Water slaters are watery relatives of the familiar garden woodlice.
There are only two likely to seen in Britain, and by looking closely at them you can actually tell them apart quite easily: the common water slater (Asellus aquaticus) has two white spots on its head, whereas the more uncommon, slightly classier, one-spotted water slater (Asellus meridianus) has only one spot on its head. It’s the common water slater that usually is found in garden ponds: we’ve never seen the one-spotted water slater in a garden pond, though given how little we know about garden ponds it wouldn’t be surprising to see it turn up.
Water slaters have to get around from pond to pond by being moved on plants, or by birds, or floods. But get around they do, although they will probably take quite a few years to arrive naturally in a new garden pond. There’s nothing to really worry about if you don’t have them, and no special need to introduce them.
Water slaters suffer from a common prejudice that they indicate pollution – they don’t. They are a perfectly normal part of the animal community of any fairly alkaline pond or stream. They don’t care for acid water though, so you don’t find them in more mountainous uplands, nor usually in the acid heathland ponds of southern England.
The reason for their polluted water reputation is that in the early days of river pollution studies they were one of the survivors when sewage was discharged pretty much untreated into rivers. This is because they naturally live in places where oxygen levels can be low – but there’s nothing unnatural about that in ponds, and in clean rivers they like the naturally silty places.