Observation

List out all morphometric and meristic features

MORPHOMETRIC AND MERISTIC CHARACTERS OF FRESHWATER FISHES USED FOR CLASSIFICATION AND IDENTIFICATION

The term, morphometry denotes the measurement of the structures and parts of organisms. The term, meristics denotes the counting of quantitative features of a fish, such as its number of fins or of scales.

I. MORPHOMETRIC MEASUREMENTS

Sharp pointed needle-like dividers or dial-reading calipers are used for taking body measurements. A Stainless steel ruler with measurements to millimeter is recommended for accurate readings.

The various body measurements of a fish are:

  1. TOTAL LENGTH

The greatest distance between the most anterior projecting part of the head (even if it is prolonged beak as in Hemiramphus) to the posterior most tip of the caudal fin, including filamentous prolongations, if any. The measurement is a straight line and is not taken over the curves of the body. Where the caudal fin has either of the lobes longer than the other, the maximum length is taken. Where the jaws are unequal, the mouth is closed and measured from the tip of the jaw that is longer.

  1. STANDARD LENGTH

The straight distance from the anterior most part of the head to the end of the vertebral column. In actual practice, the flexure line of the body over the caudal peduncle is taken as the posterior most point. Some authors however take this as mid of the caudal fin base.

  1. BODY DEPTH

The vertical measurement from a point in the body of the fish on its back where its height is the greatest to a straight line to the ventral surface or profile. The fleshy or scaly structures pertaining to fin bases are excluded. It need not necessarily be in middle of the fish.

  1. HEAD LENGTH

A Straight measurement of the distance from the tip of the snout to the most distant point on the opercular membrane on the upper angle of gill opening. Any fleshy membrane of the gill cover is also included.

  1. HEAD DEPTH OR HEIGHT OF HEAD AT OCCIPUT

The perpendicular distance measured from the midline at the occiput vertically downwards to the ventral contour of the head or the breast. In actual practice, one arm of the divider is placed at the occipital crest and the other arm below the head vertically opposite to the upper arm of the divider, to form a vertical lie. Unlike body depth this measurement is not taken up to ventral profile line.

  1. HEAD WIDTH

Straight distance across the head in a ventral position: if the opercles are dilated they are forced into a reasonably normal position.

  1. EYE DIAMETER

The distance between margins of the cartilaginous eyeball across the cornea.

  1. SNOUT LENGTH

The distance from the most anterior midpoint on the snout or upper lip to the front hard margin of the orbit.

  1. INTERORBITAL WIDTH

The least distance between the bony rims between inner margin of the eyes.

  1. PRE-DORSAL LENGTH

A straight measurement from the midpoint or tip of the snout or upper lip or the anterior most part of the head to the structural base of the anterior most dorsal fin ray. In fishes with a dorsal spine, the basal bone of the dorsal fin, also called the nuchal shield, is taken as the structural base.

  1. POST-DORSAL LENGTH

A straight line measurement from the structural base of the dorsal fin to the flexure line of the body over the caudal peduncle or the end of the vertebral column.

  1. PRE-PELVIC DISTANCE

A straight distance from the midpoint of the base of the pelvic fin to the anterior most point of head.

  1. HEIGHT OF DORSAL OR LENGTH OF DORSAL FIN SPINE

It is measured from the anterior point of junction with the body to anterior tip of the fin or spine even if the other rays do not reach this point.

  1. LENGTH OF BASE OF SPINOUS OR SOFT DORSAL ADIPOSE DORSAN ANAL FIN

The greatest distance measured in a straight line between the anterior most and posterior most points of junction with the body.

  1. LENGTH OF PECTORAL HEIGHT AND PELVIC FINS, DORSAL FIN OR SPINES

Measured between its origin or place of insertion into the body to its extreme tip.

  1. DEPTH OR LEAST HEIGHT OF CAUDAL PEDUNCLE

The least vertical distance from the dorsal to ventral profile at the narrowest part of the caudal peduncle. It is a straight measurement.

  1. LENGTH OF CAUDAL PEDUNCLE

An oblique measurement from the last point of contact of anal fin posteriorly to the end of the vertebral column or the flexure line of the body over the caudal peduncle.

  1. LENGTH OF LONGEST FIN RAY

Measured from the structural base of the longest fin ray to its tip.

19.LENGTH OF OCCIPITAL PROCESS

The distance from the point of origin or base of the occipital process on the supra occipital bone to its posterior most tip where it meets the basal bone of the dorsal fin, the latter is excluded. Where it meets the inter-neural shield, the shield is excluded.

  1. POST-ORBITAL LENGTH

The greatest distance from the posterior edge of orbit to the posterior tip of the fleshy operculum.

  1. SUB-ORBITAL WIDTH

The least distance from the lowermost margin of the orbit to sub-orbital or pre-orbital margin.

  1. WIDTH OF GAPE OF MOUTH

The greatest transverse distance across the opening of the mouth, without stretching the mouth opening.

  1. WIDTH OF PREMAXILLARY TEETH BAND

The maximum width of the band

  1. WIDTH OF BASE OF OCCIPITAL PROCESS

The maximum width at base of occipital process where it originates from the supra-occipital bone.

II. MERISTIC COUNTS

The accurate enumeration of meristic data or counts of fin rays is of diagnostic importance. Great care should be taken to count accurately the simple and branched rays and present the data in charts of frequency distribution wherever possible.

