3.1: Taxonomy - Biology

3.1: Taxonomy - Biology

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Taxonomy (which literally means “arrangement law”) is the science of classifying organisms to construct internationally shared classification systems with each organism placed into more and more inclusive groupings. This organization from larger to smaller, more specific categories is called a hierarchical system.

The taxonomic classification system (also called the Linnaean system after its inventor, Carl Linnaeus, a Swedish botanist, zoologist, and physician) uses a hierarchical model. Moving from the point of origin, the groups become more specific, until one branch ends as a single species. For example, after the common beginning of all life, scientists divide organisms into three large categories called a domain: Bacteria, Archaea, and Eukarya. Within each domain is a second category called a kingdom. After kingdoms, the subsequent categories of increasing specificity are: phylum, class, order, family, genus, and species (Figure 1).

The kingdom Animalia stems from the Eukarya domain. For the common dog, the classification levels would be as shown in Figure 1. Therefore, the full name of an organism technically has eight terms. For the dog, it is: Eukarya, Animalia, Chordata, Mammalia, Carnivora, Canidae, Canis, and lupus. Notice that each name is capitalized except for species, and the genus and species names are italicized. Scientists generally refer to an organism only by its genus and species, which is its two-word scientific name, in what is called binomial nomenclature. Each species has a unique binomial nomenclature to allow for proper identification.

Therefore, the scientific name of the dog is Canis lupus.

It is important that the correct formatting (capitalization and italics) is used when calling an organism by its specific binomial.

The name at each level is also called a taxon. In other words, dogs are in order Carnivora. Carnivora is the name of the taxon at the order level; Canidae is the taxon at the family level, and so forth. Organisms also have a common name that people typically use, in this case, dog. Note that the dog is additionally a subspecies: the “familiaris” in Canis lupus familiaris. Subspecies are members of the same species that are capable of mating and reproducing viable offspring, but they are considered separate subspecies due to geographic or behavioral isolation or other factors.


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Taxonomy, in a broad sense the science of classification, but more strictly the classification of living and extinct organisms—i.e., biological classification. The term is derived from the Greek taxis (“arrangement”) and nomos (“law”). Taxonomy is, therefore, the methodology and principles of systematic botany and zoology and sets up arrangements of the kinds of plants and animals in hierarchies of superior and subordinate groups. Among biologists the Linnaean system of binomial nomenclature, created by Swedish naturalist Carolus Linnaeus in the 1750s, is internationally accepted.

Popularly, classifications of living organisms arise according to need and are often superficial. Anglo-Saxon terms such as worm and fish have been used to refer, respectively, to any creeping thing—snake, earthworm, intestinal parasite, or dragon—and to any swimming or aquatic thing. Although the term fish is common to the names shellfish, crayfish, and starfish, there are more anatomical differences between a shellfish and a starfish than there are between a bony fish and a man. Vernacular names vary widely. The American robin (Turdus migratorius), for example, is not the English robin (Erithacus rubecula), and the mountain ash (Sorbus) has only a superficial resemblance to a true ash.

Biologists, however, have attempted to view all living organisms with equal thoroughness and thus have devised a formal classification. A formal classification provides the basis for a relatively uniform and internationally understood nomenclature, thereby simplifying cross-referencing and retrieval of information.

The usage of the terms taxonomy and systematics with regard to biological classification varies greatly. American evolutionist Ernst Mayr has stated that “taxonomy is the theory and practice of classifying organisms” and “systematics is the science of the diversity of organisms” the latter in such a sense, therefore, has considerable interrelations with evolution, ecology, genetics, behaviour, and comparative physiology that taxonomy need not have.

Scientific Classification

Classification, or taxonomy, is a system of categorizing living things. There are seven divisions in the system: (1) Kingdom (2) Phylum or Division (3) Class (4) Order (5) Family (6) Genus (7) Species.

Kingdom is the broadest division. While scientists currently disagree as to how many kingdoms there are, most support a five-kingdom (Animalia, Plantae, Protista, Monera, and Fungi) system. The lowest division is species, which consists of organisms that are capable of interbreeding to produce fertile offspring. Species are identified by two names (binomial nomenclature). The first name is the genus, the second is the species.

