Just remembering the incident makes me smile. It happened several years ago, when I was exploring the kelp forests of Wellington’s south coast. I was making my way along a rock face, and had just brushed aside a clump of seaweed. There, directly in front of me, two young seahorses, their backs to me, were peering intently into a small crevice. Both seemed oblivious to my presence. Their attention was firmly focused on a small shrimp they had bailed up in the crevice. The larger of the two seahorses appeared to be winning the race to the shrimp, blocking out the smaller one by virtue of its bulk. No matter how hard it tried, the smaller seahorse couldn’t get past.
Suddenly, the shrimp made a break for it, propelling itself past the larger seahorse with an explosive contraction of its tail. As it unbent its tail for another burst, which would take it safely out of range, the smaller seahorse “locked on” to it, and with a flick of its head sucked the shrimp down its tubular snout.
I swear the look on the face of the larger seahorse as it stared at the smaller one was one of incredulity and disgust, as if to say: “How could you?” The noisy exhalation of laughter through my regulator startled both animals, causing them to flatten against the rock, heads tucked down, bulgy eyes regarding me cautiously. I moved on.
Seahorses, both real and mythical, have been objects of human fascination for centuries. In Roman legend, they pulled Neptune’s chariot and consorted with mermaids. On a more mundane level, they were listed by Pliny the Elder as the major ingredient in a cure for baldness. In the East their equine character has been equated with medicinal efficacy as a restorer of strength and enhancer of potency.
Zoologists classify seahorses in the Syngnathidae family of bony fishes, a family which also includes pipefishes, pipehorses and seadragons, and give them the generic name Hippocampus (from the Greek words for horse and sea monster).
The latest estimate of the number of seahorse species is 32, whittled down from 120 catalogued names. One reason for this redundancy in seahorse taxonomy is the wide variation in physical traits such as colour, size and body proportions that individual seahorses can exhibit Another is the generally low rate of dispersal of young seahorses, which may lead to localised “races” occurring within a species. As scientific knowledge of seahorses is still relatively limited, the exact number of species remains to be determined.
Seahorses occur in both temperate and tropical waters, with the greatest number of species found in the Indo-Pacific region. Most are strictly marine creatures, though a few species can tolerate estuarine conditions. Seagrass, seaweed, mangroves and corals reefs are their typical habitats, and they can be found from the shallows to depths of over 100 metres. Seahorses range in size from Hippocampus ingens of the American continent—a veritable drafthorse that measures up to 40 cm from the coronet to the tip of the tail—to the Shetland pony of seahorses, H. minotaur of southeastern Australia, measuring just 1.5 cm in length.
In New Zealand waters, seahorses are represented by a single species, the large-bellied (also called potbellied) seahorse H. abdominal s. This species also occurs around south-eastern Australia, and is one of the larger seahorse species, reaching 30-35 cm in length. Coloration is highly variable, ranging from a base colour of white, grey, olive or yellow to brown and charcoal. Overlying these base colours is a pattern of blotches and bands unique to each individual. Males are more heavily marked than females. On rare occasions, entirely gold seahorses are encountered. These stunning creatures give the seaweed they cling to the appearance of an aquatic Christmas tree on which a brand new ornament has been placed.
The large-bellied seahorse has been recorded from the Three Kings in the north to the Snares in the south, in water as shallow as intertidal rockpools and as deep as 104 metres. Since they typically blend in so well with the clumps of seaweed they inhabit, they are often difficult to spot. The best time to see them is at dawn and dusk, when they are most active, or at night, when a bright torch picks the sleeping seahorse out more easily against the background weed.
Occasionally, large aggregations of hundreds of seahorses have been observed, in places such as Kawhia and Golden Bay, probably as a result of water currents and wind patterns concentrating juveniles, which congregate near the surface following birth. Fishing trawlers sometimes accidentally catch large numbers of seahorses during the course of normal fishing.
Despite the fact that they are widespread and relatively large, very little is known about H. abdominalis in New Zealand waters. We don’t know how abundant they are, how long they live for, whether they are nomadic or remain in one location for their adult lives, how often they breed, or whether they have monogamous mating arrangements, as many other seahorse species do.
The body of the large-bellied seahorse is like that of almost every other seahorse species. It has an armour-like appearance, the result of an external covering of bony plates arranged in a series of rings over the internal skeleton. Where the plates intersect, the skin is raised into small knobs, which are usually blunter and less pronounced in larger and older seahorses. Tendril-like skin growths, or filaments, sometimes occur on the head and neck region. They can grow very quickly—over the space of a few days—and are thought to aid in camouflage.
