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Diving with Seahorses

A Diver's Best Friend

Seahorses are mesmerising and magical marine creatures. It’s like they materialised from a young child’s dream with their horse like head, a tail akin to a monkey, eyes that move independently of each other and skin colour that changes like a chameleon.

They are one of 4 families in the syngnathiform family order which also includes pipefish, flag tail pipefish and seadragons. They like sheltered areas where they are well camouflaged. They camouflage themselves by changing colour quickly to blend in with their surroundings. They also allow encrusting organisms to settle on them and they can grow long skin appendages to match their surroundings even better.

During mating their skin will lighten and darken. Generally the easiest part of a seahorse to spot is the tail. They swim in an upright position with their tails down and their heads up. Their dorsal fin moves them forward and the pectoral fin controls steering and turning.

There are about 55 species of seahorses worldwide, and there may be as many sub-species. Identification can be problematic as individuals of the same species can vary greatly in appearance. The pygmy seahorse is a recently discovered relative of the common seahorse, many divers consider spotting a pygmy the highlight of their macro dive! A lot of deco time and a dive master familiar with the dive site will help in the search for these cryptic critters.

Seahorse Fact Sheet

Family name: Syngnathidae

Order name: Gasterosteiformes

Common name: Seahorse

Scientific name: Hippocampus



Distinguishing Features

Seahorses are characterised by an elongated segmented bony body with an upright posture, an unusual equine shaped head and neck and a curled tail . Instead of scales that are found on most fish, seahorses have a thin layer of skin stretched over a series of bony plates. These plates show themselves as the bony rings along their body's trunk; each species is distinguished by a specific number of rings. Another feature that is used to identify different species is the “coronet”, or crown-like spine or horn on their heads. They have no pelvic fins but most have small pectoral fins and a single dorsal fin. They have a prehensile tail (able to grasp) and a tube-like mouth with no teeth, and small gill openings.

Unlike (most) humans but similar to chameleons, seahorses can move their eyes independently of each other. They range in size from 10mm to 35cm, the larger species' being the Pacific and pot-bellied seahorses. They average 15mm in length although some are smaller, and may appear even smaller still, as their tails are generally curled around a gorgonian branch. The females are slightly larger than the males and also have longer tails.

Behaviour

Seahorses are definitely not the Michael Phelps of ocean critters. In fact they hold the title for the fish that swims the slowest. They rely on their pectoral fins for stability and steering, their swim bladder for buoyancy, and their dorsal fins beat 30-70 times per second for propulsion. Given their size, you can imagine this is not particularly effective, so you will most likely find them grasping seaweed or sea fans with their tails.

Seahorses are widespread across our oceans. They inhabit coral reefs and sea grass beds and are known to move into deeper waters in some areas during the winter to escape turbulent elements. Seahorses have small home rangers with females having a larger territory than the males, 1.4 square meters compared to 0.5 square metres. These territories do overlap and allow for courtship and breeding. They remain virtually immobile for much of the time and can change colour to better match their surroundings and to facilitate both prey capture and predator avoidance.

Seahorses have a long annual mating season during which the male pretty much remains attached to his chosen spot. During gestation the female will greet him every morning and it is thought that these daily interactions help synchronise female egg preparation at the end of male pregnancy. The morning after the ‘delivery’ the greeting is prolonged into courtship and mating.

Their digestive system, or rather lack thereof (they do not have a stomach), means that seahorses need to eat constantly and therefore spend large parts of their waking hours hunting. Some species are nocturnal and others are diurnal.

Feeding Habits

Seahorses are carnivorous and their diet is determined by their small mouths and their lack of teeth. Adult seahorses eat between 30-50 times a day and juveniles can consume up to 3,000 pieces of food per day.

Their camouflage is critical to successful hunting. Their hunting style is by ambush, they attach themselves to plants or coral, relying on their ability to change colour to blend in with their surroundings and their ability to look in different directions simultaneously. Here they remain motionless waiting for their prey to unsuspectingly pass by. At the opportune moment, they extend their neck and use their tube-like snout to suck up their prey whole like a vacuum, creating a distinctive sound. Seahorses eat small crustaceans such as amphipods, fish fry and other invertebrates.

Reproduction

There is a romatic myth that seahorses are monogamous, however this perception has been shattered and we now know that their monogamy lasts for a single breeding season. The species that can own the myth, and do mate for life, is the Australian Hippocampus whitei. This species have shown that practice makes perfect, as they are able to produce more offspring the longer they remain together.

Sea horses do have a complex and enchanting courtship ritual that lasts several days, involving a ‘dance’ where they entwine their tales while swimming side by side, or attached to a coral branch, or most mesmerising is the ‘predawn dance’ where they wheel around in unison with their noses touching while they change colour. Divers can get to witness it afresh each season!

Sea horses are very special little creatures, and procreate in a rather unusual manner, engaging in a profound biological role reversal. Being part of the Syngnathidae family, the males, perform the reproduction function typically associated with females. The female inserts her eggs through an oviduct into the male’s brood pouch where they are fertilised. Between 5 to 2,500 eggs (species dependent) may be transferred, which can take up to 8 hours.