For example, a lion is Panthera leo, a tiger is Panthera tigris. The first word is always capitalized, the second is not, and both should be italicized. Humans, of course, are Homo sapiens. The full classification for a lion would be: Kingdom, Animalia (animals) Phylum, Chordata (vertebrate animals) Class, Mammalia (mammals) Order, Carnivora (meat eaters) Family, Felidae (all cats) Genus, Panthera (great cats) Species, leo (lions).

Bacterial Taxonomy: Meaning, Importance and Levels

The science of classification of bacteria is called bacterial taxonomy. Bacterial taxonomy (G: taxis = arrangement or order, nomos = law or nemein = to distribute or govern), in a broader sense, consists of three separate but interrelated disciplines: classification, nomenclature, and identification.

Classification refers to the arrangements of bacteria into groups or taxa (sing, taxon) on the basis of their mutual similarity or evolutionary relatedness.

Nomenclature is the discipline concerned with the assignment of names to taxonomic groups as per published rules. Identification represents the practical side of taxonomy, which is the process of determining that a particular isolate belongs to a recognized taxon. It is to mention here that the term Bacterial systematics often is used for bacterial taxonomy.

But, systematics bears broader sense than taxonomy and is defined by many as the scientific study of organisms with the ultimate object of characterizing and arranging them in an orderly fashion. Systematics therefore encompasses disciplines such as morphology, ecology, epidemiology, biochemistry, molecular biology, and physiology of bacteria.

Importance of Bacterial Taxonomy:

Bacterial taxonomy, however, is important due to following reasons:

1. Bacterial taxonomy senses to be a library catalogue that helps easily access large number of books. Taxonomy therefore helps classifying and arranging bewildering diversity of bacteria into groups or taxa on the basis of their mutual similarity or evolutionary relatedness.

2. The science of bacteriology is not possible without taxonomy because the latter places bacteria in meaningful, useful groups with precise names so that bacteriologists can work with them and communicate efficiently.

3. Bacterial taxonomy helps bacteriologists to make predictions and frame hypotheses for further research based on knowledge of identical bacteria. For convenience, the bacteriologist can predict that a bacterium in question would be possessing similar characteristics to its relative bacterium whose characteristics are already known.

4. Contribution of bacterial taxonomy in accurately identifying bacteria is of practical significance. For convenience, bacterial taxonomy contributes particularly in the area of clinical microbiology. Treatment of bacterial disease often become exceptionally difficult if the pathogen is not properly identified.

Ranks or Levels of Bacterial Taxonomy:

In bacterial taxonomy, a bacterium is placed within a small but homogenous group in a rank or level. Groups of this rank or level unite creating a group of higher rank or level. In bacterial taxonomy, the most commonly used ranks or levels in their ascending order are: species, genera, families, orders, classes, phyla, and domain (Table 3.1).

Species is the basic taxonomic group in bacterial taxonomy. Groups of species are then collected into genera (sing, genus). Groups of genera are collected into families (sing, family), families into orders, orders into classes, classes into phyla (sing, phylum), and phyla into domain (the highest rank or level). Groups of bacteria at each rank or level have names with endings or suffixes characteristic to that rank or level.

Characteristics Used In Bacterial Taxonomy:

1. Classical Characteristics (Classical Taxonomy):

Several phenotypic characteristics (e.g., morphological, physiological and metabolic, ecological) and genetic analysis have been used in bacterial (microbial) taxonomy for many years.

These characteristics are assessed and the data are used to group bacteria up to the taxonomic ladder from species to domain. Classical characteristics are quite useful in routine identification of bacteria and also provide clues for phylogenic relationships amongst them as well as with other organisms.

Morphological Characteristics:

Various morphological features, e.g., cell shape, cell size, colonial morphology, arrangement of flagella, cell motility mechanism, ultra structural characteristics, staining behaviour, endospore formation, spore morphology and location, and colour are normally used to classify and identify microorganisms.

Morphological characteristics play important role in microbial classification and identification due to following reasons:

(i) They are easily studied and analysed especially in eukaryotic microorganisms and comparatively complex prokaryotes.

(ii) They normally do not vary greatly with environmental changes as they are resulted in by the expression of many genes and, therefore, are usually genetically stable.

(iii) Morphological similarity amongst microorganisms often is a good indication of phylogenetic relatedness.

Some taxonomically useful morphological characteristics and their variations are shown in Table 3.2.

Physiological and Metabolic Characteristics:

Some physiological and metabolic characteristics are very useful in classifying and identifying microorganisms because they are directly related to the nature and activity of microbial enzymes and transport proteins.

