Lined Seahorse (Hippocampus erectus): Facts, Size and Habitat

What does a lined seahorse look like?

The lined seahorse has the classic seahorse shape: a horse-like head held at a right angle to the body, a long tube-shaped snout, a deep, armored chest and belly, and a tail that curls forward. Instead of scales, its body is covered by skin stretched over about 50 bony plates arranged in rings. It has 11 rings on the trunk and roughly 34–39 on the tail. On top of the head sits a low, crown-like bump called the coronet.

Color is extremely variable. Lined seahorses may be black, gray, brown, green, yellow, orange or red, and they are usually paler on the front. Many have fine white lines tracing the neck and back, which give the species its name, and small white dots on the tail. Individuals living among floating sargassum often grow fleshy tabs and bumps on their skin that help them pass for drifting weed.

Males and females can be told apart at a glance in adults. The male has a smooth brood pouch at the base of the tail, on the front of the body, while the female has none. Males also tend to be slightly larger and have longer tails.

How big is a lined seahorse?

Seahorse size is measured as height, from the top of the coronet to the tip of the uncurled tail. The lined seahorse is one of the larger seahorses of the Americas.

Young seahorses grow quickly, reaching near-adult size in about 8–10 months. For comparison, the dwarf seahorse (Hippocampus zosterae) of Florida and the Gulf of Mexico is only about 2–2.5 cm (1 in) tall and is among the slowest animals in the sea.

Habitat and range

The lined seahorse lives along the Atlantic coast of the Americas from Nova Scotia and the Gulf of Maine south through the eastern United States, Bermuda, the Gulf of Mexico and the Caribbean to Brazil. Scientists still debate whether seahorses farther south, in Brazil, Uruguay and Argentina, belong to the same species. It is most often reported from the US Atlantic and Gulf coasts.

It is a coastal fish that likes something to hold on to. Typical homes are seagrass beds, sponge and soft coral areas, mangrove roots, oyster beds, salt marsh edges and floating rafts of sargassum seaweed. Lined seahorses also cling to human-made structures such as docks, buoy lines and crab pots. Most live in shallow water, but they have been found down to about 70–100 m (230–330 ft).

In northern parts of the range, seahorses move from shallow seagrass into deeper water as the coast cools in autumn and return in spring. The lined seahorse is the only seahorse native to Chesapeake Bay.

What do lined seahorses eat?

Lined seahorses are carnivores that eat tiny animals, mainly small crustaceans such as amphipods, copepods and mysid shrimps, along with other invertebrates and fish larvae. They have no teeth and no true stomach, so food passes through quickly and they must feed often. A growing young seahorse can spend up to 10 hours a day feeding.

They hunt by ambush. A seahorse waits, often camouflaged against vegetation, until prey drifts within about 2–3 cm (1 in) of its snout. Then it flicks its head upward and expands its snout, sucking in the prey in a few thousandths of a second. This rapid pivot feeding means a slow swimmer can catch fast, jumpy prey. Its eyes move independently, so it can watch for food in two directions at once.

How seahorses swim and hide

Seahorses are poor swimmers. A lined seahorse holds its body upright and pushes itself along by fluttering its small dorsal fin, on its back, dozens of times a second, while the pectoral fins behind the head steer. It has no tail fin. Instead, its flexible tail grips seagrass, roots or ropes like a monkey’s tail, letting the fish rest in one spot without being swept away. More detail on how this odd shape still counts as a fish is in is a seahorse a fish?

Staying still and hidden is the lined seahorse’s main defense. Its color can change over minutes to days to match its surroundings, and its bony armor makes it a hard, unappetizing mouthful for many small predators.

Courtship and pair bonds

Lined seahorses form pairs during the breeding season. Partners meet in the morning for a short greeting dance, changing color, circling and holding the same stem of seagrass with their tails. During courtship they also make clicking sounds by snapping the back of the skull against the coronet.

Popular accounts often say lined seahorses stay with one partner for life. Pair bonds are real, but how long they last in the wild has not been well studied, and in most seahorse species a pair lasts a breeding season rather than a lifetime. The evidence is weighed in do seahorses mate for life?

Male pregnancy and birth

On the day of mating, the pair rise together through the water. The female inserts a short tube, called an ovipositor, into the male’s pouch and transfers her eggs, which are about 1.5 mm across. The male fertilizes them inside the pouch, which then seals. In a Chesapeake Bay study, broods ranged from about 100 to more than 1,500 eggs.

The male’s pregnancy lasts about 20–21 days. The pouch supplies the embryos with oxygen and adjusts its fluid toward seawater as birth approaches. When the young are ready, the male grips a support with his tail and pumps his body back and forth, pushing out tiny, fully formed seahorses in bursts. In Chesapeake Bay, breeding runs from about May to October. Seahorse fatherhood is explained in more depth in do male seahorses give birth?

Lifespan, predators and threats

Lined seahorses live about 1–4 years in the wild and up to about 4–5 years in captivity. They can breed from about 4 months old, though most mature at around 8 months. Very few young survive: newborns drift away from their father with no further care and are eaten in huge numbers.

Adults are eaten by larger fish such as tuna and dolphinfish, as well as rays, sharks, crabs and seabirds. People are a bigger problem. Lined seahorses are caught accidentally in shrimp trawls, especially in the Gulf of Mexico and off Florida, and their seagrass and mangrove habitats are damaged by coastal development, pollution and boat traffic. Seahorses are also collected for dried curios, traditional medicine and the aquarium trade, although many lined seahorses sold in aquariums today are captive-bred.

Conservation status

The IUCN Red List classifies the lined seahorse as Vulnerable, with a decreasing population, mainly because of bycatch and habitat loss. All seahorses are listed under Appendix II of CITES, the international treaty on wildlife trade; the listing was agreed in 2002 and took effect in 2004. Exporting countries must show that their trade does not harm wild populations.

Protecting seagrass beds helps lined seahorses and many other coastal animals, from young fish to sea turtles. Some coastal communities and researchers also run seahorse surveys, which help fill gaps in what is known about how many lined seahorses remain.

Lined seahorses and people

The lined seahorse was named Hippocampus erectus in 1810 by the English naturalist George Perry in his illustrated book Arcana. The genus name comes from the ancient Greek hippokampos, a mythical sea-horse, and erectus means upright. Over the following century, naturalists described the same fish again under several other names, including Hippocampus hudsonius for seahorses from the waters around New York. All of these are now treated as one species.

Lined seahorses still turn up in surprising places close to people. They shelter under docks and piers from New England to Florida, and are occasionally recorded in the Hudson River estuary around New York City. Divers at popular shore sites in Florida often search the seagrass and pilings for them.

Several myths surround seahorses in general. They are true fish, not a separate kind of animal. Animals sold as “freshwater seahorses” are usually freshwater pipefishes, since no seahorse lives permanently in fresh water, although the lined seahorse does enter brackish estuaries. And seahorse fathers do not care for their young after birth: once born, the babies are on their own.

Lined seahorse look-alikes

Two other seahorses share parts of the lined seahorse’s range:

• Longsnout seahorse (Hippocampus reidi): found from North Carolina to Brazil, it has a more slender body and a longer snout, and is often dotted with small dark spots rather than pale lines.
• Dwarf seahorse (Hippocampus zosterae): a tiny species of Florida, the Gulf of Mexico, the Bahamas and Bermuda, rarely more than about 2.5 cm (1 in) tall.

North of Florida, any large seahorse seen along the US Atlantic coast is almost certainly a lined seahorse. Many other animals of these coastal waters are featured in ocean animals.