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Identification key for holocene lacustrine arcellacean (thecamoebian) taxa

Arun Kumar and Andrew P. Dalby

Plain Language Abstract

Arcellaceans, also known as agglutinated rhizopods and testate amoebae, belong to the "Thecamoebians", a subdivision of the microscopic single-celled organisms called Protozoans. The most commonly known microscopic organism, the 'Aamoeba', is closely related to arcellaceans. These organisms are characterized by the presence of different kinds of pseudopods, which are temporary protrusions ('arms') of cytoplasm (semi-liquid matter within the cell) from the cell for movement or feeding. They have a simple, rigid, and sac-like shell (calledwe call it a 'test') into which the body of the animal can be withdrawn. These tests are either secreted by the organisms themselves (autogenous) or built of agglutinated foreign particles (xenogenous), like tiny sand grains, in a self-generated cement. The autogenous tests are usually smooth, and could be made of either siliceous platelets (idiosomes), complex organic compounds (proteinaceous), or rarely chalky substances (calcareous). Since most arcellaceans live at the bottom of lakes or other bodies of water, the foreign particles (xenosomes) come from the sediment upon which they live. That could be sand grains, diatoms (an even smaller single-celled organism), or any other available particle. Their size ranges between 60-300 microns (thousandths of a millimetre), and they occur abundantly in fresh and marginally saline (brackish) waters around the world. Their fossil record is known mainly from lake sediments and peat deposits.

Arcellaceans are very useful in the study of the environmental history of lakes and their surrounding regions. Several arcellacean studies on lake sediments and peat bogs have shown that these microscopic organisms respond to changes in salinity and acidity of the water mass and to the changing climate of a region by either altering their overall shape or by leaving the hostile environment for a more equitable one. In addition to natural environmental changes, man-made changes in the environment can also be detected by the study of arcellaceans. These include: deforestation and the resulting erosion in the catchment area of the lakes; and mining and subsequent dumping of the waste rocks ('tailings') in the lakes resulting in the chemical pollution of the water and the sediment. Arcellaceans They cancould be used to monitor both short- and long-term environmental changes, and levels of pollution and remediation in the polluted lakes. Organic productivity within lakes also affects arcellaceans in terms of species composition and/or numerical abundance of a certain species or "strain". Thus, they are useful indicators of trophic levels (availability of nutrition for organic productivity) within lakes.

The analysis of arcellaceans from peat bogs provide information on the development of peatlands, the water table, and moisture content. Such information is very useful for management schemes for the forestry, agriculture and horticultural industries.

As arcellaceans produce asexually, they are uniparental organisms. Morphologically, arcellaceans They usually have simple sac-like testshape with an opening or aperture at one end. Their test may or may not have a neck or spines. These morphological parameters and other characteristics like the nature and shape of idiosomes and xenosomes, and the shape and composition of tests, are used to identifyname and classify them. Thus, arcellaceans Therefore they are grouped into specific units (species) on the basis of the shape of their tests. Under the influence of various physical or chemical environments, several arcellacean morphotypes or "strains" occur abundantly in nature. This has created serious problems in theof identification and naming of arcellaceans. There are two extreme camps on this issue: the 'lumpers' consider all arcellaceans belonging to one species; while the 'splitters' believehold that all morphotypes (strains) that occur in nature more than few times belong to a genuine species. Our opinion is that neither of these views are neither correct nor practical.

We have developed anour identification key for this very significant group of micro-organisms with the objective of providing a more scientific method of identification, thus helping new researchers to study these organisms and to standardize their classification. The key is designed to illustrate various testsforms that we find useful in our studies and provide an arcellacean identification guide for future researchers.

The following two websites provide more information on arcellaceans.

1. The Dalhousie Thecamoebian Bibliography.

2. Dr. Ken Hooper Virtual Natural History Museum, Ottawa-Carleton Geoscience Centre, Department of Earth Sciences, Carleton University, Ottawa, Canada. Arcellaceans: A new class of paleolimnological indicator.

