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Seahorses are mainly found in shallow tropical and temperate waters throughout the world, and prefer to live in sheltered areas such as seagrass beds, estuaries, coral reefs, or mangroves. In Pacific waters from North America to South America there are approximately four species. In the Atlantic, the H. erectus ranges from Nova Scotia to Uruguay. H. zosterae, known as the dwarf seahorse, is found in the Bahamas.
Three species live in the Mediterranean Sea: H. guttulatus (the long-snouted seahorse), H. hippocampus (the short-snouted seahorse) and H. fuscus (the sea pony). These species form territories; males stay within 1 square meter (11 sq ft) of their habitat while females range about one hundred times that.
Seahorses are named for their equine appearance. Although they are bony fish, they do not have scales but rather thin skin stretched over a series of bony plates, which are 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, who swim horizontally. Unusual among fish, seahorses have a flexible, well-defined neck. They also sport a coronet on the head, which is distinct for each individual.
According to Guinness World Records 2009, H. zosterae (the dwarf seahorse) is the slowest moving fish, with a top speed of about 5 feet (150 cm) per hour. They swim very poorly, rapidly fluttering a dorsal fin and using pectoral fins (located behind their eyes) to steer. Seahorses have no caudal fin. Since they are poor swimmers, they are most likely to be found resting 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 (like a chameleon).
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 specimens of H. guttulatus (though literature more commonly refers to them under the synonym of H. ramulosus), 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, H. 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 large areas of shallow-water, newly created as the result of tectonic events. The shallow water allowed the expansion of seagrass habitats that selected for the camouflage offered by the seahorses’ upright posture. These tectonic changes occurred in the Western Pacific Ocean suggesting an origin there with molecular data suggesting two later and separate invasions of the Atlantic Ocean.
The male seahorse is equipped with a brood pouch on the ventral, or front-facing, side. When mating, the female seahorse deposits up to 1,500 eggs in the male's pouch. The male carries the eggs for anywhere from 9 to 45 days until they emerge, expelling fully developed, miniature seahorses in the water. Once the seahorse babies are released into the water, the male's role is done and he offers no further care.
Before breeding, seahorses court for several days. Scientists believe the courtship behavior synchronizes the animals' movements so that the male can receive the eggs when the female is ready to deposit them. During this time they may 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 a “pre-dawn dance". They eventually engage in a “true courtship dance" lasting about 8 hours, during which the male pumps water through the egg pouch on his trunk which expands and opens to display its 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 and deposits dozens to thousands of eggs. As the female releases her eggs, her body slims while his swells. Both animals then sink back into the seagrass and she swims away.
The male releases his sperm directly into seawater where it fertilizes the eggs, which are then embedded in the pouch wall and become surrounded by a spongy tissue. The male supplies the eggs with prolactin, the same hormone responsible for milk production in pregnant mammals. The 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 gestation, which in most species requires two to four weeks, his mate visits him daily for “morning greetings”. They interact for about 6 minutes, reminiscent of courtship. The female then swims away until the next morning, and the male returns to vacuuming up food through his snout.
Research published in 2007 indicates the male releases sperm into the surrounding sea water during fertilization, and not directly into the pouch as previously thought.
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 1,500. When the fry are ready to be born, the male expels them with muscular contractions. 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 nurture their young after birth. Infants are susceptible to predators or ocean currents which wash them away from feeding grounds or into temperatures too extreme for their delicate bodies. Less than 0.5% of infants survive to adulthood, explaining why litters are so large. These survival rates are actually fairly high compared to other fish, because of their protected gestation, making the process worth the great cost to the father. The eggs of most other fish are abandoned immediately after fertilization.