Some most important physiological and metabolic characteristics used in microbial taxonomy are nutritional types, cell wall components, carbon and nitrogen sources, energy metabolism, osmotic tolerance, oxygen relationships, temperature relationships, salt requirements and tolerance, secondary metabolites, storage inclusions, etc.

Some taxonomically useful physiological and metabolic characteristics and their variations are given in Table 3.3.

Ecological Characteristics:

Ecological characteristics, i.e., the characteristics of relationship of microorganisms to their environment significantly contribute in microbial taxonomy. It is because even very closely related microorganisms may vary considerably with respect to their ecological characteristics.

For convenience, microorganisms inhabiting freshwater, marine, terrestrial, and human body environments differ from one another and from those living in different environments.

However, some of the most important ecological characteristics used in microbial taxonomy are – life cycle patterns, the nature of symbiotic relationship, pathogenic nature, and variations in the requirements for temperature, pH, oxygen, and osmotic concentrations.

Genetic analysis:

Genetic analysis is mostly used in the classification of eukaryotic microorganisms because the species is defined in these organisms in terms of sexual reproduction which occurs in them. This analysis is sometimes employed in the classification of prokaryotic microorganisms particularly those that use the processes of conjugation and transformation for gene exchange.

For example, members of genus Escherichia may conjugate with the members of genera Shigella and Salmonella but not with those of genera Enterobacter and Proteus. This shows that the members of first three genera are more closely related to one another than to Enterobacter and Proteus.

Studies of transformation with genera like Bacillus, Haemophilus Micrococcus, Rhizobium, etc. reveal that transformation takes place between different bacterial species but only rarely between genera.

This provides evidence of a close relationship between species since transformation tails to occur unless the genomes are very much similar. Bacterial plasmids that carry genes coding tor phenotypic traits undoubtedly contribute in microbial taxonomy as they occur in most genera.

2. Molecular Characteristics:

Some recent molecular approaches such as genomic DNA GC ratios, nucleic acid hybridization, nucleic acid sequencing, ribotyping, and comparison of proteins have become increasingly important and are used routinely for determining the characteristics of microorganisms to be used in microbial taxonomy.

Genomic DNA GC ratios (G + C content):

Genomic DNA GC ratio (G + C) is the first, and possibly the simplest, molecular approach to be used in microbial taxonomy. The GC ratio is defined as the percentage of guanine plus cytosine in an organism’s DNA.

The GC ratio the base sequence and varies with sequence changes as follows:

The GC ratio of DNA from animals and higher plants averages around 40% and ranges between 30 and 50%. Contrary to it, the GC ratio of both eukaryotic and prokaryotic microorganisms varies greatly prokaryotic GC ratio is the most variable, ranging from around 20% to almost 80%. Despite such a wide range of variation, the GC ratio of strains within a particular species is constant.

Genomic DNA GC ratios of a wide variety of microorganisms have been determined, and knowledge of this ratio can be useful in microbial taxonomy, depending on the situation. For convenience, two microorganisms can possess identical GC ratios and yet turn out to be quite unrelated both taxonomically and phylogenetically because a variety of base sequences is possible with DNA of a single base composition.

In this case, the identical GC ratios are of no use with view point of microbial taxonomy. In contrast, if two microorganisms’ GC ratio differs by greater than about 10%, they will share few DNA sequences in common and are therefore unlikely to be closely related.

GC ratio data are valuable in microbial taxonomy in at least two following ways:

(i) They can confirm a scheme of classification of microorganisms developed using other data. If microorganisms in the same taxon vary greatly in their GC ratios, the taxon deserves to be divided.

(ii) GC ratio appears to be helpful in characterizing bacterial genera since the variation within a genus is usually less than 10% even though the content may vary greatly between genera.

For convenience, Staphylococcus and Micrococcus are the genera of gram-positive cocci having many features in common but differing in their GC ratio by more than 10%. The former has a GC ratio of 30-38%, whereas the latter of 64-75%.

Nucleic acid hybridization (Genomic hybridization):

Nucleic acid hybridization or genomic hybridization measures the degree of similarity between two genomes (nucleic acids) and is useful for differentiating two bacteria (microorganisms). DNA-DNA hybridization is useful to study only closely related bacteria, whereas DNA-RNA hybridization helps comparing distantly related bacteria.

