Seahorses belong to the Sygnathidae family which also includes seadragons (which include the genera Phycordurus eques and Phyllopteryx taeniolatus), pipehorses (which include the genera Solegnathus and Acentronura) and pipefish (which include the genera Nerophis and Synganthus).
The name Sygnathidae literally means "tube-mouthed fish", which describes these creatures perfectly, since they have no teeth or stomachs, making them tricky to feed, since they cannot digest food as well as other animals.
Pipefish, seahorses and seadragons have more distant cousins from the Solenostomus genus which include snipefish, ghost pipefish, sea moths, trumpetfish, shrimpfish, flutemouths and paradox fish.
Seadragons are even more surreal-looking than seahorses, with their seaweed-like fronds which help to camoflage them from predators. They are sometimes hard to spot in aquariums because they blend in so well with marine foliage.
Their habitat is limited to Australia, where they are now protected by law owing to a diminishing of their numbers. There are just two species (one in each genera) - the leafy seadragon (Phycodurus eques) pictured above and the weedy sea dragon (Phyllopteryx taeniolatus) pictured below.
Seadragons are seemingly even more difficult to keep in captivity than seahorses and since they are protected by the Australian government, only approved aquariums are allowed to take specimens for breeding purposes.
Luckily seadragons seem to have escaped the Traditional Chinese Medicine trade which has decimated seahorse and pipefish populations.
There are around 200 separate species of pipefish, some of which can even live in freshwater, unlike true seahorses. Pipefish do not have the bent necks of seahorses or seadragons and look more like tiny water snakes.
While wild seahorses are monogamous, at least during one mating season, the females of many species of pipefish appear to mate with more than one male during the season.
Pipehorses look more like seahorses than pipefish since they have a slightly angled head and not a totally tubular body.
Many species of pipehorse have been used in Traditional Chinese Medicine as a remedy for removing phlegm, usually sold dried but sometimes ground up in prepared pills. However an even bigger danger to pipehorses may be trawling in seas around China - unfortunately pipehorses end up in fishermen's nets along with other catches and are often simply discarded.
It has been suggested recently by Australian researchers that pygmy pipehorses may hold the key to how seahorses evolved from fish that swam horizontally into a sea creature with vertical posture. Read more about this story in this post.