Like chameleons, seahorses possess special pigment cells called chromatophores within the skin. By contracting or expanding chromatophores of different colours, an animal can alter its colour to suit the environment. Seahorses also use colour change as a means of communication with each other, particularly during courtship. The results can be striking, with fluorescent orange and wild purple falling within the palette of some seahorses.
Unlike their relatives the pipefishes and seadragons, which hold their heads more or less in line with their bodies, seahorses hold their heads at right angles to the body—accentuating their horsy appearance. At the end of the tubular snout there is a small toothless mouth through which prey are sucked whole. Seahorses eat a variety of small crustaceans such as amphipods, copepods and brine shrimps, as well as small fish and marine worms. They are ambush predators, relying on their cryptic coloration for concealment as they stalk their pray, then pouncing and vacuuming up their victims. For such precision hunting keen eyesight is essential. Like the eyes of chameleons, seahorse eyes are capable of independent movement, enabling a seahorse to look in two directions at the same time, thus increasing its chances of detecting both prey and predators.
The seahorse approaches its prey stealthily, and with both eyes fixed on the quarry. Tucking its head slightly downwards, the seahorse then rapidly flexes it upwards, simultaneously opening its mouth and expanding its cheek cavity, which creates a strong suction force. In the blink of an eye, the prey is sucked down its snout, and the seahorse starts looking for its next snack. Because seahorses possess only a simple, stomachless gut and do not chew their prey, digestion is not very efficient. Consequently, they must consume a large quantity of prey to meet their daily nutritional needs.
If their style of feeding is unusual, seahorse respiration is even more so. In most fish, the gills consist of a series of finger-like filaments radiating out from several gill arches. Each filament is composed of a tightly packed series of thin plates called lamellae. These lamellae create a large surface area for gas exchange and the excretion of metabolic products. In contrast, the gills of seahorses are composed of a patch of squat grape-like structures on either side of the head, and the opening of the opercular bone (which covers the gills in most fish) is reduced to a small pore. Such a gill structure has a much lower surface area for gas exchange, as befits the sedate-swimming (and frequently stationary) seahorse.
The main means of seahorse propulsion is the dorsal fin, located at the centre of the back. Smaller, fanlike pectoral fins located behind the gill pores on the seahorse’s head are used for steering and as stabilisers. Tail and pelvic fins are absent, and the anal fin is reduced in size. This fin configuration lends a certain gracefulness to seahorse movement: they seem to move like helicopters from one clump of seaweed to the next.
Adult seahorses spend much of their time holding on to substrates such as seaweed fronds and sponge fingers with their prehensile tails, scanning the surrounding water for prey. As well as anchoring the seahorse, the tail may also assist in movement. Tucking the tail forwards allows the seahorse to descend faster when chasing diving prey. Alternatively, straightening the tail streamlines the seahorse when ascending through the water column.
What sets syngnathid fishes such as the seahorse apart from other fishes—indeed, what makes them virtually unique in the animal kingdom—is the extreme form of male parenting they exhibit. It is the male seahorse that becomes pregnant and carries the developing young around in a pouch until they are ready for hatching and release. Compared to mammalian parental roles, this is a strange characteristic indeed.
Seahorse courtship rituals begin with a series of colour changes and postural displays. Inflating his brood pouch to balloon-like proportions with water, and lightening it in colour to enhance its visual impact, the male will repeatedly approach his chosen female with his head tucked down and body coloration highlighted. If the female is receptive, she will reciprocate with her own colour changes and head tucking. A series of short bursts of swimming together then ensues, sometimes with tails entwined. The final part of the courtship process—which in some species occurs over several days—involves the pair swimming towards the surface. If they reach the water surface they can often be seen and heard to snap their heads upwards in a series of audible clicks.
Mating occurs when the female transfers her eggs into the male’s pouch, where he then fertilises them. Facing each other, the female swims slightly above the male and presses the base of her abdomen against the male’s pouch and squirts her eggs through the opening, which he has dilated to approximately 8-12 mm in diameter. Several bursts of egg transfer occur before the pair separate and go their own ways. Following egg transfer, the male seahorse will repeatedly contort and stretch his body in an attempt to evenly distribute the eggs within his pouch. Once the eggs are comfortably distributed, he dulls his coloration and becomes relatively reclusive, spending more time hidden among the seaweed.
In many seahorse species, individuals maintain faithful pair bonds throughout their lives. It is not known whether the New Zealand seahorse is monogamous in the wild, but captive H. abdominalis males will court multiple mates, although each individual male appears to receive his entire brood of eggs from only one female.
