June 1996
Seasonal snapshots of nature in close-up
This month's features in our
pond-dipping issue:
Regular readers of Micscape Magazine will know that the
contributors are particularly keen to sing the praises of
pond-dipping to the amateur microscopist. Youngsters have always
had a fascination for ponds, but seem to lose it as they grow
older. This is a pity because at any time of year there is a
wealth of both macroscopic and microscopic pond organisms for
naturalists both young and old to study.
In northern England where the author lives, a spell of cold
weather (our second coldest May since records began!) has slowed
the progress of early summer, so the author has concentrated this
month on some of the plant and animal organisms that can be found
in most bodies of freshwater.
Please read the important
notes on collecting which also describes
some simple precautions when pond-dipping.
For the casual observer of freshwater life, very
little equipment is needed to become familiar with some of the
commoner pond organisms, either at the pond side with a 10X hand
lens or for taking home for closer examination. A more scientific
study of the ecology of a habitat and collection of particular
organisms requires more systematic techniques and specialist
equipment which may be discussed in a later article.
A wealth of subjects to study at home can
often be found simply by transferring a small handful of weed and
water collected from the pond shallows into a sealed plastic
container. It is important to keep the container cool during
transport and back at home as well as allowing good aeration.
It is often worth studying the larger
invertebrates you may find such as beetles and dragonfly larvae
at the pond side and returning them to the pond immediately, as
the larger organisms are less likely to survive a journey home,
and the carnivorous larvae may eat the smaller organisms of
interest! The small white margarine tubs (and lids to examine the
larger organisms closely with a lens) are excellent for this.
Photographic developing trays are also useful. There are a number
of simple guides and keys available which will allow you to
identify organisms to the correct family.
The organisms below were all collected from
a pond sampled as described above with no special equipment.
Freshwater shrimps (Gammarus) are
commonly found in cleaner ponds and streams where the calcium
concentration is high enough to support them. In Yorkshire,
England where the author lives Gammarus pulex is not
found in water bodies with a calcium content of less than about
10mg per litre.
They are members of the Phylum Crustacea and
the order Amphipoda. They can reach 20 mm in length and their
habits can easily be observed by eye or 10X hand lens in a
shallow dish. They are laterally compressed and swim on their
side and typically eat organic debris. Their body and leg
structure is worth studying, which are shown in the images
heading this and the previous section.
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Walk's Contents
Algae are single-celled plants and include
microscopic forms less than 1 micron long to the giant seaweed
called kelp which can reach lengths of 60m. Many algae are very
attractive particularly if dark ground illumination is used. Most
ponds contain various filamentous algae and the green 'slime'
often found in ponds is composed of algae.
A small sample of this apparently unappealing
slime when pond-dipping can provide a wealth of microscopic fauna
and flora that have attached themselves to the algae or are
trapped in the filaments such as the smaller water fleas (see
below). The cellular structure of the algae can be examined with
modest powers of 100-200X using the traditional 'high power'
microscope rather than the low power stereo. One of the general
guides on algae may allow identification to genus.
In the sample taken by the author, some of the
algal filaments were undergoing conjugation, and is shown on the
image on the right above. This is sexual reproduction and
involves two filaments lying side by side joining by the
formation of outgrowths. The contents of one cell passes into the
other and forms a fused cell or zygote which can overwinter while
vegetative filaments die.
A group of organisms
in which the author has a particular interest is the 'water
fleas' a popular name for a group that includes Daphnia,
which are well known to aquarists as a fish food. Water fleas
were formerly grouped in the order Cladocera, although many
taxonomists now split them among four groups. However, Cladocera
is still a widely used descriptive term for them.
Cladocera are a
delight to study under the microscope and have fascinated the
author since he received his first microscope at ten years old. A
drawing made by the author when he was aged 12 is shown right as
proof! Chydorus sphaericus is a very common Cladoceran
in most ponds and may be found with similar species in the weed
samples described above. The Cladocera shown left above under
dark ground illumination is also a member of the Chydoridae
family. If mounted in a shallow cavity slide their structure,
beating heart and limb movement can be admired while they are
alive.
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Walk's Contents
Suggested reading
Pond-life - Introductory guides.
1) Collins Field Guide to Freshwater Life
by R Fitter and R Manuel. Collins, London, 1986. ISBN 0 00
219143-1. A superb guide to the macroscopic and microscopic plant
and animal life of Britain and N. Europe and enables organisms to
be identified to at least the correct family and many to genus.
2) Freshwater Invertebrates. A Key to the
Major Groups. by P S Croft. Field Studies Council, 1986. ISBN 1
85153 181 6. A good pond side guide to allow the macroscopic
(> 2mm) invertebrates to be identified to family. Excellent
for school students and can complement ref. 1.
Algae
3) A beginner's Guide to Freshwater Algae by H
Belcher and E Swale. Natural Environment Research Council 1978.
ISBN 0 11 881393 5. A good well-illustrated introduction.
Water fleas
4) A Key to the British Species of Freshwater
Cladocera by D J Scourfield and J P Harding. Freshwater
Biological Association, UK, 1994 reprint. This to the author's
knowledge is still the only guide to the British species in
print. It became a classic when first published in 1941, but
unfortunately the Freshwater Biological Association reprinted in
effect the 1958 Edition in 1994 without updating. It is still a
useful introduction but is woefully out of date.
5) Crustace Cladoceres by C Amoros. Bulletin
de la Societe Linneenne de Lyon, 53e annee, no 3, mars 1984
p72-107. Although an introduction to the French Cladocera, most
British species are covered and has a superb diagrammatic key
which is a delight to use.
Image details
Macro images of the shrimp were taken using a CCD camera
attached to the eyepiece tube of a stereo microscope using a x1
paired objective with no eyepiece. Algae and water flea images
were taken using 9x or 20x objective without an eyepiece on a
compound microscope.
The pond image was captured from a video by the Microscopy UK
Editor Maurice Smith...thanks Maurice!
Camera images were transferred to the PC using a Creative Video
Spigot capture card.
Image manipulation using Photostyler v2.0 software.
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Index
The author Dave Walker
is a UK based amateur naturalist keen to encourage people to
explore nature in close-up.
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