Protozoan Taxonomy and
Systematics
John O Corliss, University of Maryland, College Park, Maryland, USA
Secondary article
Article Contents
. Introduction
. Historical Considerations
. Major Groups of Protozoa
Taxonomy and systematics of major groups of the Protozoa, an assemblage of so-called
lower eukaryotes, refer basically to the classification of such groups, that is, their
arrangement into a hierarchy of evolutionary interrelated groups (taxa) of scientifically
named phyla, classes, orders, etc.
Introduction
The classification of protozoa and other microorganisms
above the organizational level of the bacteria has always
been dependent on microscopy because the body sizes
involved generally range from only one micrometre to one
or two millimetres in length. Any structures these species
possess, useful in comparative studies of their morphology
and thus their taxonomy and systematics, are at the cellular
and subcellular levels, and invisible to the naked eye. The
physiological properties of protozoa (and the neighbouring algae) have also played a role in the classification of
these ubiquitous eukaryotic microorganisms; and of
growing significance are the findings made possible by
research using molecular biological approaches.
Because we are continuing to learn more and more about
such minute organisms, protozoan systematics – that is,
the taxonomy (classification) and the evolutionary interrelationships of major groups of protozoa – remains a
topic of debate and change, still today. Some of the rather
large and unwieldy taxonomic groupings of past years are
particularly subject to revision with expansion of and
refinement in our knowledge about the members of those –
and related – assemblages. Paradoxically, the protozoa
themselves are becoming more difficult to define with
precision as our information about them and other
microbial assemblages increases. Thus, presenting a single
satisfactory circumscribed definition for them is not an
easy task. It is attempted towards the end of this article;
but, for sake of clarity, further background information is
first supplied.
Historical Considerations
Until well beyond the middle of the twentieth century,
protozoa were widely treated taxonomically as a (mere)
subset of the kingdom Animalia, retaining their ‘firstanimal’ definition dating from 150 years earlier. As a
phylum of unicellular animals, they were thought to exhibit
major characteristics typical of that kingdom: colourless
(i.e. no photosynthetic pigments present; yet, incongru-
. The Old ‘Phylum Protozoa’
. Modern Options Concerning the Place of Protozoa in
the Biotic World
. The New ‘Kingdom Protozoa’
ently, a goodly number of chlorophyll-possessing algal
groups were included in the phylum), phagotrophic, and
capable of independent locomotion. It is now abundantly
clear that this classical definition of protozoa is at best
misleading and incomplete, and that it requires considerable refinement.
In former times, zoologists were the principal investigators – and namers and claimers – of such microorganisms;
and they often worked with taxonomic disregard for
studies of what might actually be the same group,
sometimes even the same species, by the botanists
(phycologists and mycologists). Thus, unfortunately,
territoriality and authoritarianism also played major roles
in determining the early systematic status of the protozoa,
algae and lower fungi (Corliss, 1986).
Advances in our knowledge of the protozoa in general
have followed progress in microscopy, as mentioned
above. As early as a century ago, novel improvements in
methods of light microscopy and related techniques of
fixing and staining were already beginning to make
possible the revelation of morphological cellular characteristics that would have remained totally unrecognized
before the appearance of such technological advances.
Then, at mid-twentieth century, the use of electron
microscopical approaches in cell biology opened up a
whole new epoch of exploration in protozoology and
related fields. During the ‘Age of Ultrastructure’ (as it has
been called), a myriad of previously unknown subcellular
structures were revealed that became of immense value in
the comparative taxonomy of the lower eukaryotes
(especially protozoa and algae). Even more recently, the
exciting development of molecular biological approaches,
particularly in study of genealogical and phylogenetic
relationships within all groups of organisms, large and
small, plant or animal, has offered a refinement in
taxonomic investigations that is without parallel in past
decades.