Within the male’s brood pouch there is a jellylike lining of tissue supplied with blood vessels, like a placenta. As the eggs settle, the blood vessels of the pouch lining enlarge, and the lining gradually swells, gently surrounding each individual egg.
The egg yolk provides nutrients to the developing embryo, but the father contributes calcium (which is presumably important for skeletal development), and also provides gas exchange, osmoregulation and waste removal for the developing young. During pregnancy, the pouch environment changes from being similar to body fluid to resembling seawater in composition.
Researchers have found that the hormone prolactin, which has such an important role in milk production in women, also appears to govern the male seahorse’s pregnancy.
The duration of seahorse pregnancy is variable. The shortest I have observed in captive H. abdominalis is 18 days, the longest 69 days. It seems that brood time and brood size are somehow related, since the number of juveniles produced in the former brood was 46, while for the latter it was 721.
Juvenile seahorses hatch from their eggs within the male’s pouch, but are not usually expelled until they have completely absorbed their yolk sac. If the juveniles are dark in colour, their hatching will cause a noticeable darkening in the male’s pouch. Occasionally, the young are released prematurely, without having used up their yolk sac, or with underdeveloped body parts, such as stumpy tails and snouts. These “preemies” do not usually survive long.
Release of the juveniles usually occurs during the night. Dilating the opening to his pouch, the male repeatedly bends forward, compressing his pouch against his body to expel the young. The process, which can take up to three days, appears to be stressful to the father, for the coloration of his pouch changes from dark to light and then back again. After relieving himself of his brood, the male is ready for another pregnancy—in fact, he actively seeks a further mating. In the Australian species H. whitei, males can notch up seven consecutive 21-day pregnancies during a season.
Juvenile seahorses are completely independent of their father upon release, and are capable of capturing their own tiny prey. Although like their parents in shape, the young are much slimmer, and their heads and dorsal fins are proportionately larger. For the first 2-4 weeks of their lives they congregate near the surface, hunting for food among the zooplankton. During this pelagic phase currents may carry the juveniles away from their point of first release—the main dispersal mechanism in seahorses.
As they grow larger and heavier, the young seahorses start to descend lower in the water column and become more sedentary in nature. Their bodies start to fill out, and, like their parents, they spend more of their time among the seaweed.
Seahorse longevity varies according to the species, but one to seven years—barring predation or human capture—are common estimates. In fact, predation is thought to be a relatively minor cause of mortality among adult seahorses. As well as being masters of disguise, seahorses appear to be fairly unpalatable on account of their bony exterior. Of far more importance, especially in recent years, is fishing pressure.
The seahorse harvest is considerable, and, at present, unsustainable. In 1995, total global consumption was estimated to be 56 tonnes, or 20 million individuals. Most of these animals are destined not for the dinner plate (though stir-fried seahorse is not unknown in Oriental cuisine), but for medicinal uses.
In traditional Chinese medicine (TCM) and its derivatives, the seahorse is credited with increasing energy flows within the body and decreasing cholesterol levels. It is also said to have a curative role in such diverse ailments as asthma, impotence, goitre, kidney disorders and skin afflictions such as severe acne and persistent skin nodules.
In TCM, seahorses are sold as whole, bleached and dried animals, as a powder, or in patented pill form. Usually they are combined with a range of other animal and plant ingredients, the exact mix depending on the condition and person being treated.
Seahorses for the TCM trade are supplied by commercial fishers from by catch, and by individual fishers operating in subsistence fishing communities in places such as the Philippines and Thailand. For the latter, the sale of seahorses may represent a significant part of their livelihood.
There is mounting concern over the decline in the wild stocks of some species of seahorse. Some seahorse populations are thought to have declined by up to 50 per cent in the past five years. Yet, if anything, seahorse demand is increasing. China’s economic growth since the mid-1980s has brought the more expensive elements of traditional medicine within the reach of the masses. TCM traders speak of a “limitless demand” for seahorses.
However, the use of seahorses for medicinal purposes is by no means the only reason for seahorse decline. Their natural habitats are particularly susceptible to degradation through pollution, land reclamation and other human activities. Destructive fishing methods, such as the use of heavy bottom-trawl nets and dynamite and cyanide fishing, also have a long-lasting negative impact, as does bycatch of seahorses, especially in shrimp trawls.
Habitat disturbance and loss is thought to have a more serious impact on wild seahorse populations than direct harvesting.
On top of these pressures, each year some 200,000 dried seahorses are sold as curios, and more than half a million live animals end up in saltwater aquariums. While seahorses are attractive aquarium inhabitants, and can live out a healthy life given the appropriate care and attention, they are notoriously difficult to maintain in good condition over long periods. This is because of the relatively large amounts of live food that they require, as well as their susceptibility to various diseases.
