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Seahorse

Adventurer · ages 7–10Adapted from Wikipedia
Two Korean seahorses (Hippocampus haema) during their courtship. Seahorses often pair up and perform a dance together before they mate.

A seahorse (also written sea-horse or sea horse) is any of 47 species of small marine bony fish in the genus Hippocampus.

Seahorses have a head and neck that look a little like a horse. They also have bony plates on their bodies, stand upright, and have a curled prehensile tail. Along with pipefishes and seadragons (Phycodurus and Phyllopteryx), they belong to the family Syngnathidae.

Seahorses live in seas, estuaries where fresh and salt water meet, and lagoons.

Evolutionary history

The name of the genus comes from Ancient Greek words. The word hippókampos comes from híppos, meaning "horse," and kámpos, meaning "sea monster" or "sea animal."

Scientists have studied the bodies, shapes, and genes of seahorses. These studies show that seahorses are very changed forms of pipefish. Seahorse fossils are rare. The best-known ones are of Hippocampus guttulatus (often called H. ramulosus in books). They were found near the Marecchia River in Rimini Province, Italy. They are from the Lower Pliocene, about 3 million years ago.

The oldest known seahorse fossils are of two pipefish-like species, H. sarmaticus and H. slovenicus. They were found in a coprolitic layer in the Tunjice Hills. This is a middle Miocene lagerstätte, a place where many fossils are found, in Slovenia. These fossils are about 13 million years old.

Molecular dating suggests that pipefish and seahorses split apart during the Late Oligocene. Some scientists think seahorses may have evolved because tectonic events created large areas of shallow water. This shallow water may have helped seagrass spread. The grass could have offered camouflage for seahorses, whose bodies stand upright.

These changes likely happened in the western Pacific Ocean. This suggests that seahorses started there. Genetic data suggests that later, seahorses moved into the Atlantic Ocean two separate times.

In 2016, a study published in Nature found that the seahorse genome is the fastest-changing fish genome studied so far. Seahorses may have evolved from pipefish because of the biomechanics of catching food. Their upright posture lets them catch small shrimp from farther away than pipefish can.

Description

Spiny seahorse H. histrix from East Timor holding on to soft coral with its prehensile tail

Seahorses range in size from 1.5 to 35 cm (0.6 to 13.8 in). They are named for their equine look, with bent necks, long snouted heads, and a distinctive trunk and tail. Although they are bony fish, they have no scales. Instead, they have thin skin stretched over bony plates arranged in rings around their bodies. Each species has a different number of rings. This bony armor protects them from predators. Because of this outer skeleton, they no longer have ribs. Seahorses swim upright, moving themselves with their dorsal fin. Their close pipefish relatives swim horizontally instead. Razorfish are the only other fish that swim vertically. Their pectoral fins, located on either side of the head behind the eyes, are used for steering. They do not have the caudal fin that most fish have. Their prehensile tail is made of square-like rings. Seahorses are good at camouflage. They can grow and reabsorb spiny parts of their bodies depending on their habitat.

Unlike most fish, a seahorse has a flexible, well-defined neck. It also has a crown-like spine or horn on its head, called a "coronet," which is different for each species.

Seahorses are very poor swimmers. They flutter their dorsal fin quickly and use their pectoral fins to steer. The slowest-moving fish in the world is H. zosterae (the dwarf seahorse). It moves at a top speed of about 1.5 m (5 ft) per hour. Because they swim so poorly, they are often found resting with their prehensile tail wrapped around something that does not move. They have long snouts, which they use to suck up food. Their eyes can move independently of each other, like those of a chameleon.

Habitat

Seahorse hiding using camouflage

Seahorses live mainly in shallow tropical and temperate salt water all over the world. They live in sheltered places such as seagrass beds, estuaries, coral reefs, and mangroves. Some kinds have been found rafting on clumps of sargassum, far from where they first lived. Some may spend their whole lives around local sea fans.