Here an attempt is made to review and understand the
systematics of protozoa both from the more traditional
point of view, keeping in mind that many biologists are still
familiar mostly with older, conventional classifications of
these minute organisms, and from more modern ap-
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Protozoan Taxonomy and Systematics
proaches. The latter are complicated by the relatively
recent advent of the ‘protistological perspective’ – lumping
protozoa, algae and some other eukaryotic microorganisms together as Protista – and by their usage of more
sophisticated data. For more detailed information concerning protozoan–protistan overlapping relationships,
the reader is referred particularly to comprehensive papers
by Cavalier-Smith (1993, 1998) and Corliss (1994, 1998)
and to the insightful textbooks by Sleigh (1989) and
Hausmann and Hülsmann (1996).
Major Groups of Protozoa
For many decades (commencing with the perceptive
schemes of Otto Bütschli, published in the 1880s), it has
been convenient to assign species known as protozoa to just
four major categories or assemblages, as has been done in
numerous textbooks of biology and zoology as well as
protozoology. Despite advances and improvements in our
knowledge of eukaryotic microorganisms in general, it is
still often rather helpful, for sake of discussion under diverse
circumstances, to arrange protozoa taxonomically in such a
way. The categories generally recognized are: (1) the
amoeboid forms (the Sarcodina, in a broad sense); (2) the
flagellated forms (the Mastigophora, including groups of
autotrophic – or photosynthetic – as well as heterotrophic
species); (3) the ciliated forms (the Ciliophora, the most
stable and perhaps most circumscribed of all protozoan
assemblages); and (4) the various totally symbiotic or
parasitic forms (primarily spore-forming species that are
typically endoparasites, some highly pathogenic to their
hosts, once assigned to a very broad group called the
Sporozoa, a high-level taxon that subsequently became
divided into the Sporozoa and the Cnidosporidia).
One of the pedagogically oft-regretted but inevitable
changes, especially during the busy second half of the
twentieth century (even before the ‘protist revolution’: see
Corliss, 1986), was the tremendous expansion in the total
numbers of high-level taxonomic groups (subphyla,
classes, orders) of protozoa, while generally still recognizing the four major top divisions mentioned above. The
discovery of new and unique differentiating characteristics
useful in classification – and in evolutionary and phylogenetic studies as well – required such a multiplication and
fragmentation of taxa even though it placed/places greater
demands on (the memories of) teachers, students and
researchers alike.
The Old ‘Phylum Protozoa’
Basically, the classification scheme of the widely accepted
‘Honigberg Report’ of 1964 may be used to illustrate the
conventional situation concerning the systematics of the
2
protozoa (Table 1). Although this popular arrangement – in
comparison with the long-followed classical one of
Bütschli of 80 years earlier – contained nearly four times
the number of taxonomic units above the level of family, it
showed few novelties of great significance. For an outstanding example of its conservatism, the classification still
retained several major groups of algae, all treated as
comprising a single class, the Phytomastigophorea, of
flagellated protozoa (the Mastigophora). A subsequent
revision supported by the international Society of Protozoologists, the ‘Levine Report’ of 1980, recognized seven
separate phyla (in a subkingdom Protozoa) and increased
the number of taxa above the familial level to 229 (Table 2).
But, fundamentally, it followed the arrangement of its
predecessor (Table 1). Five years later, the well-known and
widely used Illustrated Guide to the Protozoa (Lee et al.,
1985) appeared, with its classification scheme mainly that
of the Levine Report. By then, the ‘protist revolution’ had
already been going strong for a full decade (see accounts in
Corliss, 1986, 1998) and many findings were indicating the
pressing need for a fresh look at the old persisting problems
of how to treat the systematics of the conventional/
traditional protozoan, algal and fungal assemblages of
microorganisms.
One reason for including the above discussion here,
besides the fact that those classifications of 20–40 years ago
are still accepted by many biologists, is to help bridge the
gap between such neoclassical systems and the suggested
recent arrangements of the 1990s (see below). The
organisms involved, and often their common group names
as well, have not changed over the decades, but our ideas
concerning their most likely interrelationships at the higher
taxonomic levels have done so, primarily by fresh analyses
of the continuing accumulation of data of high phylogenetic and evolutionary significance from precise ultrastructural and molecular biological investigations
(Coombs et al., 1998).
