There are many representatives of this phylum found underground, often in water; some of them are particularly ancient and found only in caves.
Ostracods mostly live in freshwater. They have a two part carapace (like a shell) but because they are small and difficult to identify there are few recordings.  Ostracods have been seen in small pools in Rift Cave, Reed’s Cave and elsewhere although they have not been identified further.   

Three types of Copepods have been identified in Devon caves.  Paracyclops fimbriatus has been recorded from Baker’s and Pridhamsleigh Cavern; Acanthocyclops bisetosus has been identified from Tornewton Cave; and an unidentified Macrocyclops species has also been found in Pridhamsleigh Cavern.  Because they are very small (about 1mm or less) and difficult to identify, their importance in the underground ecosystem is probably underestimated.  Copepods are possibly the most widespread type of crustacean found in subterranean waters and an examination of most cave pools will often find tiny ‘specks’ moving about in the water and silt at the bottom.

Woodlice are terrestrial Isopods. The Common Shiny Woodlouse, Oniscus asellus, is common and has been identified from several caves (e.g. Brixham Cavern, Haw Cave, Dart Garage Cave). Woodlice are usually near the thresholds of caves. One woodlouse, Androniscus dentiger, appears quite happy in the cave – it is a more delicate pink colour than the usual woodlouse and is frequently found under stones and in rotting piles of wood (and outside in your compost heap). It has been widely recorded from many caves in Devon.
The freshwater hog lice (Asellus spp.) are aquatic creatures similar in appearance to woodlice and found in streams and ponds.   Asellus meridianus has been recorded in Pridhamsleigh Cavern.

Androniscus dentiger, Corbridge Cavern

Amphipods, whose bodies are flattened laterally (sideways) include the shrimps. Devon caves contain two species of troglobitic cave shrimp.
The first is Niphargus aquilex which has a long thin body and can grow to 15mm long, although most specimens tend to be smaller.  This is the most common species in the genus, occurring widely in groundwater across southern England and Wales.  As well as caves and mines it also occurs in springs and wells.  Elsewhere in the country it is only rarely found in caves, this niche usually being occupied by the more robust Niphargus fontanus.  In Devon it appears widespread in many caves, particularly after heavy rain. Niphargus fontanus does not occur in Devon and it might be that N. aquilex is occupying an ecological niche from which it is excluded by this species in caves further north.  Sometimes after heavy rain quite large numbers of N. aquilex can be seen in the pools between the Pit and the Lake in Pridhamsleigh Cavern, especially if there haven’t been many recent visitors to the cave.  They are able to leave their habitat if it dries up, moving over the thin layer of wet mud on rocks and retreating down into the water table via small crevices, which it’s long thin body is well adapted to fit through.  There are usually a few specimens to be seen swimming around in Mystery Pool (Pridhamsleigh Cavern). 

Niphargus aquilex in Rift Cave

The second species, Niphargus glenniei is Devon’s speciality.  It was first discovered in Pridhamsleigh Cavern in 1948 and until 2000, Devon was the only place in the world from which it was recorded.  In 2000 it was discovered in several wells in the west of Cornwall and it has since been recorded from wells in the east of the county and from Carnglaze Caverns, an underground slate mine near Liskeard.  Niphargus glenniei is much smaller than N. aquilex (3-5mm) and in addition to wells and springs, it is known from fourteen caves in Devon (including Napp’s in the north) and four mines.  There are particularly good populations, which appear to be permanent, in the gour pools in Easter Chamber (Reed’s) and Cascade Chamber (Afton) as well as in Penn Recca and Haytor Iron Mine.  Niphargus species are omnivores, eating whatever source of food they can find, including dead animals, decaying plant matter and even ingesting silt to extract the micro-flora (fungi & bacteria) associated with it.

Niphargus glenniei in Reed’s Cave

Recent work in Swildon’s Hole, on the Mendips, found specimens of the tiny troglobitic shrimp Microniphargus leruthi in the small streams of the North-west Stream Passage, Passchendaele and Black Hole Series.  These are the first confirmed records of this species from Britain.  It has subsequently also been identified from boreholes, wells and springs in Dorset, Gloucestershire, Isle of Wight and North Devon and was recently discovered, along with Niphargus glenniei in the sump in Sweetwater Pot, Berry Head.  The species was discovered in two boreholes in Ireland in 2006 and subsequent work found it deep in riverine gravels and several caves in the Burren.  Its next nearest occurrence is in the caves of the Ardennes in Belgium, Luxembourg and the upper Rhine basin in Germany. It was suspected that it might occur in Britain and had been overlooked in the past, due primarily to its small size (1.5 to 2mm).  It looks like a tiny Crangonyx subterraneus.

<	The recently discovered Microniphargus leruthi

The freshwater shrimp commonly seen outside is Gammarus pulex. It does get washed into caves and like many other creatures left in the dark they eventually turn white. They are rare in the pools fed by seepage water alone. They are scavengers, browsing on microscopic plants, animals, and decomposing material. Due to the lack of active stream passage in most Devon caves Gammarus is rarely recorded underground, although they have been recorded in Penn Recca mine.  On the surface Gammarus are much more active at night than during the day hours.