Hypogean Crustacea ecology
|Cave pool at the end of Stal Passage,
Quaking House Cave, Somerset
(photo courtesy of William Kromhaut)
The British hypogean Crustacea is impoverished in comparison to that of mainland Europe. It has generally been accepted that this is the result of the last glaciation (known as the Devensian in Britain and the Midlandian in Ireland). At its maximum extent, between 25000 and 15000 years ago it covered most of Ireland, all of Scotland and Wales and north-west England, with permafrost conditions present in the south. It is assumed that the ice would have sterilised the environment beneath it and would have removed sources of food from the surface environment. This theory would suggest that most of the British sygobitic Crustacea populations were established by animals migrating from un-glaciated areas in Europe. The main pattern of distribution of the British species would seem to support this hypothesis, with most records occurring in the southern part of England. It is thought that the cave systems of South Wales were re-colonised by migration from areas such as the Mendip Hills in North Somerset.
However, there are several notable exceptions to this, including Antrobathynella stammeri in the Midland Valley of Scotland and several records of Niphargus aquilex north of the glacial limit. Similarly in Ireland most of the Niphargus irlandicus records are north of the Midlandian glaciation. Current thinking is that the hypogean fauna might have survived in un-frozen groundwater within the tundra (tundral refugia) and in groundwater beneath the ice-sheet (sub-glacial refugia). The recent discovery of two endemic species of hypogean Crustacea in Iceland, Crymostygius thingvallensis and Crangonyx icelandicus (Krisjánsson & Svavarsson, 2007) proves that stygobitic Crustacea can survive beneath ice. It is now thought the populations of South Wales survived beneath the ice in the ancient cave systems there, rather than having become established by re-colonisation at the end of the Devensian. Proudlove et. al. (2003) discusses the geological and geomorphological explanations for the distribution of the British stygobitic Crustacea.
The habitats from which the subterranean Crustacea have been recorded include; the interstitial water in the gravels of rivers and streams (the hyporheic zone); underground aquifers in chalk and other strata; and pools and streams in caves and mines. It is believed that the true habitats of these species are the small channels of deep phreatic water in underground geological strata. Their presence in caves and mines is usually a result of the organisms being washed out of fissures in the rock, into streams and pools following heavy rain. The relatively few individuals seen in caves are not isolated but form part of a larger, unseen population.
The entomologist Frank Howarth has classified the subterranean habitats into three categories, based on size; the microcavernous habitat (>1mm in diameter); the mesocavernous habitat (1 - 200mm in diameter); and the macrocavernous habitat (>200mm in diameter) (Chapman, 1993). Since only the macrocavernous habitat is accessible to cavers one can understand why so little is known about a group of animals in which the major populations dwell beyond the reaches of direct human study.
When washed into caves the local conditions usually determine the fate of hypogean Crustacea. If conditions are favourable and good cover and / or a good food supply are present, then the cave habitat will become a home, as demonstrated by the numerous populations of cavernicolous (cave-dwelling) Crustacea throughout the British Isles. If conditions are unfavourable then the animal can either end up falling prey to other aquatic denizens of the cave (e.g. flatworms) or, if the cave atmosphere is humid enough, it can migrate, either to other pools, larger waterbodies, or back to the mesocavernous habitat. The high moisture content of the air, within many caves, means that the sharp division between the aquatic and terrestrial habitats on the surface are not so clear-cut beneath the ground. Many aquatic organisms in caves have been observed moving over a thin film of water on rocks. A population of the Freshwater Shrimp (Gammarus pulex) has been observed living on a vertical rock face above the stream-way in Manor House Farm Swallet on the Mendips. There was only a thin film of water running down the rock (not even enough to cover the shrimps themselves) and the animals were moving vertically up and down the rock face between small puddles of water, collected on tiny projections. Similarly, during an initial visit to Pridhamsleigh Cavern (near Buckfastleigh, Devon) after heavy rain, many of the cave pools were inhabited by quite large numbers of Niphargus aquilex; whilst a visit ten days later showed most of the same pools to be empty. The ability to move between pools in caves would explain the surprising habitats in which subterranean Crustacea appear, such as water-filled hollows in mud, left by cavers' boots.
Relatively little is known about the ecology of the hypogean Crustacea due to their cryptic habitat, with most studies having been carried out on European species. Most of the information below has been taken from Gledhill et. al. (1993).
The recent Groundwater Animals UK project collected a whole suite of groundwater chemical parameters in order to identify any link between preferred water chemistry and the occurrence of stygobitic Crustacea. A study aquarium was also recently (2010 to 2013) set up in Joint-Mitnor Cave in the Higher Kiln Quarry (Buckfastleigh, South Devon) in order to study certain aspects of the auto-ecology of Niphargus glenniei and Niphargus aquilex (Knight and Johns, 2015).
|The aquarium in Joint-Mitnor Cave, South Devon.