A seahorse (also written sea-horse and sea horse) is any of 47 species of small marine bony fish in the genus Hippocampus. Having a head and neck suggestive of a horse, seahorses also feature segmented bony armour, an upright posture and a curled prehensile tail. Along with the pipefishes and seadragons (Phycodurus and Phyllopteryx) they form the family Syngnathidae. They can be found in seas, brackish estuaries and lagoons.
Evolutionary history Etymology The genus name comes from the Ancient Greek itself from meaning "horse" and meaning "sea monster", , . or "sea animal".
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 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.
Molecular dating implies that pipefish and seahorses diverged 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 would have allowed the expansion of seagrass habitats that served as camouflage for the seahorses' upright posture. These tectonic changes occurred in the western Pacific Ocean, pointing to an origin there, with molecular data suggesting two later, separate invasions of the Atlantic Ocean. In 2016, a study published in Nature found the seahorse genome to be the most rapidly evolving fish genome studied so far.
The evolution of seahorses from pipefish may have been an adaptation related to the biomechanics of prey capture. The unique posture of the seahorse allows them to capture small shrimps at larger distances than the pipefish is capable of.
Description
Seahorses range in size from . They are named for their equine appearance, with bent necks and long snouted heads and a distinctive trunk and tail. 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 bodies. Each species has a distinct number of rings. The armor of bony plates also protects them against predators, and because of this outer skeleton, they no longer have ribs. Seahorses swim upright, propelling themselves using the dorsal fin, another characteristic not shared by their close pipefish relatives, which swim horizontally. Razorfish are the only other fish that swim vertically. The pectoral fins, located on either side of the head behind their eyes, are used for steering. They lack the caudal fin typical of fishes. Their prehensile tail is composed of square-like rings. They are adept at camouflage, and can grow and reabsorb spiny appendages depending on their habitat.
Unusual among fish, a seahorse has a flexible, well-defined neck. It also sports a crown-like spine or horn on its head, termed a "coronet", which is distinct for each species.
Seahorses swim extremely poorly, rapidly fluttering a dorsal fin and using pectoral fins to steer. The slowest-moving fish in the world is H. zosterae (the dwarf seahorse), with a top speed of about per hour.Guinness Book of World Records (2009) Since they are poor swimmers, they are most likely to be found resting with their prehensile tail wound around a stationary object. They have long snouts, which they use to suck up food, and their eyes can move independently of each other like those of a chameleon.
Habitat
Seahorses are mainly found in shallow tropical and temperate salt water throughout the world, from about 45°S to 45°N. They live in sheltered areas such as seagrass beds, estuaries, coral reefs, and mangroves. Some species can be found rafting in clumps of sargassum far away from their original ranges, some may spend their entire lifetimes around local sea fans.
Four species are found in Pacific waters from North America to South America. In the Atlantic, Hippocampus erectus ranges from Nova Scotia to Uruguay. H. zosterae, known as the dwarf seahorse, is found in the Bahamas.
Colonies have been found in European waters such as the Thames Estuary.
Two species live in the Mediterranean Sea: H. guttulatus (the long-snouted seahorse), and H. hippocampus (the short-snouted seahorse). These species form territories; males stay within of habitat, while females range over about one hundred times that.
Feeding habits
Seahorses use their long snouts to eat their food with ease. 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. Seahorses feed on small crustaceans floating in the water or crawling on the bottom. With excellent camouflage, seahorses ambush prey that floats within striking range, sitting and waiting until an optimal moment. Mysid shrimp and other small crustaceans are favorites, but some seahorses have been observed eating other kinds of invertebrates and larval fish. One study found that their distinctive head morphology gave them a hydrodynamic advantage by minimizing interference while approaching evasive prey. Thus the seahorse can get very close to the copepods on which it preys. Seahorses have three distinctive feeding phases: preparatory, expansive, and recovery. During the preparatory phase, the seahorse slowly approaches the prey while in an upright position, after which it slowly flexes its head ventrally. In the expansive phase, the seahorse captures its prey by simultaneously elevating its head, expanding the buccal cavity, and sucking in the prey item. During the recovery phase, the jaws, head, and hyoid apparatus of the seahorse return to their original positions. The phased prey capture mechanism is known as pivot-feeding.
The amount of available cover influences the seahorse's feeding behaviour. For example, in wild areas with small amounts of vegetation, seahorses will sit and wait, but an environment with extensive vegetation will prompt the seahorse to inspect its environment, feeding while swimming rather than sitting and waiting. Conversely, in an aquarium setting with little vegetation, the seahorse will fully inspect its environment and makes no attempt to sit and wait.
Reproduction
The male seahorse is equipped with a brood pouch on the ventral, or front-facing, side of the tail. When mating, the female seahorse deposits up to 1,500 eggs in the male's pouch. The male carries the eggs for 9 to 45 days until the seahorses are released into the water fully developed, but very small. The male often mates again within hours or days during the breeding season.
Courtship Before breeding, seahorses may court for several days. 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. 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 "predawn 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 drift upward snout-to-snout, out of the sea grass, often spiraling as they rise. They interact for about 6 minutes, reminiscent of courtship. 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 sea grass and she swims away.
Phases of courtship Seahorses exhibit four phases of courtship that are indicated by clear behavioral changes and changes in the intensity of the courtship act. Phase 1, the initial courtship phase, typically takes place in the early morning one or two days before physical copulation. During this phase the potential mates brighten in colour, quiver, and display rapid side-to-side body vibrations. These displays are performed alternately by both the male and the female seahorse. The following phases, 2 through 4, happen sequentially on the day of copulation. Phase 2 is marked by the female pointing, a behaviour in which the female will raise her head to form an oblique angle with her body. In phase 3 males will also begin the same pointing behaviour in response to the female. Finally, the male and female will repeatedly rise upward together in a water column and end in mid-water copulation, in which the female will transfer her eggs directly into the male's brood pouch.
Phase 1: Initial courtship This initial courtship behaviour takes place about 30 minutes after dawn on each courtship day, until the day of copulation. 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. There is repeated reciprocal quivering. This starts when the male approaches the female, brightens and begins to quiver. The female will follow the male with her own display, in which she will also brighten and quiver about 5 seconds later. As the male quivers, he will rotate his body towards the female who will then rotate her body away. 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. However, the female will shift her tail attachment site, causing the pair to circle their common hold-fast.
Phase 2: Pointing and pumping This phase begins with the female beginning her pointing posture, by leaning her body towards the male, who will simultaneously lean away and quiver. This phase can last up to 54 minutes. Following phase 2 is a latency period (typically between 30 minutes and four hours), during which the seahorses display no courtship behaviour and females are not bright; males will usually display a pumping motion with their body.
Phase 3: Pointing – pointing
The third phase begins with the females brightening and assuming the pointing position. The males respond with their own brightening and pointing display. This phase ends with the male departing. It usually lasts nine minutes and can occur one to six times during courtship.
Phase 4: Rising and copulation The final courtship phase includes 5–8 bouts of courtship. 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. During the first bout, following the facing behaviour, the seahorses will rise upward together anywhere from 2 to 13 cm in a water column. During the final rise, the female will insert her ovipositor and transfer her eggs through an opening into the male's brood pouch.
Fertilization