Questions surrounding reproductive roles
Reproduction is energetically costly to the male. 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, Bateman's principle suggests that the lesser contributor takes the role of the aggressor. Male seahorses are more aggressive and sometimes “fight” for female attention. According to Amanda Vincent of Project Seahorse, only males tail-wrestle and snap their heads at each other. This discovery prompted further study of energy costs. To estimate the female’s direct contribution, researcher Heather D. Masonjones, associate professor of biology at the University of Tampa, chemically analyzed the energy stored in each egg. To measure the burden on the male, Masonjones measured its oxygen consumption. By the end of incubation, the male consumed almost 33% more oxygen than before mating. The study concluded that the female's energy expenditure while generating eggs is twice that of males during incubation confirming the standard hypothesis.
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. Given an unlimited number of ready and willing partners, males have the potential to produce 17 percent more offspring than females in a breeding season. Also, females have “time-outs” from the reproductive cycle that are 1.2 times longer than those of males. This seems to be based on mate choice, rather than physiology. When the female’s eggs are ready, she must lay them in a few hours or eject them into the water column. Making eggs is a huge cost to her physically, since they amount to about a third of her body weight. To protect against losing a clutch, the female demands a long courtship. The daily greetings help to cement the bond between the pair.
One common misconception about seahorses is that they mate for life. Many species of seahorses form pair bonds that last through at least the breeding season. Some species show a higher level of mate fidelity than others. However, many species readily switch mates when the opportunity arises. H. abdominalis and H. breviceps have been shown to breed in groups, showing no continuous mate preference. Many more species mating habits have not been studied, so it is unknown how many species are actually monogamous, or how long those bonds actually last.
Although monogamy within fish is not common, it does appear to exist for some. In this case, the mate guarding hypothesis may be an explanation. This hypothesis states “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. Though not proven, males could have taken on this role because of the lengthy period the females require to produce their eggs. If males incubate while females prepare the next clutch (amounting to 1/3 of body weight), they can reduce the interval between clutches.
Seahorses feed on small crustaceans floating in the water or crawling on the bottom. With excellent camouflage and a lot of patience, seahorses ambush prey that float within striking range. Mysid shrimp and other small crustaceans are favorites, but some seahorses have been observed eating other kinds of invertebrates and even larval fish. While feeding they produce a distinctive click each time a food item is ingested. The same clicks are heard with social interactions.
Threats of extinction
Most seahorse species are data deficient, and there is insufficient information to make an assessment about their risk of extinction. Because the seahorse population is unknown, there is a risk of losing more seahorses because of the lack of information about how many are dying each year, how many are being born, the number used for souvenirs, etc. Coral reefs and seagrass beds are deteriorating, reducing viable habitats for seahorses.
While many aquarium hobbyists keep seahorses as pets, seahorses collected from the wild tend to fare poorly in home aquaria. Many eat only live foods such as brine shrimp and are prone to stress, which damages their immune systems and makes them susceptible to disease.
In recent years, however, captive breeding has become more popular. Such seahorses survive better in captivity, and are less likely to carry diseases. They eat frozen mysidacea (crustaceans) that are readily available from aquarium stores, and do not experience the stress of moving out of the wild. Although captive-bred seahorses are more expensive, they take no toll on wild populations.
Seahorses should be kept in an aquarium to themselves, or with compatible tank-mates. Seahorses are slow feeders, so fast, aggressive feeders will leave them without food.
Seahorses can co-exist with many species of shrimp and other bottom-feeding creatures. Gobies also make good tank-mates. Keepers are generally advised to avoid 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 H. aimei was not a real species, but a name sometimes used for Barbour's and Hedgehog seahorses. 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 it is believed that they have different medicinal properties from aquarium seahorses. Seahorses are also used as medicines by the Indonesians, the Central Filipinos, and many other ethnic groups.
The problem may be exacerbated by the growth of pills and capsules as the preferred method of ingesting medication. 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. 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, adding to the pressure on the species.