After the ‘act’ the male attaches himself by his tail to a nearby coral or seaweed to wait out the gestation period, which varies between 9-45 days. It is common in the fish world for the males to take care of the eggs by guarding them or fanning them to keep them clean and provide enough oxygen. But seahorses take paternal care to an extreme unknown elsewhere in the animal kingdom. A capillary system provides oxygen and nutrients to the young. When the time arrives, his body will contort with contractions for up to 12 hours while he expels perfectly formed miniature versions of himself. This generally occurs during the night and amazingly he is ready for fertilisation the next morning when his mate returns!

Life Cycle

Teeny tiny seahorses start their life when they are expelled from their father’s pouch. Despite the extreme lengths he went to to protect them during their gestation, they are now on their own and no longer receive any form of parental nurturing. They immediately rise to the surface where they swallow air to fill up their swim bladder which then seals up. During the first 2-3 weeks of their lives they drift along in the plankton layer of the ocean. Here they are at the mercy of the elements and predators. The relatively large litters (compared to their size) and protection received from their fathers during gestation slightly compensate for the low survival rate of the juveniles - less than 1 in a 1,000 fry survive to adulthood.

Smaller seahorses mature at a younger age compared to the larger species. This ranges between 3 months and a year. With so many different species, dispersed across the globe, their breeding season varies according to their location. The specific season is influenced by their environment, such as light, temperature and food availability. The seahorses’ lifespan is not known in their natural habitat. In aquaria they have been recorded to survive from 1 to 4 years.

Predation

Seahorses superb camouflage capabilities and unpalatable bony plates and spines means that few predators specifically target them as food. Unfortunately they are consumed via opportunistic foraging. Invertebrates, fishes, turtles, waterbirds and marine mammals have all been found to have consumed seahorses. Pelagic fish stomach contents have also included seahorses which suggests that they may be found in the open ocean more than previously thought.

Distribution

They are found all over the world and inhabit coral reefs and sea grass beds. They have been recorded at depths as low as 0.5m and as deep as 75m.

The Barbigants pygmy seahorse (hippocampus barbiganti) can be found all over Indonesia in various colours and at all depths. They can be found in the muricella sea fans in Papua New Guinea, Philippines and Malaysia. These fans have bulbous red polyps as do the pygmies. This, along with their small size, is what makes them so difficult to spot.

Denise Hackett recently discovered a new pygmy species in Indonesia. It's named after her, Hippocampus Denise, but it's often called the 'plucked chicken pygmy seahorse' due to its unusual appearance with a lack of the typical bumps (tubercles). Hippocampus Denise is normally found in light yellow gorgonians which, like the pygmy, are less bulbous with smaller polyps.

The weedy pygmy seahorse is an even newer discovery. First recorded in the Banda Sea in 2000, they are now regularly seen at Wakatobi and the Lembeh Strait. The Raja Ampat area is another good place to find them. This species is the smallest and most cryptic. They seem to move around more than other species making them even harder to pin point.

Ecological Considerations

Pygmy and common seahorses such as the tigertail, are sensitive to stress and too much harassment from divers is certainly not good for them. They particularly dislike bright lights and will turn their heads away if a diver shines a dive light at them. Placing a red filter (or red plastic bag with an elastic band over it) on to a dive light will give a softer diffused light that they will be less sensitive to and should enable photographers to get better pictures.

Threats

Their numbers around the world are diminishing. They are used as a traditional medicine in China, Japan, Korea, Indonesia and the Philippines. Dried seahorses are viewed as a cure for a range of conditions such as asthma and skin disease, but they are most commonly used as a treatment of sexual dysfunction. Asian nations consume around 45 tons of dried sea horse annually. That's equates to about 16 million of them!

One problem is that many of the harvested seahorses are juveniles and have not had a chance to breed and reproduce. A large specimen can sell for up to US$ 550 per pound in Hong Kong so it is clearly big business. There is no clinical proof that they have any medicinal benefits but the Chinese have been using them as a fundamental part of their traditional medicine for centuries and there is a strong conviction that there is a benefit.

They are also being over-collected for the aquarium trade in Europe, North America and Japan. Furthermore, their habitat is under threat. Sea grass areas, mangroves and coral reefs are declining world wide.

Protective Measures

The Knysna Seahorse is listed as endangered on the IUCN Red List of Threatened Species and 9 other species are categorised as vulnerable. Some seahorses, such as the big-belly seahorse is on the precipice between vulnerable and endangered. Many others have not yet been categorised due to lack of data. CITES (Convention on International Trade in Endangered Species) has urged countries to join their mission is saving seahorses. 175 nations have agreed that seahorse exports need to be limited to sustainable levels, however what this means and how to implement these controls are still a work in progress. Fortunately some more progressive nations do have unilateral protective measures in place. For example, harvesting them in South Africa has been outlawed, partly as a result of the collapse in the numbers of the endemic Knysna Seahorse.

Dive Sites for Seahorses

More detailed information on seahorse diving destinations:

Caribbean Sea / Atlantic Ocean

Oceania

Pacific Ocean

Red Sea / Middle East / Indian Ocean

South East Asia

Dive The World Recommendations: Solomon Islands, Indonesia, Philippines and Malaysia.

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