1. DNA-DNA hybridization:

Double-stranded DNA isolated from one bacterium is dissociated into single strands at appropriate temperature which are made radioactive with 32 P, 3 H, or 14 C.

Similarly, double-stranded DNA isolated from other bacterium is dissociated into single strands which are not made radioactive. The non-radioactive single- stranded DNA molecules are first allowed to bind to a nitrocellulose filter and unbound strands are removed by washing.

Now the filter with bound strands of DNA is incubated with the radioactive single-stranded DNA under optimal conditions of annealing. Annealing is an interesting feature of single-stranded DNA in which the strands, on cooling, tend to re-associate to form double-helix structure automatically.

Annealing occurs optimally when the temperature is brought to about 25°C below the melting temperature (Tm) in a solution of high ionic concentration.

However, during incubation the radioactive single strands of DNA hybridize with non-radioactive single strands of DNA depending on their homology in the base sequence. Then the filler is washed to remove unbound radioactive DNA molecules and the radioactivity of the hybridized radioactive DNA molecules is measured.

Following this, the amount of radioactivity in the hybridized radioactive DNA molecules is compared with the control which is taken as 100%, and this comparison gives a quantitative measure of the degree of complementarity of the two species of DNA, i.e., homology between the two DNAs. The procedure is schematically shown in Fig. 3.1.

Though there is no fixed convention as to how much hybridization between two DNAs is necessary to assign two bacteria to the same taxonomic rank, 70% or greater degree of complementarity of the two DNAs is recommended for considering the two bacteria belonging to the same species.

By contrast, degree of at least 25% is required to argue that the two bacteria should reside in the same genus. Degree of complementarity to the range of 10% or less denotes that the two bacteria are more distantly related taxonomically.

DNA-DNA hybridization is a sensitive method for revealing subtle differences in the genes of two bacteria (other microbes also) and is therefore useful for differentiating closely related bacteria.

DNA homology studies have been conducted on more than 10,000 bacteria belonging to about 2,000 species and several hundred genera. It has proved to be a powerful tool in solving many problems of bacterial taxonomy, particularly at species level.

2. DNA-RNA hybridization:

DNA-RNA hybridization helps compare, unlike DNA-DNA hybridization, distantly related bacteria (microorganisms) using radioactive ribosomal RNA (rRNA) or transfer RNA (tRNA).

It becomes possible because the DNA segments (genes) transcribing rRNA and tRNA represent only a small portion of the total DNA genome and have not evolved as rapidly as most other genes encoding proteins (i.e., they are more conserved in comparison to genes encoding proteins).

Among the different rRNAs, the 16S rRNA of prokaryotes and the analogous 18S rRNA of eukaryotic organisms have been found to be most suitable for comparison of their sequences is taxonomic studies. One of the major impacts of rRNA studies on taxonomy, for convenience, is the recognition of three major domains—the Archaea, the Bacteria, and the Eukarya by Woese and Colleagues in 1990.

DNA-RNA hybridization technique is similar to that employed for DNA-DNA hybridization. The filter- bound non-radioactive ssDNA is incubated with radioactive rRNA, washed, and counted.

An even more accurate measurement of complementarity is obtained by finding the temperature required to dissociate and remove half the radioactive rRNA from the filter the higher this temperature, the stronger the DNA-rRNA complex and the more similar the base sequences.

However, DNA-RNA hybridization has been done with thousands of bacteria for relevation of their taxonomic relationships. Such studies were made with pure cultures of bacteria till 1997-98, but since then, techniques have been developed to recover rRNA genes directly from natural habitats. This has come to be called as community analysis of rRNA from natural bacterial community.

Nucleic acid sequencing:

Nucleic acid (DNA and RNA) sequencing is another molecular characteristic that helps directly compare the genomic structures. Most attention has been given to the sequencing of 5S and 16S rRNAs isolated from the 50S and 30S subunits of 70S prokaryotic ribosome, respectively.

As mentioned in DNA-RNA hybridization, rRNAs are almost ideal for the studies of bacterial (microbial) evolution and relatedness because:

(i) They are essential to ribosomes found in all bacteria,

(ii) Their functions are same in all ribosomes, and

(iii) Their structure changes very slowly with time, i.e., they are more conserved.

The procedure of rRNA sequencing involves the following steps:

(i) rRNA is isolated from the ribosome and purified,

(ii) Reverse transcriptase enzyme is used to make complementary DNA (cDNA) using primers that are complementary to conserved rRNA sequences,

(iii) The cDNA is amplified using polymerase chain reaction (PCR) and finally,

(iv) The cDNA is sequenced and the rRNA sequence deduced from the results.