Modern Options Concerning the Place
of Protozoa in the Biotic World
There are a number of ways in which modern biologists are
viewing the overall placement of the protozoa with respect
to other major groups of organisms. Several of these are
not truly taxonomic in their approach: for example,
arrangement of species of protozoa based solely on their
nutritional preferences or requirements, or considering
protozoan groups as representing simply an evolutionary
grade or level of cellular organization between the
anucleate prokaryotes (essentially the bacteria) and the
well-known ‘higher’ (nucleated) eukaryotic forms of life
(particularly the multicellular and multitissued plants and
animals). And popular today, especially among workers
employing cladistic approaches in their investigation of
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Protozoan Taxonomy and Systematics
Table 1 Taxonomic classification of the single phylum Protozoa according to the Report
published by Honigberg et al. (1964), but omitting ordinal and subordinal names. The total number
of suprafamilial categories reached 140, nearly four times the number in vogue among earlier
protozoologists well into the twentieth century. (Modified and abbreviated from Corliss, 1998.)
Subphylum I. Sarcomastigophora
Superclass 1. Mastigophora
Class 1. Phytomastigophorea
Class 2. Zoomastigophorea
Superclass 2. Opalinata
Superclass 3. Sarcodina
Class 1. Rhizopodea
Subclass (1) Lobosia
(2) Filosia
(3) Granuloreticulosia
(4) Mycetozoia
(5) Labyrinthulia
Class 2. Piroplasmea
Class 3. Actinopodea
Subclass (1) Radiolaria
(2) Acantharia
(3) Heliozoia
(4) Proteomyxidia
phylogenetic interrelationships of life forms, is the idea of
clades, distinct evolutionary lines or lineages but ones often
with unknown or nonpostulated possible taxonomic
relationships with other lines of organisms (microorganisms or macroorganisms) (see Patterson, 1999).
The only true taxonomic options today are the inclusion
of the protozoa with other eukaryotic microorganisms in a
single kingdom called Protista or the recognition of a
number of kingdoms (one called the Protozoa) within the
overall assemblage of the Eukaryota. These two approaches require further discussion.
Recognition of a single kingdom to contain all
eukaryotic microorganisms (and some of their larger but
yet basically unicellular or single-tissued relatives) is a
choice, first voiced more than 120 years ago by E. Haeckel
and others, still popular today among a number of
biologists. In this option, a neoHaeckelian kingdom
Protista (or ‘Protoctista’) contains commingled phyla,
divisions and classes of organisms formerly representing
the protozoa, the algae and taxa of lower fungi. It is given
equal standing with the neighbouring kingdoms of the
prokaryotic Bacteria, on the one hand, and of the (other)
eukaryotic moieties known as the Fungi, Plantae and
Animalia, on the other hand (Margulis et al., 1990;
Margulis and Schwartz, 1998).
Subphylum II. Sporozoa
Class 1. Teleosporea
Subclass (1) Gregarinia
(2) Coccidia
Class 2. Toxoplasmea
Class 3. Haplosporea
Subphylum III. Cnidospora
Class 1. Myxosporidea
Class 2. Microsporidea
Subphylum IV. Ciliophora
Class 1. Ciliatea
Subclass (1) Holotrichia
(2) Peritrichia
(3) Suctoria
(4) Spirotrichia
This five-kingdom arrangement represents a pragmatic
one that is quite attractive pedagogically (easy to teach at
all levels in the educational hierarchy) and is convenient for
general nonspecialist usage and for information retrieval
systems. But it is becoming evident that, as our store of
relevant data continues to expand, the ‘single kingdom
Protista’ hypothesis lumps together lines of microscopic
organisms that are proving to be not very closely related
evolutionarily and includes still other lines that are being
shown to have closer affinities with forms assigned to one
of the other three eukaryotic kingdoms. And the protozoa
are not sufficiently given their own identity as an
independent high-level taxonomic assemblage.