  1. FN RAYS

All paired and median fins in bony fishes have long, mobile filament like prolongations called rays. The movements of the fins are due to the action of the muscles, these movements being possible of the articulations and often flexibility of these rays. The term “ray” also applies to spines whether they are included within the membrane of a fin or not.

Three chief types of fin rays are encountered:

(i) SOFT RAY – Are often very thin, flexible and most often branched.

(ii) HARD RAYS – Are a number of soft rays united solidly to compose hard rays which are rigid and sometimes sharply pointed. These are formed of cartilage, and when freed from the membranous sheath holding them, clearly show the juxtaposed soft rays. Some of perches and sisorids have such hard rays. The outermost caudal fin ray called the principal ray is also a hard ray are often preceded by hard rays.

(iii) SPINOUS RAYS – Are made of bone tissues, and are harder and stronger than rays. These are usually not covered by the membrane of the fin and often are with serrations or teeth. They are common in catfishes. These are cited in the fin formulae by capital roman numerals (I, II, III etc). Hard and soft rays can be distinguished easily. Soft rays are so constructed that they can be readily split into right and left in the median. Hard rays on the other hand do not have such cleavage flexure.

  1. SIMPLE RAY

Is either soft or hard, flexible but without any branching at its tip or elsewhere; in the anal fine often these rays remain within the membranous covering and should be teased out. These are indicated in the fin formulae by lower-case roman numerals (i, ii, iii etc). The rays of a fin are not at all uniform. Hard rays and branched soft rays or un-branched and branched soft rays may be found together in two fins one following the other as in perches.

  1. BRANCHED RAY

Branches either from the base or middle or tip of the ray. These are stated in the fin formula by Arabic numerals.

  1. RUDIMENTARY RAY

Is an undeveloped ray grading into different stages of a developed one. If included in the count, it should be specifically mentioned. Generally it is excluded.

  1. CAUDALFIN RAYS

Are counted from the outermost to the inner ones including the smallest one at the inner end of the fin base. A good magnification is often needed for this count.

  1. COUNTING FIN RAYS

In the case of the rayed dorsal fin, the posterior most ray is taken as the first ray and the count is done towards the anterior direction. In the case of anal fin the direction is from anterior to posterior.

  1. LATERAL LINE SCALE COUNTS

The number of scales with perforation along the lateral line up to the structural caudal base or end of hypural plate or at the flexure line of the body over the caudal peduncle. The scales wholly on the caudal fin base or beyond this point without such perforations are excluded. If the lateral line is divided as in loaches, cichlids and anabantids, the lateral line count includes all the pored scales in the upper and lower line: only the count on the lower line starts from the scale following that directly below the last one on the upper line. Indicated as L.L. or L.I. in fin formulae. If the lateral line is incomplete or absent, or if all the scales do not have pores or perforations, the scales themselves may be counted along the normal course of line. Near the had the first scale to b counted is one which touches the pectoral girdle and which is followed by a scale which does not touch it. Transverse scale counts are taken as scale rows crossing an imaginary vertical line. Scales from the anterior base of fins are generally taken. The number and nature of the striae and pores on the scales are important.

  1. SCALES FROM DORSAL FIN TO LATERAL LINE

Generally the count is taken from the insertion of the first dorsal fin including the small scales, counting downward and backward following the natural scale row to but not including the lateral line itself. The small scales at base of the fin are however included.

  1. SCALES BELOW LATERAL LINE OR FROM LATERAL LINE TO PELVIC BASE

Are counted as above but from below upwards and forward from base of pelvic to lateral line. Small scales are included. The scale nearest the pelvic fin in counted as half when, it is so. When counting upward and forward, if the scale rows are found wavy backward and forwards, the backward rows are taken.

  1. PREDORSAL SCALES

The number of scales from the origin of the dorsal fin to the occiput. All scales which wholly or partly intercept the straight midline running from the origin of the dorsal fin to occiput is taken, Indicated as P.D.S in fin formulae.

  1. PREANAL SCALES

Counted from the anterior most base of the anal fin to the vertical base of the occiput or end of the scale rows on the head.

  1. SCALES IN LATERAL OR LINEAR SERIES

Where the lateral line is absent, the number of transverse rows of scales between the occiput and the structural base of the caudal fin is counted. Indicated as L.r. in fin formulae.

  1. CIRCUMPEDUNCULAR SCALES

Scales around the least depth of caudal peduncle. It is always better and useful to indicate the number of circumpeduncular scales above and below the lateral line.

  1. BRANCHIOSTEGAL RAYS

Numerous tiny tin bones arranged fanwise from the lower edge of the opercle to the ventral surface of the head and covered by the branchiostegal membrane. Care should be taken to include the most anterior branchiostegals which are likely to be very short, slender and hidden.

  1. GILL RAKERS

Thin needle like filamentous prolongations on the gill arches. For count purposes, the rakes on the first arch is taken. If the counts of both the upper and lower limbs of the gill arch are taken they are separated by a plus (+) sign; generally the lower limb alone is considered. If rudiments are included this fact should also be mentioned.

  1. PHARYNGEAL TEETH

“Throat teeth” borne on two bones which are modified fifth gill arches, each with one to three rows of teeth. The in each row are counted and are indicated in formula in order from left to right. For example 2,5 – 4,2 would indicate that the pharyngeal bone of the left side has two teeth in the outer row, five in the inner whereas the right bone has only four teeth in the inner row and two in the outer. The formula 3-3 would show that no teeth are developed in the outer row. The number and series of pharyngeal teeth are of classificatory value.

Last modified: Thursday, 28 June 2012, 12:00 PM