Shotgun sequencing and other genome sequencing techniques have led to the characterization of many prokaryotic genomes (approximately 100) in a very short time and many more are in the process of being sequenced. Direct comparison of complete genome sequences undoubtedly will become an important tool in determining the classificatory categories of prokaryotes.

Ribotyping is a technique which measures the unique pattern that is generated when DNA from a bacterium (all other organisms also) is digested by a restriction enzyme and the fragments are separated and probed with a rRNA probe.

The technique does not involve nucleic acid sequencing. Ribotyping has proven useful for bacterial identification and has found many applications in clinical diagnostics and for the microbial analyses of food, water, and beverages.

Ribotyping is a rapid and specific method for bacterial identification it is so specific that it has been given the nickname ‘molecular fingerprinting’ because a unique series of bands appears for virtually any bacterium (any organism).

In ribotyping, first the DNA is isolated from a colony or liquid culture of the bacterium to be identified. Using polymerase chain reaction (PCR), genes of DNA for rRNA (preferably 16S rRNA) and related molecules are amplified, treated with one or more restriction enzymes, separated by electrophoresis, and then probed with rRNA genes.

The pattern generated from the fragments of DNA on the gel is then digitized and a computer used to make comparison of this pattern with patterns from other bacteria available from a database.

Comparison of proteins:

The amino acid sequences of proteins are direct reflections of mRNA sequences and therefore closely related to the structures of the genes coding for their synthesis. In the light of this, the comparisons of proteins from different bacteria prove very useful taxonomically.

Although there are many methods to compare proteins, the most direct approach is to determine the amino acid sequences of proteins with the same function.

When the sequences of proteins of the same functions in two bacteria are similar, the bacteria possessing them are considered to be closely related. However, the sequences of cytochromes and other electron transport proteins, histones, heat-sock proteins, and a variety of enzymes have been used in taxonomic studies.

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A taxonomy code is a unique 10-character code that designates your classification and specialization. You will use this code when applying for a National Provider Identifier, commonly referred to as an NPI.

Yes, if you want to become a Medicare provider.

To become a Medicare provider and file Medicare claims, you must first enroll in the Medicare program. To enroll, you must have an NPI. And to get an NPI, your application will need to include the taxonomy code that reflects your classification and specialization.

Note: Applications for NPIs are processed through the National Plan & Provider Enumeration System, or NPPES.

To find the taxonomy code that most closely describes your provider type, classification, or specialization, use the National Uniform Claim Committee (NUCC) code set list.
Note: You may select more than one code or code description when applying for an NPI, but you must indicate one of them as the primary code..

Note: You may select more than one code or code description when applying for an NPI, but you must indicate one of them as the primary code.

CMS has created a crosswalk of taxonomy codes that links the types of providers and suppliers who are eligible to apply for enrollment in the Medicare program with the appropriate Healthcare Provider Taxonomy Codes. View the complete data set on, where you can select various download formats to view the entire list.

The code set is published and released twice a year, in January and July.

If you need help identifying your taxonomy code, or have other questions about the enrollment process, please contact us.

What is Systematics

Systematics refers to the study and classification of organisms for the determination of the evolutionary relationship of organisms. Therefore, the systematics consists of both taxonomy and evolution. Systematics uses morphological, behavioral, genetics, and evolutionary relationships between organisms. By using these characteristic features, systematics describes an organism by means of classification, name, cladistics, and phylogenetics. Cladistics refers to the classification of organisms based on the branching of different lineages from a common ancestor. Phylogenetics refers to the study of the history of evolution and the relationship among groups of organisms. Phenetics refers to the characteristics of organisms excluding the phylogenetics. The relationships of the organisms are presented by phylogenetic trees. Both phylogenetics and phenetics are described in figure 2.

Figure 2: Phylogenetics and Phonetics

Taxonomy is one of the components of systematics. Therefore, during the description of organisms by systematics, the binomial nomenclature is also used. Furthermore, systematics identifies biological enemies of organisms that act as a biological control.

Any errors in OTT should be assumed to have been introduced by the Open Tree of Life project until confirmed as originating in the source taxonomy.

Download locations are for the particular versions used to construct OTT 3.0. For new work, current versions of these sources should be retrieved.