The second major alternative available to us today is to
consider the protists, overall, as best assignable to a
number of separate kingdoms, one of which has now been
labelled with the familiar name Protozoa (Cavalier-Smith,
1993; Corliss, 1994). The composition of this protozoan
assemblage – distinctly separated from the four other
eukaryotic assemblages or kingdoms recognized, the
Chromista (taxonomic home for most lines of algae),
Plantae, Fungi and Animalia (see Cavalier-Smith, 1998;
Corliss, 1998, 2000) – is considered below in further detail.
Two points need emphasis here. (1) A formal high-level
taxonomic category called ‘Protista’ is no longer needed or
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Protozoan Taxonomy and Systematics
Table 2 Taxonomic classification of the animal subkingdom Protozoa according to the Report
published by Levine et al. (1980), but here omitting all categories below the rank of class. The total
number of suprafamilial categories reached 229, nearly double the number endorsed in the preceding Honigberg Report of 1964 (see Table 1). (Modified and abbreviated from Corliss, 1998.)
Phylum I. Sarcomastigophora
Subphylum 1. Mastigophora
Class (1) Phytomastigophorea
(2) Zoomastigophorea
Subphylum 2. Opalinata
Class (1) Opalinatea
Subphylum 3. Sarcodina
Superclass 1. Rhizopoda
Class (1) Lobosea
(2) Acarpomyxea
(3) Acrasea
(4) Eumycetozoea
(5) Plasmodiophorea
(6) Filosea
(7) Granuloreticulosea
(8) Xenophyophorea
Superclass 2. Actinopoda
Class (1) Acantharea
(2) Polycystinea
(3) Phaeodarea
(4) Heliozoea
recognized; all former protists (spelled with a lower case
‘p’) can be reassigned to various of the five kingdoms
mentioned immediately above. (2) The (new) kingdom
Protozoa, while containing many conventional groups of
protozoa, is purged of others and thus is not identical in
overall taxonomic composition to the old phylum Protozoa embraced in very popular classifications of both
distant and more recent past years (for example, see
preceding sections of this article; see also Honigberg et al.,
1964, and Levine et al., 1980; and contrast the contents of
Table 1 and Table 2 with that of Table 3).
The New ‘Kingdom Protozoa’
Of the two options briefly described above, many biologists
now favour the second one, the concept of dividing up
major taxa of former protozoa (and of algae and lower
fungi, too), with new assignments to several different
kingdoms, one of which is identified as the kingdom
4
Phylum II. Labyrinthomorpha
Class (1) Labyrinthulea
Phylum III. Apicomplexa
Class (1) Perkinsea
(2) Sporozoea
Phylum IV. Microspora
Class (1) Rudimicrosporea
(2) Microsporea
Phylum V. Ascetospora
Class (1) Stellatosporea
(2) Paramyxea
Phylum VI. Myxozoa
Class (1) Myxosporea
(2) Actinosporea
Phylum VII. Ciliophora
Class (1) Kinetofragminophorea
(2) Oligohymenophorea
(3) Polyhymenophorea
Protozoa. The first person to do this systematically and in
some detail was T. Cavalier-Smith, starting in the 1980s
(see full discussions in Cavalier-Smith, 1993, 1998). The
kingdom Protozoa may be briefly described (after Corliss,
1994, 2000) as follows.
