Introduction
Meiofauna are small benthic invertebrates that live in both
marine and fresh water environments. The term Meiofauna loosely defines a
group of organisms by their size, larger than Microfauna but smaller
than Macrofauna , rather than a taxonomic grouping. In practice that is
organisms that can pass through a 1 mm mesh but will be retained by a 45
μm mesh, but the exact dimensions will vary from researcher to
researcher. Whether an organism will pass through a 1 mm mesh will also
depend upon whether it is alive or dead at the time of sorting.
The term meiobenthos was first coined in 1942 by
Mare, but organisms that would fit into the meiofauna
category have been studied since the 18th century. A good comprehensive
text on meiofauna is Introduction to the study of meiofauna by Higgins and Thiel.
Collecting the Meiobenthos
Meiofauna are most commonly encountered in sedimentary environments in both
marine and fresh water environments, from the littoral to the
deep-sea . They can also be found on hard substrates living on
algae, the phytal environment, and sessile invertebrates
(barnacles, mussel beds, etc.).
Sampling methodologies
Sampling the meiobenthos is clearly dependent upon the environment and
whether quantitative or qualititative samples are required. In the
sedimentary environment the methodology used also depends on the physical
morphology of the sediment. For qualititative sampling within the littoral
zone, for both coarse and fine sediment, a bucket and spade will suffice.
In the sub-littoral and deep water some form of grab (like the Van Veen
grab) is required. For the quantitative sampling of sedimentary
environments at all depths a wide variety of samplers have been devised.
The simplest is a plastic syringe with the end cut off to form a piston
corer which can be deployed in the littoral zone, or in the sub-littoral
using SCUBA gear. Generally the deeper the water the more complicated the
sampling process becomes. For sampling the meiofauna on hard substrates,
phytal and epizooic environments, the only practical methodology is to cut
or scrape off a known area of substrate and place it in a plastic bag.
Extraction methodologies
There are a wide variety of methods for extracting meiofauna from the
samples of their habitat depending upon whether live or fixed specimens are
required. For extracting live meiofauna one has to contend with the large
number of species that cling or attach themselves to the substrate when
disturbed. In order to get the meiofauna to release their grip there are
three methodologies available. The first, an simplest, is osmotic shock,
this is achieveded by submerging the sample in fresh water (clearly this
will only work for marine samples) for a few seconds. This will case the
organisms to release they can then be shaken free from the substrate and
filtered out through a 45 μm mesh and immediately returned to fresh
filtered seawater. Many organisms will come through this process unharmed
as long as the osmotic shock does not last too long. The second
methodology is the use of an anaesthetic. The preferreded solution for
meiobenthologists is isotonic magnesium chloride (7.5g MgCl2
· 6H2O in 100 ml of distilled water). The sample is
immersed in the isotonic solution and left for a period of 15 min, after
which the meiofauna are shaken free of the substrate and again filtered out
through a 45 μm mesh and immediately returned to fresh filtered
seawater. The third methodology is Uhlig's seawater ice
technique. This relies on the organisms moving ahead of a front of ice
cold seawater moving down through the sample ultimately forcing them out of
the sediment. It is most effective on samples from temperate and tropical
regions.
For major studies where large numbers of samples are collected concurrently
samples are normally fixed using 10% formalin solution and the meiofauna
extracted at a later date. There are two main extraction methodologies.
The first, decantation, works best with coarse sediments. Samples are
shaken in an excess of water, the sediment is briefly allowed to settle and
the meiofauna filtered off. The second methodology, the floatation
technique, works best with finer sediments were the mass of the sediment
particles is close to that of the meiofauna. The best solution to use is
the colloidal silica, Ludox™. The sample is stirred into the
Ludox™ solution and left to settle for 40 min, after which the
meiofauna are filtered out. With both methodologies repeated extractions
should be made (at least three) with each sample to ensure that at least
95% of the fauna is extracted.
Meiofaunal Taxa
Based on the scheme of Nielsen (2001). Meiofaunal taxa
appear in bold text.
- Protozoa
- Animalia -- Eumetazoa
- Cnidaria
- Triploblastica -- Bilateria
- Protostomia -- Spiralia
- Schizocelia
- Sipuncula
- Articulata
- Bryozoa
- Paranchymia
- Gnathifera
- Cycloneuralia
- Duterstomia
References
Higgins, R.P. and Thiel, H. (1988) Introduction to the study of meiofauna.
Smithsonian Institution Press, Washington D.C. ISBN 0-87474-488-1
Mare, M.F. (1942) A study of a marine benthic community with special
reference to the micro-organisms. Journal of the Marine Biological Association of the United Kingdom, 25:517-554.
Nielsen, C. (2001) Animal evolution: interrelationships of the living phyla.
Oxford University Press. ISBN 0-19-8506821-3
Uhlig, G., Thiel, H. and Gray, J.S. (1973) The quantitative separation of
meiofauna. Helgoländer wissenschaftliche Meeresuntersuchungen,
11: 178-185.
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