List of species
- H. abdominalis, big-belly seahorse, southern and eastern Australia and New Zealand (Lesson, 1827)
- H. alatus, winged seahorse (Kuiter, 2001)
- H. algiricus, West African seahorse (Kaup, 1856)
- H. angustus, narrow-bellied seahorse (Günther, 1870)
- H. barbouri, Barbour's seahorse (Jordan & Richardson, 1908)
- H. bargibanti, pygmy seahorse, western Pacific (Whitley, 1970)
- H. biocellatus, false-eyed seahorse (Kuiter, 2001)
- H. borboniensis, Réunion seahorse (Duméril, 1870)
- H. breviceps, knobby seahorse, southern and eastern Australia (Peters, 1869)
- H. camelopardalis, giraffe seahorse (Bianconi, 1854)
- H. capensis, cape seahorse (Boulenger, 1900)
- H. colemani (Kuiter, 2003)
- H. comes, tiger tail seahorse (Cantor, 1850)
- H. coronatus, crowned seahorse (Temminck & Schlegel, 1850)
- H. denise, Denise's pygmy seahorse (Lourie & Randall, 2003)
- H. erectus, lined seahorse, western Atlantic (Perry, 1810)
- H. fisheri, Fisher's seahorse (Jordan & Evermann, 1903)
- H. fuscus, sea pony (Rüppell, 1838) (Indian Ocean)
- H. grandiceps, big-head seahorse (Kuiter, 2001)
- H. guttulatus, long-snouted seahorse (Cuvier, 1829)
- H. hendriki, eastern spiny seahorse (Kuiter, 2001)
- H. hippocampus, short-snouted seahorse, the Mediterranean and the Atlantic (Linnaeus, 1758)
- H. histrix, thorny seahorse, Indo-Pacific (Kaup, 1856)
- H. ingens, Pacific seahorse, eastern Pacific (Girard, 1858)
- H. jayakari, Jayakar's seahorse (Boulenger, 1900)
- H. jugumus, collared seahorse (Kuiter, 2001)
- H. kelloggi, great seahorse (Jordan & Snyder, 1901)
- H. kuda, common seahorse (Bleeker, 1852)
- H. lichtensteinii, Lichtenstein's seahorse (Kaup, 1856)
- H. minotaur, bullneck seahorse (Gomon, 1997)
- H. mohnikei, Japanese seahorse (Bleeker, 1854)
- H. montebelloensis, Monte Bello seahorse (Kuiter, 2001)
- H. multispinus, northern spiny seahorse (Kuiter, 2001)
- H. pontohi (Lourie and Kuiter, 2008)
- H. procerus, high-crown seahorse (Kuiter, 2001)
- H. queenslandicus, Queensland seahorse (Horne, 2001)
- H. reidi, longsnout seahorse, Caribbean coral reefs (Ginsburg, 1933)
- H. satomiae, Satomi's pygmy seahorse (Lourie and Kuiter, 2008)
- H. semispinosus, half-spined seahorse (Kuiter, 2001)
- H. severnsi (Lourie and Kuiter, 2008)
- H. sindonis, Shiho's seahorse (Jordan & Snyder, 1901)
- H. spinosissimus, hedgehog seahorse (Weber, 1913)
- H. subelongatus, West Australian seahorse (Castelnau, 1873)
- H. trimaculatus, longnose seahorse (Leach, 1814)
- H. whitei, White's seahorse, eastern Australia (Bleeker, 1855)
- H. zebra, zebra seahorse (Whitley, 1964)
- H. zosterae, dwarf seahorse, Gulf of Mexico and the Caribbean (Jordan & Gilbert, 1882)
Pygmy seahorses are less than 15 millimeters (0.59 in) tall and 17 millimeters (0.67 in) wide members of the genus. Previously the term was applied exclusively to the species H. bargibanti but since 1997, discoveries have made this term obsolete. The species H. minotaur, H. denise, H. colemani, H. pontohi, H. severnsi and H. 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. Molecular analysis (of ribosomal RNA) of 32 Hippocampus species found that H. 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 accounts for why most species have only been noticed and classified since 2001.
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