seahorses survive better in captivity, and they are less likely to carry diseases. These seahorses will eat prepackaged, frozen mysis shrimp that are readily available from aquarium stores, and they do not experience the shock and stress of being taken out of the wild and placed in a small aquarium. Although captive-bred seahorses are more expensive, they survive better than wild seahorses, and take no toll on wild populations.
Seahorses should be kept in an aquarium to themselves, or with compatible tank-mates. Seahorses are slow feeders, and in an aquarium with fast, aggressive feeders, the seahorses will be edged out in the competition for food. Special care should be given to ensure that all individuals obtain enough food at feeding times.
Seahorses can co-exist with many species of shrimp and other bottom-feeding creatures. Fish from the goby family also make good tank-mates. Some species are especially dangerous to the slow-moving seahorses and should be avoided completely: eels, tangs, triggerfish, squid, octopus, and sea anemones.
Animals sold as "freshwater seahorses" are usually the closely related pipefish, of which a few species live in the lower reaches of rivers. The supposed true "freshwater seahorse" called Hippocampus aimei was not a real species, but a name sometimes used for individuals of Barbour's seahorse and Hedgehog seahorse. The latter is a species that can be found in brackish waters, but not actually a freshwater fish.
Use in Chinese medicine
Seahorse populations are thought to have been endangered in recent years by overfishing and habitat destruction. The seahorse is used in traditional Chinese herbology, and as many as 20 million seahorses may be caught each year and sold for this purpose. Medicinal seahorses are not readily bred in captivity as they are susceptible to disease and have somewhat different energetics from aquarium seahorses. Seahorses are also used as medicines by the Indonesians, the Central Filipinos, and a whole host of other racial and ethnic groups around the world.
The problem may be exacerbated by the growth of pills and capsules as the preferred method of ingesting medication as they are cheaper and more available than traditional, individually tailored prescriptions of raw medicinals but the contents are harder to track. Seahorses once had to be of a certain size and quality before they were accepted by TCM practitioners and consumers. But declining availability of the preferred large, pale and smooth seahorses has been offset by the shift towards prepackaged medicines, which make it possible for TCM merchants to sell previously unused juvenile, spiny and dark-coloured animals. Today almost a third of the seahorses sold in China are prepackaged. This adds to the pressure on the species.
- Genus Hippocampus
- Big-belly seahorse, Hippocampus abdominalis Lesson, 1827 (New Zealand and south and east Australia)
- Winged seahorse, Hippocampus alatus Kuiter, 2001
- West African seahorse, Hippocampus algiricus Kaup, 1856
- Narrow-bellied seahorse, Hippocampus angustus Günther, 1870
- Barbour's seahorse, Hippocampus barbouri Jordan & Richardson, 1908
- Hippocampus bargibanti Whitley, 1970 (West Pacific area (Indonesia, Philippines, Papua New Guinea, Solomon Islands, etc)
- False-eyed seahorse, Hippocampus biocellatus Kuiter, 2001
- Réunion seahorse, Hippocampus borboniensis Duméril, 1870
- Short-head seahorse or knobby seahorse, Hippocampus breviceps Peters, 1869 (south and east Australia)
- Giraffe seahorse, Hippocampus camelopardalis Bianconi, 1854
- Knysna seahorse, Hippocampus capensis Boulenger, 1900
- Hippocampus colemani Kuiter, 2003
- Tiger tail seahorse, Hippocampus comes Cantor, 1850
- Crowned seahorse, Hippocampus coronatus Temminck & Schlegel, 1850
- Denise's pygmy seahorse, Hippocampus denise Lourie & Randall, 2003
- Lined seahorse, Hippocampus erectus Perry, 1810 (east coast of the Americas, between Nova Scotia and Uruguay)
- Fisher's seahorse, Hippocampus fisheri Jordan & Evermann, 1903
- Sea pony, Hippocampus fuscus Rüppell, 1838 (Indian Ocean)
- Big-head seahorse, Hippocampus grandiceps Kuiter, 2001
- Long-snouted seahorse, Hippocampus guttulatus Cuvier, 1829
- Eastern spiny seahorse, Hippocampus hendriki Kuiter, 2001
- Short-snouted seahorse, Hippocampus hippocampus (Linnaeus, 1758) (Mediterranean Sea and Atlantic Ocean)
- Thorny seahorse, Hippocampus histrix Kaup, 1856 (Indian Ocean, Persian Gulf, Red Sea, and the Far East)
- Pacific seahorse, Hippocampus ingens Girard, 1858 (Pacific coast of North, Central and South America)
- Jayakar's seahorse, Hippocampus jayakari Boulenger, 1900
- Collared seahorse, Hippocampus jugumus Kuiter, 2001
- Great seahorse, Hippocampus kelloggi Jordan & Snyder, 1901
- Spotted seahorse, Hippocampus kuda Bleeker, 1852
- Lichtenstein's seahorse, Hippocampus lichtensteinii Kaup, 1856
- Bullneck seahorse, Hippocampus minotaur Gomon, 1997
- Japanese seahorse, Hippocampus mohnikei Bleeker, 1854
- Monte Bello seahorse, Hippocampus montebelloensis Kuiter, 2001
- Northern spiny seahorse, Hippocampus multispinus Kuiter, 2001
- Hippocampus pontohi Lourie and Kuiter, 2008
- High-crown seahorse, Hippocampus procerus Kuiter, 2001
- Queensland seahorse, Hippocampus queenslandicus Horne, 2001
- Longsnout seahorse, Hippocampus reidi Ginsburg, 1933 (Caribbean coral reefs)
- Satomi's pygmy seahorse, Hippocampus satomiae Lourie and Kuiter, 2008
- Half-spined seahorse, Hippocampus semispinosus Kuiter, 2001
- Hippocampus severnsi Lourie and Kuiter, 2008
- Shiho's seahorse, Hippocampus sindonis Jordan & Snyder, 1901
- Hedgehog seahorse, Hippocampus spinosissimus Weber, 1913
- West Australian seahorse, Hippocampus subelongatus Castelnau, 1873
- Longnose seahorse, Hippocampus trimaculatus Leach, 1814
- White's seahorse, Hippocampus whitei Bleeker, 1855 (east Australia)
- Zebra seahorse, Hippocampus zebra Whitley, 1964
- Dwarf seahorse, Hippocampus zosterae Jordan & Gilbert, 1882 (Gulf of Mexico and the Caribbean)
Pygmy Seahorses are small (less than 15mm in height) members of the genus. Previously the term was applied exclusively to the species Hippocampus bargibanti but more recent discoveries have made this term obsolete. Since 1997 the species Hippocampus minotaur, Hippocampus denise, Hippocampus colemani, Hippocampus pontohi, Hippocampus severnsi and Hippocampus satomiae have been described. Other species that are believed to be unclassified have also been reported in books, dive magazines and on the internet. They can be distinguished from other species of seahorse by their 12 trunk rings, low number of tail rings (26–29), the location in which young are brooded in the trunk region of males and their extremely small size (less than 15mm tall and 17mm long). Molecular analysis (of ribosomal RNA) of 32 Hippocampus species found that Hippocampus bargibanti belongs in a separate clade from other members of the genus and therefore that the species diverged from the other species in the "ancient" past.
Most pygmy seahorses are well camouflaged and live in close association with other organisms including colonial hydrozoans (Lytocarpus and Antennellopsis), coralline algae (Halimeda) sea fans (Muricella, Annella, Acanthogorgia). This combined with their small size is believed to account for why most species have only been noticed in recent years.
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