Environmental Engineering Reference
In-Depth Information
Mayfly
Habrophlebia fusca
Meniscus midges
Dixa spp.
Empid fly
Tachydromia spp.
Water beetle
Helophorus brevipalpis
Water beetle
Helophorus laticollis
Water beetle
Hydraena gracilis
Rove beetle
Atheta spp.
Mite
Parasitus spp.
Mite
Pergamasus crassipes
Well-shrimp
Niphargus fontanus
The crustacean isopod Proasellus cavaticus , recorded from Barland Sink at the top of the Bishopston
Valley, is a truly underground animal found usually in the water film flowing over stalagmite slopes or
on stones in streams. Like the well-shrimp and a number of other inhabitants of the dark zone it is blind
and lacking pigmentation. Again it is also found mainly in Mendip and South Wales. Guzzle Hole lower
down the valley has produced records of a related species, P. meridianus , which is not a confirmed cave
dweller. Jefferson was intrigued by this and thought that the underground stream warranted further study,
but this never occurred.
Theassociationofthetwotypicallycave-dwellinganimals,thewell-shrimpand Proasellus cavaticus ,
in Mendip and South Wales seems to indicate that these animals are survivors of a preglacial British un-
derground fauna. As described in Chapter 3, South Wales was subject to repeated glaciation and yet true
cave-dwelling crustaceans are commoner in this area than in any other part of Britain. Two possible ex-
planations have been put forward for this: either there has been recolonisation since the last retreat of
the glaciers or the animals survived beneath the ice cover. The similarity of the Mendip and South Wales
faunas suggests a recolonisation from Mendip. The two areas are not far apart, and in immediate postgla-
cial times they were not separated by water, as the upper part of the Bristol Channel was then dry land.
There are some problems with the theory, however, as the two faunas are not identical. The alternative
explanation ofsurvival beneath theice alsoraises furtherquestions, although Gowerwasneartheedgeof
the ice sheet and this would probably have aided survival in that it allowed food from the surface to reach
the animals below.
The most important members of the terrestrial communities in the dark zone are Collembola (spring-
tails), several species ofwhicharecommonly foundonmudbankswithin theGowercaves. Morethan70
species of Collembola have been found in British caves, but only 15 are widespread, with the most fre-
quently recorded being Tomocerus minor . In the Gower caves this position is occupied by the troglophile
Anurida granaria . Almost nothing is known about the ecology of springtails in British caves, although
theyappeartobe'baselevel'consumersgrazingonbacteriaandmicrofungiinthesediments.Inturnthey
arepreyforvariouscarnivorousmitesandbeetles.Itisnotknownwhetherthespeciesfoundunderground
are genetically the same as those species found outside the caves, or whether some are isolated in caves
and are de facto troglobites but unrecognisable as such. Clearly much work is required, and genetic or
molecular studies will be required to answer these questions.
The common cave gnat Speolepta leptogaster is particularly widespread and numerous in the caves.
Like many animals that live permanently in caves, the gnat can be found in the deep threshold as well as
in the dark zone, but it is only rarely recorded from outside caves and it may be a troglobite. The thin,
translucent larvae, up to 14 millimetres long, live on damp cave walls and feed on microorganisms and
fungalmaterial.Otherspeciesrecordedincludewaterbeetlesandnon-bitingmidges Dixa spp.,commonly
known as 'meniscus midges' because the larvae of these aquatic insects live in the meniscus of the water.
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