Stunning images can be seen from today at London Zoo's aquarium, as part of a photo competition supporting Project Seahorse, the international campaign to conserve these special fish.

They need conserving. Most of the world's 35 species are in trouble, according to Heather Koldewey, the aquarium curator who is Project Seahorse's assistant director. "They're in trouble because they live in the most vulnerable of marine habitats, the coastal environment - coral reefs, estuaries, mangrove swamps and seagrass beds," she said. " These are all being hard hit by pollution and coastal development."

Seahorses are also threatened by over-fishing, Dr Koldewey said. They are being targeted for use in traditional Asian medicine, as live pets, and for the souvenir trade.

An estimated 30 million seahorses are traded every year, by between 70 and 80 countries - which makes this the largest wildlife trade issue, in terms of numbers.

They were the first marine fish to be listed by Cites - the Convention on International Trade in Endangered Species - being placed on its appendix 2, which means the trade has to be monitored and traders have to prove it is sustainable. But this is difficult, said Dr Koldewey, because little is known about some species, and indeed new species are still being discovered - two have been identified in the past three years.

They are also threatened as an accidental catch in many fishing operations, particularly shrimp trawling.

Project Seahorse, a global initiative which focuses on seahorses as an emblem of the health of the seas, was established in 1996 and is a partnership with the Zoological Society of London, the University of British Columbia in Canada and the John G Shedd Aquarium (USA). It is supported by the Belgian chocolate company Guylian, which is sponsoring the current exhibition.

Seahorses have heads at right angles to the body and fully prehensile tails. They are masters of camouflage, changing colour and growing skin filaments to blend in with their surroundings.

Seahorses have no stomach or teeth. They suck in prey through a tubular snout and pass it through. But they are most extraordinary because it is the male seahorse which becomes "pregnant" - carrying the female's fertilised eggs inside his body. Most species of seahorses are monogamous, forming pair bonds that last the entire breeding season.

Two species are found in British waters, the short-snouted seahorse, Hippocampus iippocampus and the long-snouted seahorse, H Guttulatus. When you've finished looking at the photographs, you can delve into the aquarium and see H Guttulatus in the flesh.

The exhibition, Guylian Seahorses of the World is in London Zoo's aquarium, is free to Zoo visitors and is open daily, 10am to 5.30pm, from 2 April until 1 June at London Zoo, Outer Circle, Regent's Park, London, NW1 4RY