The kingdom comprises predominantly unicellular,
plasmodial or colonial protists (eukaryotic microorganisms) that are mostly phagotrophic or osmotrophic,
colourless, lacking cellulosic cell walls, and microscopic
in body size. Included species that are capable of
photosynthesis (but some are nutritionally mixotrophic)
typically possess unique cytosolic plastids with stacked
thylakoids, no stored starch, and surrounded by three
membranes. Nearly universally present are tubular mitochondrial cristae; when mitochondria are absent, they are
typically replaced by hydrogenosomes. Golgi bodies and
peroxisomes are widely present. Flagellar mastigonemes, if
present, are never rigid or tubular. Free-living (typically
independently motile via pseudopodia, flagella, or cilia)
and symbiotic species are numerous from a great variety of
habitats. As one of the two (of five) eukaryotic kingdoms
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Protozoan Taxonomy and Systematics
Table 3 The 14 phyla, including authorships and dates of their names, comprising the kingdom Protozoa Goldfuss, 1818, with
an indication of the kinds and numbers of protists included in each phyletic taxon (based on abbreviated characterization data
from Corliss, 1994, 1998, 2000)
Archamoebae Cavalier-Smith, 1983
Neomonada Cavalier-Smith, 1997
Rhizopoda von Siebold, 1845
Mycetozoa de Bary, 1859
Foraminifera d’Orbigny, 1826
Heliozoa Haeckel, 1866
Radiozoa Cavalier-Smith, 1987
Percolozoa Cavalier-Smith, 1991
Euglenozoa Cavalier-Smith, 1981
Dinozoa Cavalier-Smith, 1981
Metamonada Grassé, 1952
Parabasala Honigberg, 1973
Apicomplexa Levine, 1970
Ciliophora Doflein, 1901
Large, benthic, microaerobic amoebae, amitochondriate, allegedly primitive
forms, with endosymbiotic bacteria; species few in number, all free-living in
fresh water
Often small, free-living, marine heterotrophic flagellates and amoeboflagellates;
small group, still ill-defined
Typically amoeboid, with differing kinds of pseudopodia, some flagellated forms;
naked or with tests or thecae; 4 5000 species found in soil, fresh- or saltwater
habitats
Plasmodial slime moulds (cellular and acellular), some very large; aerial (stalked)
fruiting bodies produce spores; c. 850 species, mostly in decaying vegetation; a
few symbiotic forms
Amoeboid forms in tests (usually calcareous), with alternation of haploid sexual
and diploid asexual generations; shells of extinct species reach 15 cm in
diameter; reticulate pseudopodia for feeding and locomotion; mostly marine,
with some 45 000 species (largest phylum in the kingdom) of which c. 90% are
fossil forms
Mostly freshwater group of the classical ‘actinopod sarcodinids’, with slender
radiating axopodial type of pseudopodia used in food capture; c. 100 species,
many stalked
Marine, spherical forms, typically planktonic, often with elaborate symmetrical
shell pierced by stiff axopodia; three major subgroups, with total of nearly
12 000 species (c. 65% fossil forms), second largest phylum in the kingdom
Small heterotrophic flagellates or amoeboflagellates, c. 100 species, some poorly
known
The old ‘Euglenophyta’, mainly free-living, freshwater ‘phytoflagellates’,
4 1000 species, plus Kinetoplastidea (parasitic trypanosomes plus free-living
bodonids), 4 600 species; commonly with discoidal mitochondrial cristae and
a paraxial rod in their main flagellum
Dinoflagellates, unique biflagellated protists, mostly marine planktonic, one-half
pigmented forms; some thecate; a few colonial; cortical alveoli; about half
found as fossils; total species c. 4500, with some 100 described as parasites;
many orders
Biflagellated to multiflagellated forms, typically digestive tract parasites (insects
to humans); c. 300 species; hydrogenosomes in place of mitochondria
Parasitic multiflagellated forms, amitochondriate and with prominent parabasal
(Golgi) apparatus; 4 400 species, in intestines of woodroaches to humans
Essentially the ‘Sporozoa’ of old; unique complex of apical organelles; all
symbiotic, with many minute species as harmful endoparasites (e.g. in birds,
livestock, humans: outstandingly, malaria); cortical alveoli; 4 5000 species in
three major classes
All heterokaryotic (micro- and macronuclei); usually multiciliate, phagotrophic,
relatively large protists found mostly free-living in diverse fresh-/saltwater/soil
habitats; others symbiotic or epibiotic, mostly in or on invertebrate hosts; often
complex oral ciliature; cortical alveoli; many exhibit sexual phenomenon of
conjugation; asexual reproduction by transverse fission; third largest
protozoan phylum: c. 8000 species in 8–10 classes, many orders
composed purely of protistan forms, the Protozoa –
counting fossil as well as extant species – embrace nearly
40% of all protists (numbering some 213 000 species)
described to date.
