Number of species
Our best guess is that there are about 3,000 species of earthworms globally. You may find it surprising that we don't really know, but there are many countries that have not been properly sampled. This includes the UK. Unfortunately in the past they haven’t commanded anywhere close to the amounts of money for research that, for instance, mammals have, but with their importance being recognised more and more this will hopefully change. Societies like ourselves will hopefully also start to make a difference!
There are a number of species globally are really more than one species, but taxonomists haven’t yet named them properly. We refer to them as species complexes. This means that of the earthworm species we do know about, there may in fact be many more.
Earthworms are very diverse and very versatile and are found in nearly all terrestrial ecosystems.They can live in the soil, under rocks and logs, in puddles, lie dormant wrapped around themselves covered in mucus if soils become too dry and they can even migrate up the trees to live in the epiphytes which grow there. That makes them a little harder to sample!
Earthworms also come in a range of different sizes. Megascolides australis, found in Gippsland in Australia, has been reported to get up to 3 metres in length, but there are also tiny species, the adults of which are only 1 cm in length. Earthworms also come in a variety of colours. There is a bright blue earthworm, Eutrigaster sporadonephra, found in tropical countries which can live in bromeliads up in the trees as well as in rotting logs on the ground. But you don’t have to go abroad to find colourful earthworms. There is a stripey earthworm you can find in your compost heap called Eisenia fetida and even a green earthworm, Allolobophora chlorotica, you can find in your lawns and borders.