A seahorse (also written sea-horse and sea horse) is any of 46 species of small marine bony fish in the genus Hippocampus.

[6] Having a head and neck suggestive of a horse, seahorses also feature segmented bony armour, an upright posture and a curled prehensile tail.

[10][11] These species form territories; males stay within 1 m2 (10 sq ft) of habitat, while females range over about one hundred times that.

The best known and best studied fossils are specimens of Hippocampus 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.

This has led to speculation that seahorses evolved in response to large areas of shallow water, newly created as the result of tectonic events.

Scientists believe the courtship behavior synchronizes the animals' movements and reproductive states, so that the male can receive the eggs when the female is ready to deposit them.

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 drift upward snout-to-snout, out of the sea grass, often spiraling as they rise.

During this phase the males and females will remain apart during the night, but after dawn they will come together in a side-by-side position, brighten, and engage in courtship behaviour for about 2 to 38 minutes.

During phase 1 the tails of both seahorses are positioned within 1 cm of each other on the same hold-fast and both of their bodies are angled slightly outward from the point of attachment.

Each bout of courtship begins with both the male and female anchored to the same plant about 3 cm apart; usually they are facing each other and are still bright in colour from the previous phase.

Infants are susceptible to predators or ocean currents which wash them away from feeding grounds or into temperatures too extreme for their delicate bodies.

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.

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.

The study concluded that the female's energy expenditure while generating eggs is twice that of males during incubation, confirming the standard hypothesis.

However, they are slow to consume their food and have extremely simple digestive systems that lack a stomach, so they must eat constantly to stay alive.

With excellent camouflage seahorses ambush prey that floats within striking range, sitting and waiting until an optimal moment.

[44] Mysid shrimp and other small crustaceans are favorites, but some seahorses have been observed eating other kinds of invertebrates and even larval fish.

In a study of seahorses, the distinctive head morphology was found to give them a hydrodynamic advantage that creates minimal interference while approaching an evasive prey.

Conversely, in an aquarium setting with little vegetation, the seahorse will fully inspect its environment and makes no attempt to sit and wait.

[50] Additionally, bycatch in many areas causes high cumulative effects on seahorses, with an estimated 37 million individuals being removed annually over 21 countries.

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.

They eat frozen mysidacea (crustaceans) that are readily available from aquarium stores,[53] and do not experience the stress of moving out of the wild.

[55] Seahorses require vertical swimming space to perform reproductive functions and to prevent depth-related health conditions like gas bubble disease, so a refugium that is at least 20 inches by 51 centimeters deep is recommended inside an aquarium.

[56] Animals sold as "freshwater seahorses" are usually the closely related pipefish, of which a few species live in the lower reaches of rivers.

Despite a lack of scientific studies or clinical trials,[58][59] the consumption of seahorses is widespread in traditional Chinese medicine, primarily in connection with impotence, wheezing, nocturnal enuresis, and pain, as well as labor induction.

Declining availability of the preferred large, pale, and smooth seahorses has been offset by the shift towards prepackaged preparations, which makes it possible for TCM merchants to sell previously unused, or otherwise undesirable juvenile, spiny, and dark-coloured animals.

Dried seahorse retails from US$600 to $3000 per kilogram, with larger, paler, and smoother animals commanding the highest prices.

[24] Most pygmy seahorses are well camouflaged and live in close association with other organisms including colonial hydrozoans (Lytocarpus and Antennellopsis), coralline algae (Halimeda), and sea fans (Muricella, Annella, and Acanthogorgia).