THE HYPOGEAN CRUSTACEA OF DEVON.
In 1812 a blind shrimp was obtained from a newly shunk well in the square of St.
Barthlomew's Hospital. It was given the name of Gammarus subterraneus by Leach in the
Edinburgh Encyclopaedia. This was probably the first published record of a subterranean
amphipod (Bate, 1859).
In the middle decades of the last century naturalists discovered more of these colourless and
eyeless shrimps. These initial discoveries were all made in water pumped from wells and
hence the animals acquired the name of 'well shrimps'. Since these initial investigations,
much more has been learnt about these elusive denizens of underground waters. It is
currently known that there are ten species of Crustacea dwelling within the subterranean
waters of the British Isles. These species are listed below.
BATHYNELLIDAE:
ASELLIDAE:
Antrobathynella stammeri
Bathynella natans
Proasellus cavaticus
CRANGONYCTIDAE:
Crangonyx subterraneus
NIPHARGIDAE:
Niphargus aquilex
Niphargus fontanus
Niphargus glenniei
Niphargus kochianus kochianus
Niphargus kochianus irlandicus
Niphargus wexfordensis
Of the species listed above, Niphargus kochianus irlandicus and Niphargus wexfordensis are
known only from Ireland. Much research still remains to be done on the British hypogean
Crustacea, as highlighted by the fact that Niphargus wexfordensis was only described in 1994
(Karaman, Gledhill & Holmes; 1994) and that there is still doubt over the occurrence of
Bathynella natans in the British Isles, specimens possibly having been mis-identified in the
past.
The habitats from which the subterranean Crustacea have been recorded include; the
interstitial water deep in the gravels of rivers and streams; underground aquifers in chalkland;
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 rock strata. Their presence in
caves and mines is usually a result of the organisms being washed out of the cracks 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 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 moist 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. The
author himself has observed a population of the Freshwater Shrimp (Gammarus pulex), living
on a vertical rock face above the streamway 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 in crannies. Similarly, during a visit to
Pridhamsleigh Cavern, near Buckfastleigh, 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. Similar observations have been recorded by other
researchers and 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.
Of the ten species of hypogean Crustacea known from the British Isles, only three are known
from the county of Devon: Bathynella natans, Niphargus aquilex and Niphargus glenniei.
Bathynella natans
In the past Bathynella natans has been collected from a variety of locations in the British
Isles. However, in 1964 and 1984, Dr. T. Gledhill of the Freshwater Biological Association
(FBA) collected specimens of Antrobathynella stammeri from Yorkshire and Ireland and
subsequent taxonomic research has led Dr. Gledhill and others to assume that
Antrobathynella stammeri is in fact the only species present in Britain, previous specimens
being incorrectly identified as Bathynella natans. However, since examination of earlier
collected specimens has not been possible, previous records of Bathynella natans have to
remain as such. Bathynella natans is widespread across Europe and could well be present in
Britain. Only recent collection of specimens will confirm if this is the case (Gledhill et. al,
1993 & Gledhill pers. Comm.)
Bathynella natans is a very small, eyeless crustacean, approximately 1mm long and 0.1 mm
in diameter. It is usually colourless and has a long, thin body, highly developed for the
interstitial habitat in which it normally occurs. The body is elongate with fourteen trunk
segments (8 thoracic and 6 abdominal) and the head is longer than broad. Each of the
thoracic limbs, with the exception of the seventh, is two-branched.
Vejdovsky first discovered the species in 1880 in a well in Prague. It was described but not
classified until 1899, when Calman placed it in the Syncarida, a crustacean group first
recorded in the fossil state. The species was not found again until 1913, when Chappuis
found it in a well near Basle and Calman published a thorough description based on the
Chappuis specimens. It is known to occur throughout Europe, although it was not discovered
in Britain until 1927, when A.G. Lowndes found two specimens in a tub which caught
drippings from one of the tunnels in the Bath oolite stone quarries at Corsham, in Wiltshire
(Gledhill & Driver, 1964).
Little is known about the biology of Bathynella, although observations have shown that
locomotion consists of a combination of swimming and crawling movements. It is believed
to feed on detritus, bacteria and fungi (Gledhill et.al, 1993).
Within Devon Bathynella natans has been collected in 1961, by G.M. Spooner, from one of
two springs emerging in fields at Moortown near Tavistock. Gravel from the spring water's
point of issue was washed and sieved. Specimens were also found when gravel dug from a
depth of two feet, on a gravel spit beside the River Plym, was washed and sieved. The
Moortown site is 560' OD, on the slope of Dartmoor, whilst the River Plym site was just
upstream of Cadover Bridge, at 690', in the middle of the moors (Spooner, 1961). These
observations prove that Bathynella tolerates the acidic, peaty, very soft water of Dartmoor,
which is unsuitable for most crustaceans. Generally they require the higher concentrations of
calcium, found in hard waters, for the formation of their carapaces.
In the 1960s Dr. Gledhill also collected specimens from the Plym above Cadover Bridge.
However, at the time he also identified them as Bathynella natans, before the current
developments into the group's taxonomy (Gledhill pers. comm.).