Here is a brief GLOSSARY for some of the terms that we use:

Acuminate - tapering to form a spine.
Agglutination - foreign particles bound together by cement into a test.
Aperture - opening of the test. It is standard procedure to figure the specimens aperture up.
Autogenous - a test secreted by the organism.
Frustule - diatom shell.
Fundus - the end of the test opposite the aperture (also called the aboral end).
Holocene - Recent, less than about 8-10 thousand years old.
Idiosomes - agglutinated foreign particles of the test made from other organic material.
Lacustrine - pertaining to lakes.
Strain - infraspecific morphological variant.
Test - shell, or external hard part, of a protozoan.
Xenogenous test - test composed of agglutinated foreign particles.
Xenosome - agglutinated foreign particles of the test made from mineral grains.

Resumen en Español

Claves de identificación para los taxones holocenos lacustres de arcelinos (Tecamebas)

Los arcelinos (Tecamebas), protozoos principalmente de agua dulce, han sido estudiado desde hace más de un siglo. No obstante, durante los diez últimos años se ha comprendido mejor su utilidad como indicadores paleoambientales, conforme los investigadores han reconocido que ciertos morfotipos (razas) prefieren determinados microambientes. Desafortunadamente, la naturaleza monoclonal del grupo y su morfología sencilla han creado numerosas confusiones taxonómicas que amenazan su utilidad en la investigación paleolimnológica. Las claves aquí presentadas tratan de ayudar tanto a los nuevos investigadores sobre el grupo como a su normalización taxonómica.


Résumé en Français

Clé d’identification des arcellacés (thécamibes) lacustres Holocènes.

Les arcellacés (thécamibes), des protozoaires surtout d’eau douce, ont étés étudiés depuis plus d’un siècle. Cependant, leur utilité comme indicateur paléoenvironnemental est de plus en plus reconnue depuis la dernière décennie, à mesure que les chercheurs reconnaissent les microenvironnements préférés de différents morphotypes (races). Malheureusement, la nomenclature monoclonale du groupe et la simplicité de leur morphologie créent une profonde confusion taxonomique, menaçant leur utilité pour les recherches paléoenvironnementales. Cette clé fut créée pour faciliter l’étude du groupe par les nouveaux chercheurs, et pour en standardiser la taxonomie.

MOTS CLÉS: thécamibe(s), arcellacé(s) Holocènes, clé d’identification

Deutsche Zusammenfassung

Bestimmungsschlüssel für Holozäne lakustrische Arcellacea (Thecamoebia)

Arcellacea (Thecamoebia), die vor allem Süsswasser-Protozoen sind, werden seit mehr als einem Jahrhundert untersucht. Ihr Wert als Paläo-Umweltindikator hat sich während der letzten zehn Jahre verbessert, nach dem Bearbeiter festgestellt haben, dass bestimmte Morphotypen bestimmte Mikrohabitate bevorzugen. Leider hat die monoklonale Natur der Gruppe und ihre einfache Morphologie zu erheblicher taxonomischer Verwirrung geführt, die die Nützlichkeit der Gruppe für paläolimnologische Forschung in Frage stellt. Der hier vorgestellte Bestimmungsschlüssel wurde erarbeitet, um neuen Bearbeitern zu helfen und um die Taxonomie zu vereinheitlichen.

Riassunto in Italiano


Le Arcellacee (tecamebe), in gran parte protozoi d'acque dolci, sono state studiate per oltre un secolo. Durante gli ultimi dieci anni la loro utilità come indicatori paleoambientali è stata compresa più a fondo perchè i ricercatori hanno riconosciuto che certi morfotipi (razze) preferiscono microambienti ben distinti. Sfortunatamente, la natura monoclonale e la morfologia semplice di questo gruppo hanno portato ad una considerevole confusione tassonomica, minacciando il loro utilizzo negli studi paleolimnologici. Questa chiave identificativa è stata creata per aiutare i nuovi ricercatori che studiano il gruppo e per uniformare la tassonomia.


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