In an effort to redress the alarming imbalance between human impacts on seahorses and the ability of seahorse wild stocks to maintain their own viability, an international collective called Project Seahorse is spearheading a series of initiatives in seahorse conservation. The aim is to encourage and enable the people involved in seahorse fishing to sustainably manage, rather than overexploit, their wild stocks of seahorses. In addition, Project Seahorse is building up a global database on seahorse biology, as well as fostering communication and cooperation between the “stakeholders” in seahorse trade: fishers, traders, medical practitioners and biologists.
Some successes have already been achieved. In the Philippine village of Handumon, local fishers, with the assistance of Project Seahorse workers, have established a marine sanctuary in front of their village. No fishing is allowed in the sanctuary, and villagers patrol the area to prevent poaching.
In fishing areas, where once pregnant male seahorses were taken straight from the water for drying, they are now held in sea cages until they release their young, which escape through the cage mesh.
Villagers participate in the tagging and gathering of data on seahorses in order to gain a better understanding of the ecology of the animals.
Handumon’s examplehas led to neighbouring villages getting involved, and seven new marine protected areas or sanctuaries were set up in 1998.
Compared with the Southeast Asian situation, overexploitation of seahorses is not yet an issue in New Zealand waters. The large-bellied seahorse is not fished on either a commercial or traditional basis, and is not a gazetted species in the Quota Management System, which is the regulatory mechanism for ensuring sustainable fishing. However, H. abdominalis can be caught as bycatch during fishing operations and then sold to Licensed Fish Receivers (LFR) for export. The same applies to other syngnathid species within New Zealand waters such as pipefish.
Although seahorses and pipefish are often incidentally caught, most fishers appear to keep them for sale only when they are caught in large numbers. Consequently, although both seahorses and pipefish may be caught each year, their recorded sale to LFRs fluctuates markedly. For example, according to Ministry of Fisheries records, in 1995 no seahorses were sold to LFRs, but 5451 kg of pipefish were. In 1997, 71 kg of seahorses were sold to LFRs, but no pipefish were.
Given New Zealand’s economic structure, our extensive and often remote coastline, and the difficulty of actually finding H. abdominalis in its seaweed habitat, it is unlikely that overall wild stocks of our seahorse could be affected to the extent seen in Southeast Asia if commercial fishing were allowed. However, there is the potential for stocks in easily accessible areas to be seriously depleted through exploitation or environmental disturbance. This would be of particular concern if local seahorse populations were found to be genetically distinct from one another—a conclusion which research in the Philippines shows to be quite possible.
Given the problems associated with fishing wild seahorse stocks, aqua-culture would seem to be a better way to tap into the lucrative seahorse market. The Chinese established land-based seahorse “ranches” in the late 1950s, but with the switch to a more capitalist economy in the mid-1980s most of these facilities appear to have became economically untenable. Seahorse farming is being tried in Australia, England, Indonesia, Malaysia, the Philippines, Thailand, Vietnam and New Zealand, but most ventures are still in their infancy, and there are formidable technical difficulties to overcome.
My own work over the past two years has involved finding ways to overcome the problems of seahorse mortality when the animals are kept in captivity. Tank design, hygiene and the type and quantity of food all have a bearing on survival. For example, if they eat too much they digest it even less efficiently than usual. Like many wild animals, they will take whatever is on offer, and in captivity this habit can lead to problems. Finding an optimum feeding regime is just one of the challenges facing the would-be seahorse farmer.
On the positive side, I have found that H. abdominalis reaches sexual maturity in a single year, and at NIWA’s Wellington laboratory I have been able to raise two generations of seahorses from an original wild stock captured in 1997.
The interest in Hippocampus being shown by the fishing industry, conservationists, aquarium enthusiasts and the public at large bodes well for the future of the seahorse, provided we find out more about them and balance commercial interests against the survival and needs of the animals In New Zealand, a doctoral student is about to begin a study into ecological aspects of the large-bellied seahorse around Wellington, while a survey programme is being set up by the Underwater Association of New Zealand to try to determine seahorse distribution and population sizes around the country.
In the past, marine conservation has garnered little public sympathy compared, say, with efforts to save high-profile endangered land animals. London Zoo curator Heather Hall, a co-founder of Project Seahorse, has made the comment that most people regard fish as food, not wildlife. “We’re just lucky that seahorses are about as cute as you get in the fish world,” she says. Promote seahorse conservation, and you may save a whole raft of other marine creatures.
The tide may be turning for the denizens of the deep.