Four kinds live in Pacific waters from North America to South America. In the Atlantic, Hippocampus erectus lives from Nova Scotia to Uruguay. H. zosterae, called the dwarf seahorse, lives in the Bahamas. Groups of Short-snouted Seahorse have been found in European waters, such as the Thames Estuary in England. Two kinds live in the Mediterranean Sea: H. guttulatus (the long-snouted seahorse) and H. hippocampus (the short-snouted seahorse). Seahorses set up their own territories. Males stay within about 1 m2 (10 sq ft) of their home area, while females roam over about one hundred times that much space.

Feeding habits

Seahorses rely on stealth to ambush small prey such as copepods. They use pivot feeding to catch the copepod, which involves rotating their snout at high speed and then sucking in the copepod.

Seahorses use their long snouts to eat. They are slow eaters. Their digestive systems are very simple and have no stomach, so they must eat almost all the time to stay alive. They feed on small crustaceans that float in the water or crawl on the bottom. With excellent camouflage, seahorses wait for prey to drift within reach. Mysid shrimp and other small crustaceans are their favorite foods, but some have been seen eating other invertebrates and larval fish. Their head shape helps them get very close to the copepods they hunt. Each catch has three steps: getting ready, opening the snout, and returning to rest. This method is called pivot-feeding.

The number of plants around them changes how seahorses feed. In wild areas with few plants, seahorses sit and wait for food. In places with many plants, they look around and feed while swimming. In an aquarium with few plants, seahorses carefully check their surroundings and do not sit and wait.

Reproduction

See also: Animal sexual behavior § Seahorse

Seahorse life-cycle

The male seahorse has a pouch on the front of its tail. The female puts up to 1,500 eggs into the male's pouch, and he carries them for 9 to 45 days. Before they mate, seahorses may spend several days in courtship. They change color, swim side by side, and move together. Near the end, the female uses her ovipositor to place her eggs in the male's pouch. This happens during copulation, when the pair rises together in the water column while both hold on to the same hold-fast. Inside the pouch, the eggs are fertilized. The pouch helps the young grow and gets them ready for life in the sea. Scientists think this protected way of fertilizing eggs may lower sperm competition among males. Pipefishes, which are related to seahorses, may have similar methods, but this has not been proven.

The male releases the young, called fry, by pushing them out with muscle movements, usually at night. Many young do not survive, so males often release large numbers. Reproduction takes a lot of energy from the male. Bateman's principle suggests that the partner who spends less energy takes on the role of the aggressor. Amanda Vincent of Project Seahorse found that only males tail-wrestle and snap at each other. Many seahorses form pair bonds that last through the breeding season, but many species switch mates easily. Scientists are still studying whether monogamy happens in some species. One idea, the mate-guarding hypothesis, suggests that males stay with one female because caring for the young is especially helpful for them.

Seahorses in phase 1 of courtship

Threats of extinction

Destructive fishing is damaging coral reefs and seagrass beds. This leaves fewer safe homes for seahorses. Every year, many seahorses are caught by fishermen in many countries.

Protecting seahorses is a worldwide concern. It is closely linked to protecting their homes, including seagrass beds such as Posidonia oceanica and Zostera marina, mangroves, coral reefs, estuaries, and seaweeds. According to Woodall et al., human activities can harm seahorses. Building homes, businesses, and tourist areas along coasts can damage their habitats. Nets and trawls can also catch seahorses by accident. Seahorses are hard to study because there are few of them, they are spread out, and they are hard to see. Scientists use maps of where seahorses live to find important areas, possible dangers, and places where more research is needed.

Aquaria

Seahorses (Hippocampus erectus) at the New England Aquarium

Many aquarium hobbyists keep seahorses as pets. However, seahorses taken from the wild often do poorly in home tanks. Many eat only live foods, such as brine shrimp, and they get stressed easily. Stress weakens their immune systems, which makes them more likely to get sick. In recent years, captive breeding has become more common. Captive-bred seahorses survive better and are less likely to carry diseases. They can eat frozen mysidacea (crustaceans) from pet stores. Feeding microalgae such as Tisochrysis lutea to young seahorses has been shown to help them grow much heavier. Captive-bred seahorses cost more, but they do not harm wild populations.