As characterized above, the refined kingdom Protozoa,
large though it is, is considerably more discriminating and
more restricted in its boundaries than was the ‘old’ phylum
Protozoa. The latter, traditionally, contained a number of
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Protozoan Taxonomy and Systematics
major taxa (e.g. at class and ordinal levels) that have now
been eliminated. For example, purged from the ‘new’
kingdom have been half a dozen algal groups, the curious
labyrinthomorphids, the choanozoa, the opalinids, the
myxozoa, and most recently, the microsporidia. All species
of these taxa fail to possess major distinguishing features of
the kingdom Protozoa listed above and/or have unique
morphological, physiological or molecular characteristics
of their own beyond discussion here. Supported by recent
molecular (e.g. rRNA sequencing information) as well as
morphological and biochemical findings, these other
protists have been placed in one or another of the
neighbouring kingdoms Chromista, Fungi, Plantae or
Animalia (Corliss, 1998, 2000). The 14 phyla (containing
nearly 83 000 species, about 60% of which are fossil and
12% symbiotic forms) that then remain in the refined
kingdom Protozoa are listed, with abbreviated descriptions, in Table 3. Representative genera are not mentioned,
for sake of brevity within the table, but names of many such
genera are included in the long papers by Cavalier-Smith
(1993) and Corliss (1994).
Several additional observations might be helpful concerning some of the major differences between this new
classification of the Protozoa and the older, more
conventional ones discussed on preceding pages.
Two distinctive eukaryotic algal lines – the euglenids
and the dinoflagellates – are retained as Protozoa, but they
have strikingly different characteristics (including presence
of three, not two, plastidic membranes and frequent
exhibition of nonphotosynthetic modes of nutrition,
among others) from other major assemblages of algae
now appearing in the separate kingdoms Chromista (e.g.
the chrysophytes, phaeophytes, haptomonads, cryptomonads and lesser heterokontic groups) and Plantae (e.g. the
prasinophytes, chlorophytes (with such long-claimed
‘protozoa’ genera as Volvox and Chlamydomonas), ulvophytes, charophytes, rhodophytes and glaucophytes). In
fact, the ‘euglenophytes’ of old are phylogenetically closely
related to the colourless kinetoplastideans (bodonids and
parasitic trypanosomatids), the combining single phyletic
name now being the Euglenozoa.
The dinoflagellates (Dinozoa), furthermore, are linked
with the ciliates (Ciliophora) and the sporozoa (Apicomplexa) in a supraphyletic assemblage often designated as
the Alveolata because of their common possession of
cortical alveoli in their pellicles, a character not particularly known in other phyla of protists.
Mostly on the basis of very recent rRNA data, but also
supported by other differentiating characteristics, the longlinked ‘protozoan-cnidosporidian’ groups, the enigmatic
microsporidia and myxosporidia, turn out to be not only
not closely related to each other but also not members of
the kingdom Protozoa. The first, the Microspora, along
with chytrids (but not with oomycetes and hyphochytriomycetes, which are now in the Chromista), are probably
primitive or degenerate protistan members of the kingdom
6
Fungi, while the complex Myxozoa are better classified in
the Animalia (perhaps close to the cnidarians).
Until rather recently, the Archamoebae (with genus
Pelomyxa and possibly others such as the important
parasite of humans, Entamoeba) were considered as very
primitive protists in a kingdom of their own. Latest studies
(see references in Cavalier-Smith, 1998; Corliss, 2000)
suggest that, while probably composed of very ancient
species (now without mitochondria or hydrogenosomes),
the phylum belongs more appropriately in the revised
kingdom Protozoa, as seen in Table 3.
The unique Choanozoa, collar-celled, loricate, colourless flagellates with flattened mitochondrial cristae, presumably at the crossroads of origins of both kingdoms
Animalia and Fungi, have been removed from the
Protozoa and tentatively placed in the Animalia (Corliss,
2000), where another protistan group and former protozoan phylum, the Myxozoa, is also now located (see above).
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