Why Seahorses?
Seahorse biology

Species Spotlight

The long-snouted seahorse, found in Europe, is often found on seagrasses, macroalgae and colonial invertebrates.

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Seahorse biology is at the core of our work. Project Seahorse began when Dr. Amanda Vincent and Dr. Heather Koldewey, both studying seahorses, formed the organization to help protect them and their habitats in 1996.

Seahorse biology remains a key part of our work, as we study these creatures and share what we learn.

Taxonomy

The actual number of seahorse species is not clear, and there’s disagreement. More than 120 species names have been proposed for seahorses over the past 200 years, but many have turned out to be synonyms for the same species (list of synonyms). That’s partly because seahorses can change colour and grow skin filaments to blend in with their surroundings.

Another problem is a lack of understanding of how individuals vary in shape, size, and form, or how that’s affected by habitat, geographic location, temperature, and age. Many scientists who thought they’d found a new species were just renaming one that had already been described.

Previous attempts to revise the seahorse genus sometimes added to the confusion because they were limited in scope or did not examine the specimens on which the original species description was based. Project Seahorse currently recognizes 47 species, although that number is likely to increase with further research. See Project Seahorse's position on seahorse taxonomy.

Read about Project Seahorse's position on seahorse taxonomy.

Distribution

All seahorses are marine species, usually living among seagrass beds, mangrove roots and coral reefs in shallow temperate and tropical waters. Some species, such as the endangered South African seahorse H. capensis, can also be found in estuaries.

Seahorses range roughly from 50 degrees north to 50 degrees south latitude, with most species occurring in the Western Atlantic Ocean and the IndoPacific region.

Habitat degradation is a real threat to seahorse populations as they mainly inhabit shallow, coastal areas which are highly influenced by human activities.

Survival

Natural lifespans for seahorses are virtually unknown. Most estimates come from laboratory or aquarium observations. Known lifespans for seahorse species range from about one year in the smaller species to an average of three to five years for the larger species.

Watch H. denise, one of the smallest known species of seahorse, eating (Windows Media or MPEG):

Adult seahorses have few predators because of their excellent camouflage, a sedentary life style, and unappetizing bony plates and spines. They have been found in the stomachs of large pelagic fishes such as tuna and dorado. They are also eaten by skates and rays, penguins, and other water birds. A seahorse has even appeared in the stomach of a loggerhead sea turtle. Crabs may be among the most threatening predators, with damaged tails indicating a narrow escape. Young seahorses are the most vulnerable to being eaten by other fish.

For some populations of seahorses, humans are the greatest predator.Seahorses are voracious predators, relying entirely on live, moving food. They are opportunitistic predators, sitting and waiting until prey come close enough and then sucking them rapidly from the water with their long snouts. They will ingest anything small enough to fit into their mouths -- mostly small crustaceans, fish fry and other invertebrates.

Growth

Seahorses have heads at right angles to the body and prehensile tails which wrap around seagrass stems, corals, sticks, or any other suitable natural or artificial object. These traits, along with a pouch for the young and eyes that swivel independently of each other, lend to the unique nature of these fish.

Instead of the scales found on most fish, seahorses have a thin layer of skin stretched over a series of bony plates visible as rings around the trunk. These rings are useful in identifying species, as are the cheek spines and coronet (a crown-like group of spines on the top of the head). In addition to the spines, some species also have bony bumps or skin filaments protruding from these bony rings.

Seahorses have no stomach or teeth. They suck in prey through a tubular snout and pass it through an inefficient digestive system. Like other fish, they breathe through gills, extracting oxygen from the water.

Unlike other fish, however, the gills are small and compacted. Seahorses swim using the propulsive force of a quickly oscillating dorsal fin, and use the pectoral fins on either side of the body for steering and stability. They are more adapted to maneuverability than speed.

Seahorse heights are measured from the tip of the tail to the top of the coronet, or "crown" (see figure above, and our technical bulletin on measuring seahorses).

Seahorse sizes vary with species, ranging from the large Australian big-bellied seahorse (Hippocampus abdominalis, which can surpass 30 cm in height) to the tiny pygmy seahorse (H. denise, less than 2 cm). Their weights vary with reproductive stage, increasing a great deal when they have ripe eggs (females) or are pregnant (males).

Seahorses are masters of camouflage, changing colour and growing skin filaments to blend in with their surroundings. Short-term colour changes may also occur during courtship displays and daily greetings. Male and female seahorses can be told apart by the presence of a brood pouch on the male.

Watch H. bargibanti blend into its surroundings:

Reproduction

The male seahorse, rather than the female, becomes pregnant. This unusual mode of reproduction is the most extreme form of male parental care yet discovered.

Sexual maturity in males is usually determined by the presence of a brood pouch. Male seahorses are able to become pregnant any time during the breeding season, which varies with species, and most likely depends on water temperature. Lunar and monsoon patterns may affect the timing of the breeding season.

Most seahorse species are monogamous, forming pair bonds that last the entire breeding season. Pair bonds are reinforced by daily greetings in which the female and male change colour, promenade and pirouette together. This dance lasts several minutes, and then they separate for the rest of the day. The greetings occur throughout the male pregnancy. Once the male has given birth and it is time to re-mate, sometimes only hours later, this greeting is extended into a courtship that, for one species, lasts up to nine hours.

The female inserts her ovipositor into the male’s brood pouch, where she deposits her eggs, which the male fertilizes. The fertilized eggs then embed in the pouch wall and become enveloped with tissues. The pouch acts like the womb of a female mammal, complete with a placental fluid that bathes the eggs, and provides nutrients and oxygen to the developing embryos while removing waste products. The pouch fluid is altered during pregnancy from being similar to body fluids to being more like the surrounding seawater. This helps reduce the stress of the offspring at birth.

Pregnancy lasts between two and four weeks, the length decreasing with increasing temperature. At the end of gestation the male goes into labour (usually at night), pumping and thrusting for hours to release his brood. Young are miniature adult seahorses, independent from birth, and receive no further parental care. Newborns of most species measure between seven and 12 millimeters. The number of young released averages about 100 to 200 for most species, but can be as low as five for the smaller species, or as high as 1,500.

Movement

Seahorses swim using the propulsive force of a quickly oscillating dorsal fin, and use the pectoral fins on either side of the body for steering and stability. They are more adapted to maneuverability than speed, and therefore rely primarily on camouflage to avoid detection from predators.