They are among the most graceful and intriguing animals of the ocean and their strictly monogamous lifestyle breaks one of the golden rules of biology - it is the male rather than the female who is left holding the baby.

Scientists have unravelled another mystery of the seahorse and in the process discovered that these unusual fish may be more vulnerable to environmental pollution than was previously believed.

Until now it was thought the eggs and sperm of seahorses are protected against external pollutants because the creatures engage in a form of internal fertilisation, where the eggs and sperm come together inside a body cavity.

However, an anatomical study of the male has found that seahorses release sperm directly into seawater before these delicate "germ cells" of the body are brought quickly back into the protective environment of the male's incubation pouch.

Scientists believe that this key observation could explain why seahorses may be more vulnerable to environmental pollutants, such as heavy metals like mercury, than some marine biologists had thought possible.

Unlike the sex roles in the vast majority of animals, the male seahorse looks after the fertilised eggs in a special brood sac on the front of his abdomen, which works much like the womb of a female mammal.

The fertilised eggs get embedded into the wall of the pouch and are bathed in a fluid that provides nutrients and oxygen. In effect, the male seahorse becomes pregnant and gives birth to live offspring - the only male in the animal kingdom to do so.

It was known for many years that female seahorses deposit eggs directly inside the male's incubation pouch during the close contact of the breeding ritual. It was also assumed, but never proven, that it was in this incubation pouch that the male directly released his sperm to ensure that he, and he alone, fertilised the eggs.

However, Katrien van Look and colleagues at the Institute of Zoology in London looked again at the delicate male anatomy of the common or yellow seahorse, and found that this was not the case. The sperm was released externally into the surrounding seawater, as it is in most other species of fish.

"We found that the sperm duct is actually external which means that the sperm has to go outside into seawater before it can enter the pouch," Dr Van Look said.

"This means that seawater is in direct contact with the sperm, as well as with the inside of the pouch when it is opened to let it in. This means that any environmental pollution in the seawater will also be in direct contact with the sperm and eggs."

Previous studies have shown that relatively high levels of mercury in seawater can inhibit the motility - the swimming ability - of seahorse sperm. So, if sperm comes into direct contact with seawater, it makes it more vulnerable to similar environmental pollutants, Dr Van Look said.

Bill Holt, head of science at the Zoological Society of London, said the discovery was particularly important for the conservation of the seahorse as it furthers our understanding of how this vulnerable fish reproduces in the wild.

"This helps us to understand the implications of environmental change, including global warming and pollution, on the ability of the seahorse to reproduce," Professor Holt said.

Of the 33 species of seahorse, nine are listed as vulnerable - including the yellow seahorse - and one is classified as endangered. Too little is known about the remainder to know whether they are at serious risk from the environmental threats.

However, it is known that seahorses are fished extensively for the trade in traditional Asian remedies, as well as for aquarium pets. Many are also caught and killed as bycatch from other marine activities, such as shrimp fishing.

In addition, Dr Van Look and her colleagues discovered that seahorses produced two distinct types of sperm - type one with a small, elongated head, and type two with a much larger head.

"The research suggests that the type two sperm does not take part in the fertilisation process, but that it is a relic of a previous reproductive strategy in which the male seahorse fertilised the eggs in the water before they entered his brood pouch," she said.

Tim Birkhead, of Sheffield University, who is an evolutionary biologist with a special interest in sperm biology, said the study's findings could lead to a better understanding of the unusual breeding pattern of seahorses.

The strict monogamy of the seahorse, as well as the role played by the male in brooding the fertilised eggs, must have arisen for an evolutionary reason, Professor Birkhead said.

"It provides further confirmation that promiscuity doesn't come cheap. Monogamous species, like the seahorse, save a considerable amount of energy by not having to invest in making testes or sperm," he said.

"The mystery from this study is the occurrence of two types of sperm. That's a real puzzle. All the other animal species with multiple sperm types are promiscuous.

"Then next big questions for seahorse researchers are to figure out whether other species also have two sperm types, and what their function is."

Conventional evolutionary theory states that it should be females of any species who look after the eggs and their young because they provide a far bigger initial investment in the offspring compared to males - since an egg is much larger than a sperm.

However, if the male can be assured that any eggs have been fertilised by his own sperm, and not the sperm of another competing male, then there may be an advantage in him looking after the eggs.

Some biologists have suggested there is something about the ecology of the seahorse habitat which also makes it conducive for males to benefit from monogamy and egg-brooding.

Seahorses cannot swim very well - they use their prehensile tail to hang on to seaweed - and so it may be that once they have found a mate, it makes sense to stick with that mate and concentrate on a life of egg-brooding monogamy.

The facts

* Seahorses are true fish with a dorsal fin on the lower body and pectoral fins on the head near their gills. They are members of the "bony" class of fish, the same as salmon and tuna.

* They live in coastal areas, from Australia to the Caribbean, and prefer seagrass beds and coral reefs where they can hide easily among the vegetation.

* Despite having no teeth or stomachs, they are voracious predators. They suck prey into their mouths and pass it through an inefficient digestive system.

* Seahorses can roll their eyes independently of one another to search for prey, but are not good swimmers.

* About 33 species are recognised today.

* Seahorses range in size from 16mm to 35cm. They can change colour to blend in with their surroundings.

* About 20 million are caught each year for Chinese medical remedies, despite the trade being illegal.

* Captive-bred seahorses survive better than seahorses taken from the wild.