Niphargus aquilex
The Niphargus group has representatives throughout Europe. Its members are eyeless and
colourless and most bear a resemblance to the ubiquitous Freshwater Shrimp of surface
waters. They occur in a variety of subterranean aquatic habitats where they are generally
believed to be saprophagous (i.e. they feed on animal and plant derived detritus). However,
they can be predacious on other invertebrates if the opportunity presents itself, including
juveniles of their own species (Gledhill et. al, 1993). Niphargus fontanus has been observed
feeding on the small isopod Proasellus cavaticus in South Wales caves (Glenniei, 1956) and
bait traps containing cheese and meat have provided good results in the past (Gledhill et. al,
1993). Often there appears to be no obvious organic matter in many of the pools in which
Niphargus has been found in caves, which has led to research into alternate sources of
nutrition.
Several researchers on Niphargus have shown that clay and / or silt, with its associated
bacteria and fungi, is an important source of organic nutrients and is the primary food source
for the continental species Niphargus rhenorhodanensis (Gledhill et. al, 1993). Endogenous
(i.e. from within the cave system) organic matter, from chemoautotrophic bacteria, has been
suggested to be of important value within the food chains of some cavernicolous ecosystems.
Some exogenous (i.e. from outside the cave system) organic matter might enter pools in
solution in seepage water and it has been suggested that this matter could be absorbed
directly in solution (Jefferson, 1969).
Niphargus aquilex is the commonest British niphargid, occurring in wells and other
subterranean waters in southern England and Wales. It has been recorded from many
locations south west of a line drawn from the East Coast of Kent to the Wirral Peninsula in
Cheshire. There are also some recent records from north Lincolnshire and a very old record
(1893) exists of the species from Hartlepool, County Durham. It is probably widely
distributed in ground water and is liable to be found where this reaches the surface (e.g.
springs) (Gledhill et.al, 1993). It has been found in low-lying ground (e.g. from under the
Sphagnum moss cover of a mash near Wellington) where the soil-water zone is continuous
with the phreatic, or ground-water, zone and it is thought that, in England at least, the species
may be beginning to invade the soil-water zone (Glennie, 1953).
Niphargus aquilex was first discovered in 1852 by Professor Westwood, who obtained
specimens of a subterranean amphipod from a location near Maidenhead. He described them
initially as being Niphargus stygius, a species then known from continental Europe.
However, in 1855 Schiodte re-examined the material and described them as a new species,
which he named Niphargus aquilex (Gledhill et. al, 1993).
Within Devon, Niphargus aquilex has been recorded from various South Devon caves, wells
near Plymouth, the Shutterton Brook (near Dawlish Warren) and from the River Erme.
Niphargus glenniei
Niphargus glenniei can usually be easily separated from the other British niphargids by virtue
of its much smaller size. Niphargus glenniei attains sexual maturity at 2.5 to 4mm long (most
specimens tend to be 3mm in size), whilst other British Niphargus species are sexually
mature at a minimum size of 4 to 6mm (most specimens found tend to be at least 8mm)
(Spooner, 1952). The Irish species, Niphargus wexfordensis, is the nearest species from the
British Isles to Niphargus glenniei in size, attaining sexual maturity at 4.5mm. Niphargus
glenniei also shows a variety of other more detailed taxonomic differences from the other
British Niphargus and until recently it used to be placed in a separate genus, Niphargellus. It
has more similarities to the continental species Niphargellus arndti and Niphargellus nolli
than to any of the other British niphargids (Karaman et. al,1994)Niphargus glenniei
Niphargus glenniei was first observed on 19th April 1948 by Brigadier E.A. Glennie of the
Cave Research Group, in company with Miss Mary Hazleton (biological recorder for the
Cave Research Group), who actually captured the first specimen. About six specimens were
seen in a small, shallow, mud-lined pool about 250 to 300 feet from the entrance of
Pridhamsleigh Cavern. The six specimens were observed to move about very actively and
disappeared into the mud when disturbed. A young female was captured by Miss Hazleton
and is now the holotype for the species, deposited at the British Museum for Natural History.
The co-type is a young female collected from the Deep Well in Prid on 21st June 1948. The
species was first described by G.M. Spooner, of the Plymouth Marine Labs, in 1951,
following his examination of the specimens collected by Brigadier Glennie, after whom the
species was named (Spooner, 1952).
Glennie (1967) states that "Niphargus glenniei is a highly interstitial form", as suggested by
its thin body and small size and that the species is "frequent in the Pridhamsleigh and
Buckfastleigh caves in the Devonian limestones in South Devon, where its true home is in the
very disturbed rock."
Until recently, the county of Devon was thought to be the only locality world-wide from
which Niphargus glenniei is known. It is believed to be endemic to England, is known from
only 6-15 10km squares on the O.S. national grid and is afforded a Red Data Book K
conservation status (Bratton, 1992).
Within Devon it has been recorded from Napps Cave, near Berrynarbor in the north of the
county and from seventeen sites in South Devon. These sites include: a spring issuing
through metamorphosed slate at Moortown, near Tavistock; superficial riverine gravels in a
spit on the upper reaches of the River Plym, at Cadover Bridge on Dartmoor; a well at
Moorgate (on the edge of Dartmoor, near Wrangton); and several caves and mines. Most of
these latter observations have been in the vicinity of Buckfastleigh, although recent
discoveries have been made in the Chudleigh, Torquay and Brixham Devonian limestone
outcrops. The altitudes the species has been recorded from vary from near sea level, at Napps,
to 700 feet O.D. on Dartmoor, where the species appears to tolerate the soft, acidic, moorland
waters, as with Bathynella.
In late 2000, the species was discovered in a well near Land’s End. A recent investigation by
the author has found it at a second site in the area. It is likely to occur at other locations
between West Cornwall and the Devon border.
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