Curated additions from the Open Tree amendments-1 repository, commit 12c810f5. These taxa are added during OTU mapping using the curator application.

Taxonomy from: SILVA 16S ribosomal RNA database, version 115. See: Quast C, Pruesse E, Yilmaz P, Gerken J, Schweer T, Yarza P, Peplies J, Glöckner FO (2013) The SILVA ribosomal RNA gene database project: improved data processing and web-based tools. Nucleic Acids Research 41 (D1): D590-D596. Web site:
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Index Fungorum. Download location: derived from database query result files provided by Paul Kirk, 7 April 2014 (personal communication). Web site:
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Taxonomy from: Schäferhoff, B., Fleischmann, A., Fischer, E., Albach, D. C., Borsch, T., Heubl, G., and Müller, K. F. (2010). Towards resolving Lamiales relationships: insights from rapidly evolving chloroplast sequences. BMC evolutionary biology 10(1), 352.. Manually transcribed from the paper and converted to OTT format.
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World Register of Marine Species (WoRMS) - harvested from web site using web API over several days ending around 1 October 2015. Download location:

Taxon identifiers are carried over from OTT 3.0 when possible

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Taxonomy of Finfish

The study of organic diversity has changed its objectives and enlarged its scope in the course of history as it happens in any branch of science. Our knowledge of biodiversity is incomplete. Only 1.70 million of the earth’s estimated 10 - 100 million species have been scientifically erected, named and classified. In the marine biota, 340000 species are known including many unnamed species. It would be impossible to deal with the enormous diversity if it were not ordered and classified. Systematic zoology solves this problem and develop many methods and principles to make this task possible.

The systematic zoology is the science that discovers names, determines relationships, classifies and studies evolution of living organisms. It is an important branch in biology and is considered to be one of the major subdivisions of biology having a broader base than genetics, biochemistry and physiology. Systematics includes taxonomy and the term taxonomy is derived from the Greek word ‘taxis’ - arrangement and ‘nomos’ - law. The name taxonomy was first proposed by Candolle (1813). Taxonomy is defined as the theory and practice of classifying organisms. On the whole systematics is a synthesis of many kinds of knowledge, theory and method applied to all kinds of classification of organisms.

In taxonomy, the terminology classification overlaps with identification. The term identification and classification are often confused among taxonomists. The phraseology classification refers ordering of animals into groups on the basis of their relationship. The population or groups of population are classified at all levels of taxon. In the identification of a species, the individuals are placed by deductive procedure to each taxon.

Taxonomy is classified into three stages. They are ‘alpha taxonomy’ which emphasis only description of new species and its arrangement in comprehensive genera. In ‘beta taxonomy’ the relationships are worked out on the species level and on higher categories. In ‘gamma taxonomy’ emphasis is given to intra specific variations and its evolutionary relationship and casual interpretation of organic diversity.

Taxonomy of Finfish

Finfish taxonomy is the only subject in ichthyology which deals with populations, species and higher taxa. No other branch in fisheries science occupies itself in a similar manner with this level of integration in the organic world. The contribution of finfish taxonomy to fisheries science has been both direct and indirect. For conservation and management of our fishery resources the identification of finfishes is vital. Ichthyotaxonomical study reveals numerous interesting evolutionary phenomena in piscine phylogeny and the study is most indispensable for culturing fish fauna. The correct identification of a particular candidate finfish for aquaculture is very important for successful culture practices. On the whole taxonomic study on finfishes furnishes the urgently needed information about species and it cultivates a way of thinking and approaching of all biological problems which are much needed for the balance and well being of fish biology as a whole.

Pisces are the most numerous, highly diversified groups exhibiting enormous diversity in their morphology, in the habitats they occupy and in their biology. Fishes constitute almost half the total number of vertebrates (Nelson, 1976). According to Cochen (1970), the estimated number of fishes is about 20,000 - 22,000 and Nelson’s (1976) estimate is 18,818 living fishes. Fishes can be simply defined as aquatic poikilothermic vertebrates and have gills throughout their life span and limbs if any in the shape of fins. Most fishes fall into one of six broad categories. They are rover-predator, lie-in-wait predator, surface-oriented fish, bottom fish, deep bodied fish and eel like fish. Thus their ecological diversity is reflected in variety of body shapes and means of locomotion they possess. The modern living fishes could be broadly classified into two categories namely, elasmobranchs and teleosts.