Taxonomic key

 Taxonomic  key is a device by which each specimen in a group of specimens may be identified. This can be achieved by presenting diagnostic characters in a series of alternative choices. These keys are based on similarities and dissimilarities in characters, generally in a pair called couplet. Each statement in a taxonomic key is referred to as a lead. For categorizing each taxonomic rank, such as family, genus, species, etc., different keys are used. It is also useful in identification of unknown organisms.

The use of keys in identification is old indeed. The classification of animals by Aristotle was usually presented in the form of simple dichotomous alternatives e.g., “bloodless versus with blood” etc. Although the term key was developed by John Ray, Jean Baptiste Lamarck, is the person who developed the dichotomous key. Metcalf (1954) provides some hints for constructing keys.

The construction of keys is a laborious and time-consuming task, involving the selection and shifting of the most useful and clearly diagnostic characters. Ideal key characters apply equally to all individuals of the population irrespective of their age and sex. They are external and can be observed directly and without special equipment.

A good key is strictly dichotomous, not offering more than two alternatives at any point.

Dichotomous keys- choice is made only between 2 alternate options called couplet are called dichotomous keys • Contrasting characters are chosen which divide the full set of sps into smaller groups • Each time number of sps are eliminated so that the choice is narrowed down to the correct sps Dichotomous- Greek origin: dich- two; temnein- to cut • Dichotomous keys- choice is made only between 2 alternate couplet are called dichotomous keys

1.Bracket Key: The most commonly used key by the taxonomists is the bracket key. This key has the advantage that the couplets are composed of alternatives which are side by side for ready comparison. It is also economical in space. It may be run forward or backward with equal facility by following the numbers which indicate the path that the various choices follow. The numbers in brackets are responsible for running backward

Advantage: When properly constructed one can quickly and easily run through this key both forwardand backward. It is also more economical in terms of space because it is un-indented.

Disadvantage: The main disadvantage in both simple bracket and simple non bracket key types is that the relationship of the divisions is not apparent to the eye like indented key.

2. Indented key/ yoked key- indents the leads of the couplet a equal distance from the left margin • The two choices are labelled 1 and1’ or 1a and 1b . It has the advantage that the relationship of the various divisions is apparent to the eye.

Advantage: This type of key is advantageous in the sense that the relationship of various divisions is quite apparent to the eye and can be used in reverse also. It serves good purpose for keys to higher taxa or comparative keys.

Disadvantage: When the key is short, it works alright. But in long keys, the alternatives get widely separated and take more space. Moreover this can lead to much white space in print, and consequently little remaining room for lead text and illustrations.

3.Multi access key: A  multi-access key is an identification key which overcomes the problem of the more traditional single-access keys (dichotomous or polytomous identification keys) of requiring a fixed sequence of identification steps.

A multi-access key enables the user to freely choose the characteristics that are convenient to evaluate for the item to be identified. It is relatively a new alternative to dichotomous keys and becoming increasingly popular, especially because of the ease of computerizing them, is termed multiple access or poly clave or synoptic key. The advantage of these keys is that they allow the user to enter the key at any point. This key is based on the identification of organisms by a process of elimination. In a written poly clave key, there is a series of characters and character states. Each state is followed by a number or code for the species that possess that feature. The user needs to select any character and then copy down the list of species that possess the feature. Then the user has to select another character and eliminate any species that is not common to both lists. This process has to be continued until the specimen is identified.

4.Computer aided Interactive Key: Multi-access keys are more commonly used as computer-aided, interactive keys. In recent years, computer technology has been used for constructing keys. Figures , photographs etc can be fed into computer with the help of appropriate programmes. It is much more easier and less time consuming to compare the characters of unknown taxa with known taxa.Scanners of computer can observe and record the characters of undetermined taxa and compare these with those of already known • DELTA( Description Language for Taxonomy) is a useful system of programmes for recording taxonomic descriptions for processing by computer. • It generates conventional identification keys.

 1.      A.  Asymmetrical.porous body, usually soft to touch . . . . . .Porifera

B.  with symmetry, and has definite structures ………………go to 2

 2.      A. Radial symmetry. . . . . . .  . . . . . .  . . ………….. . . . .  . . . .. go to 3

B. Bilateral symmetry or worm-like, or has a shell  . . …….. . . go to 4

 3.      A. Body soft, may have stinging tentacles………………………Cnidaria

B. Body hard and spiny……………………………………….Echinodermata

 4.    A. 3 or more pairs of appendages, or no appendages ……  go to 5

B. 2 pairs of appendages, most have backbone………. . . . . . . . . . . . . . . .Chordata

 5.            A. Body with jointed appendages & exoskeleton  . .. . . . . . . . . . Arthropoda

B.     Body without jointed appendages . . .. . . . . . . .  . . . . . . . . . . .  . . . . go to 6

 6.     A. Body soft, flattened & worm-like . . . . . . . . . . . . . . . . . . . . . . . .  Platyhelminthes

B. Body otherwise . . . . . .. . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . .. . . . . . go to 7

 7.     A. Body with noticeable segments (rings), worm-like . . . . . . . . . . . . . . . . .Annelida

B. Body not obviously segmented . . . . . . . . . . . . . . . . . . . . .  . . . . . . . . . . . . . go to 8

 8.       A. Body long, slender, and smooth, worm-like . . . .. . . .  . . …. . . . . Nematoda

B. Body enclosed in shell &/or with tentacles……....Mollusca