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. 

Five types of Copepods have been identified in Mendip caves.  Paracyclops fimbriatus and Cyclops agilis have both been recorded from Swildon’s Hole; Cyclops languidus has been identified from Eastwater; and both Cyclops viridis and Cyclops bicuspidatus have been found in Pen Park Hole and Swildon’s.  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.

Isopoda (means 'same footed') are crustaceans with seven pairs of legs of similar size and form such as the woodlouse.
The Common Shiny Woodlouse, Oniscus asellus, is common in caves and has been identified from Elm Cave, Lamb Leer Cavern, Mouse Hole and others. Another species, Trichoniscus pygmaeus has been recorded from Elm 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 on the Mendips.

Androniscus dentiger in Sidcot Swallet

The freshwater hog lice (Asellus pp.) are aquatic creatures similar in appearance to woodlice and found in streams and ponds.   Asellus meridianus has been found in Swildon’s Hole and unidentified Asellus have been recorded in Great Oones Hole and Stoke Lane Slocker.  These specimens have no doubt been washed in from then surface. The one in the photograph is from the lake in Pen Park Hole.

The troglobite Isopod Proasellus cavaticus is usually found in cave streams on the underside of stones, on the thin film of water flowing over flowstone, and in pools. They are white, eyeless, up to 8mm long, look like undernourished woodlice and have been found in most of the major Mendip caves, including Cuckoo Cleeves, GB, Gough’s, Longwood Swallet, Cuthbert’s, Swildon’s, Tyning’s, Charterhouse Cave, Manor Farm, Waterwheel Swallet and  Eastwater, as well as the Bath Stone mines at Corsham, Wiltshire.  There are usually one or two specimens to be seen in the gour pools in Barnes’ Loop, Swildon’s.  They are often relatively inactive and so hard to spot. In Welsh caves they are preyed on by the cave shrimp Niphargus fontanus and some flatworms (e.g. Dendrocoeleum) and it is thought that the same might occur in Mendip caves.

Proasellus cavaticus in GB Cave.

Amphipoda means "different-footed", and refers to the different forms of appendages these crustaceans have. Their bodies are flattened laterally (sideways) and the order includes the shrimps. Mendip caves contain five species of troglobitic cave shrimp.

The troglobite cave shrimp, Niphargus fontanus is by far the most widespread species and has been found in many Mendip caves, as well as in the lake in Pen Park Hole and the Wiltshire Box stone mines.  Specimens can often be seen in pools in Gour Hall, St. Cuthbert’s and in the Trouble Series in Swildon’s.  Occasionally specimens might also be seen in Barnes’ Loop.  It lives in the flooded parts of caves as well as above the water table. It is an omnivore ingesting silt to extract organic matter as well as preying on Proasellus. They are fairly active and so easier to spot in pools and small streams. It is able to swim and hide in crevices and so can cope with living in a small stream that is liable to floods. They also seem to be able to leave their habitat if it dries up and return when it floods again.

Niphargus fontanus, GB Cave

Niphargus aquilex 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.   It is widespread in caves in Devon but elsewhere in the country it is only rarely found in caves, this niche usually being occupied by the more robust Niphargus fontanus. 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.  On the Mendips there is a single record from Rickford Farm Cave and it is also known from Holwell Cavern, another site from which N. fontanus appears to be absent. 

Niphargus aquilex, Rickford Farm Cave

Niphargus kochianus kochianus is generally regarded as an inhabitant of the deeper groundwater in this country and thus the fact that it has been recorded from three caves in the Mendip area is a surprise.  It is far more common in deep wells and boreholes.  The first record is from the Plantation Junction area of the streamway in St. Cuthbert’s Swallet in 1966.  There are two records, both in 1951 from Holwell Cavern, when N. aquilex was also recorded.  It is thought that both of these records could be misidentifications but these were in the days when Glenniei and Hazelton were actively running a cave research biological group and the former would have checked the specimens.  Glenniei was certainly aware of the appearance of N. kochianus, having collected specimens from wells elsewhere.  All other records from St. Cuthbert’s are of Niphargus fontanus and recent (1998 & 2004) visits to Holwell Cavern have only found N. aquilex.  The third site for N. kochianus is Pen Park Hole, where the species was recorded in 1957 and more recently in 2004 and 2007.  It can be present in fairly large numbers in the lake, which is thought to connect with the deep phreatic water table. 

Crangonyx subterraneus is another species of cave shrimp, distantly related to the Niphargus species.  A male and a juvenile were collected from the flooded rift in the Skeleton Pit in Gough’s Cave in 1966.  This species is more usually found in springs, wells and boreholes and its only other cave record is from the lake in Ogof Pant Canol in South Wales.

Recent work (2010) in Swildon’s Hole found specimens of the tiny 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 at wells, springs and boreholes in North Devon, Dorset, Isle of Wight and Gloucestershire and from a second cave in South Devon.  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.  It 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.

Microniphargus leruthi

All of the above species of cave shrimps 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.

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, although specimens can occasionally be seen in Barnes’ Loop. They are scavengers, browsing on microscopic plants, animals, and decomposing material. On the surface Gammarus are much more active at night than during the day hours.   They are common in most of the Mendip cave streams that originate on the surface and appear to be able to thrive in the environment.   A spectacular population can be seen in the Manor Farm streamway close to the climb up into the NHASA Gallery, where a thin trickle of water flows down the cave wall.  The Gammarus are quite happy living in the thin trickle of water that barely covers their bodies, swimming from ledge to ledge on the vertical rock face. 
Another species of surface-dwelling shrimp Crangonyx pseudogracilis has been recorded at various points in the Swildon’s stream, from sump 9 to the surface, along with much larger numbers of Gammarus.