Seahorses need calm tanks with gentle water flow and peaceful tank mates, because they are slow eaters. Many shrimp and other bottom-feeding creatures can share a tank with them, and gobies also make good tank mates. Keepers are generally advised to avoid eels, tangs, triggerfish, squid, octopus, and sea anemones. Good water quality is very important, so seahorses should not be added to a new tank. Poor water can show up as clamped fins, less eating, erratic swimming, and gasping at the surface. Seahorses also need space to swim upward and downward, so a refugium at least 20 inches (51 centimeters) deep is recommended. Animals sold as "freshwater seahorses" are usually pipefish. The so-called "freshwater seahorse" H. aimei is not a valid species. It is a synonym sometimes used for Barbour's and hedgehog seahorses. These fish can be found in estuarine environments, but they are not really freshwater fish.

Consumption

Dried seahorse

Seahorse populations are thought to be endangered because of overfishing and habitat destruction. Seahorses are used in traditional Chinese medicine. People use them for health problems such as impotence, wheezing, nocturnal enuresis, pain, and labor induction. Scientists have not studied these uses much. Many seahorses are caught each year to be sold for medicine. Popular types include H. kellogii, H. histrix, H. kuda, H. trimaculatus, and H. mohnikei. Some people in Indonesia and the central Philippines also eat seahorses. Since 2002, the buying and selling of seahorses between countries has been controlled under CITES.

The problem may be growing. Many people now take seahorses as pills and capsules. Pills are cheaper and easier to find than traditional medicines made from whole seahorses. However, it is harder to know what is inside them. In the past, TCM practitioners and buyers only accepted seahorses of a certain size and quality. Large, pale, and smooth seahorses are now harder to find. Pills allow sellers to use young and darker seahorses that were once thrown away. Seahorses are very expensive. The larger, paler, and smoother ones cost the most.

Species

H. kuda, known as the "common seahorse"
H. subelongatus, known as the "West Australian seahorse"

Scientists count 47 seahorse species in the genus Hippocampus. This count is based on the newest overall review of the group and later discoveries:

H. whitei, known as "White's seahorse"

Pygmy seahorses

Main article: Hippocampinae

Hippocampus satomiae (Satomi's pygmy seahorse) attached to coral

Pygmy seahorses are small members of the genus. They are less than 15 mm tall and 17 mm wide. For a long time, the name was used only for the species H. bargibanti. Since 1997, new discoveries have changed this. Several other species have been described, including H. minotaur, H. denise, H. colemani, H. pontohi, H. severnsi, H. satomiae, H. waleananus, H. nalu, and H. japapigu. Other possible species that are not yet unclassified have also been reported. Pygmy seahorses can be told apart from other seahorses by their small size, their 12 trunk rings, their 26 to 29 tail rings, and the place on the male's trunk where the young are carried.

Most pygmy seahorses are well camouflaged. They live close to other living things, including colony-forming hydrozoans (Lytocarpus and Antennellopsis), coralline algae (Halimeda), and sea fans (Muricella, Annella, and Acanthogorgia). Their camouflage and small size explain why most species were only noticed and classified after 2001.

Quiz time!

4 questions about Seahorse

Question 1 of 4

Which seahorse carries the eggs in a special pouch?

Pictures

A Arabian seahorse (Hippocampus jayakari), photographed in Egypt.
A male seahorse carrying eggs in his pouch at the New York Aquarium in Brooklyn. In seahorses, it is the father who carries and cares for the developing young.
Seahorses and scorpions on a skewer, sold as a snack at Wangfujing Snack Street in Beijing, China.
A diagram showing a male seahorse (Hippocampus comes) carrying developing young in its pouch. In seahorses, the male, not the female, becomes pregnant.

Keep exploring

Horse
Prehensile tail
Osteichthyes
Genus
Estuary
Pipefish

This article is a child-friendly adaptation of the Wikipedia article on Seahorse, available under CC BY-SA 4.0.

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