A seahorse the size of a pea (and the other bizarre species new to science)

They include a tiny snake no more than four inches long, a snail with a shell that twists in four directions and a palm tree that flowers itself to death.

These are just some of the more than 18,500 species formally described as new to science in 2007 – the latest year such information is available.

From this inventory, researchers involved in the task of naming and describing newly discovered species drew up a list of the "top 10" animals, plants and microbes that illustrate the wondrous – and often bizarre – diversity of life on Earth. They include a bacterium that lives in hairspray, a stick insect that is nearly two feet long and a pea-sized seahorse.

There are an estimated 1.8 million species that have been formally named and described over the past 300 years since Swedish scientist Carolus Linnaeus started his binomial system of taxonomic classification. But this number is widely recognised as a gross underestimation of the true total of lifeforms. Estimates for the number of living species on the planet vary from two million to 100 million – although most scientists believe it to be closer to 10 million.

Biologists named and described 18,516 species in 2007, an increase on the 16,969 species described in 2006. Of these, 75 per cent were invertebrate, 11 per cent were vascular plants and nearly 7 per cent were vertebrates, such as the tiny snake. The scientists for the first time included prokaryote bacteria, such as the microbe that Japanese researchers discovered living in hairspray.

"Most people do not realise just how incomplete our knowledge of Earth's species is, or the steady rate at which taxonomists are exploring that diversity," said entomologist Quentin Wheeler, the director of the International Institute for Species Exploration at Arizona State University, which has compiled the annual report known as the State of Observed Species.

"Charting the species of the world and their unique attributes are essential parts of understanding the history of life. It is in our own self interest as we face the challenges of living on a rapidly changing planet," said Professor Wheeler.

An international committee of experts, chaired by Janine Caira from the University of Connecticut, selected the "top 10" new species based on assessment of each creature's unique attributes and surprising features. Among the most intriguing plants on the list are a caffeine-free coffee plant and a palm tree from north-western Madagascar that produces a huge, spectacular inflorescence with countless flowers in a magnificent display just before it collapses and dies.

The research centres involved in compiling the list of new species said that of all the reasons to document life in this way, "perhaps the most compelling is curiosity about the diversity of life analogous to our quest to map the stars of the Milky Way and the contours of the ocean floor".

The decaffeinated coffee plant

Coffee charrieriana is the first recorded caffeine-free coffee plant from Central Africa. It is named after coffee expert Professor Andre Charrier of the Institut de Recherche pour le Développement in France, a centre of coffee research.

The wild West:

The eerily white ghost slug, Selenochlamys ysbryda, was found in Cardiff. Its discovery in such as in a densely-populated area was a surprise, said scientists.

The oldest mother of them all

This is the oldest known vertebrate that can be shown to have given birth to live young, as opposed to laying eggs. The species, Materpiscis attenboroughi, lived about 380 million years ago and its fossil was discovered at a site in Western Australia.

The pygmy of the seas

This tiny seahorse, Hippocampus satomiae, grows no bigger than a pea, with a length of just over half an inch (13.8mm) and an approximate height of 0.45ins(11.5mm). This pygmy species was found near Derawan island off Kalimantan in Indonesia and is named after the diver, Satomi Onishi, who collected the samples.

The smallest snake on earth

The Barbados threadsnake, Leptotyphlops carlae, is just 4.1 inches long, making it probably the world's smallest snake. It was discovered in St Joseph's parish, Barbados.

The world's longest insect

The body of Phobaeticus chani can grow to 14 inches (36.6cm), which when added to its legs gives it an overall length of more than 22 inches. The insect, which resembles a stick, was found in Borneo, Malaysia.

The blue wonder This beautiful species of damselfish, Chromis abyssus, was discovered in a deep-reef habitat off the coast of the Pacific island of Ngemelis in Palau. Its discovery highlights just how little is known of deep reefs.

The death-flower palm

Only 100 specimens of Tahina spectablilis, a gigantic species of palm that dies soon after its flowers appear, have been found in its wild habitat in Madagascar. Its seeds have since been distributed widely and it is now a highly-prized ornamental plant.

Hair-raising bacteria

Unusual "extremophile" bacteria called Microbacterium hatanonis was found living in hairspray by Japanese scientists.

The snail with a twist

This Malaysian snail, Opisthostoma vermiculum, is unusual in that its shell twists along four different axes. It lives in the limestone hills of Malaysia.