Meristic refers to the countable features of fish that are useful in\identifying them. Different species of fish have varying numbersof fin rays, scales, barbells, vertebrae and other features that can be used for their identification. The meristics are discrete variables,which mean that there are either 4 or 5 dorsal fin spines not 4.5.Meristic traits are often described in a shorthand notation calleda meristic formula. Meristics are often combined with details onanatomy and morphometrics. Meristic characteristics of a fish canbe compared to those listed in key reference books, descriptionsand in dichotomous keys to help us work out which species we arelooking at.A meristic study is an often difficult task, as counting the featuresof a fish is not easy. Much of the meristic analyses are performedon dead fish that have been preserved in alcohol or formalin. These
traits are less easily observed on live fish, though it is possible. Onvery small fish, a microscope may be required.The meristic counts commonly used in fish identification andtaxonomic studies are fin rays count, lateral line scales count,vertebral count, branchiostegal rays count, pyloric caeca count,gill rackers count, pharyngeal teeth count, etc.
a. Counting fin rays
Counting fin rays is a shorthand method of describing the numberand way of arrangement of the fin rays of a bony fish fin, whichis comparable to the floral formula of flowers. As described inabove section, fin rays are of two main types: hard and soft rays.All of the hard and soft rays in all fins are counted separately(Figure 4.19), including the tiny ones on the inside of the pelvicfins, which may not easy to detect on a live fish. When makingcounts it is important to examine the base of each fin where eachfin rays inserts into the body. Since soft-rays are often branched,examining them at the tips would result in two or more counts fora single ray. The last ray of dorsal and anal fins in most fishes isusually divided to the base and may look like two rays, but it shouldbe counted as a single ray. As with most things in Ichthyology, finray counts are written in a strict formula known as fin formula.Different fish species have their own fin formula. Below is a guidefor expressing fin formula.
dorsal fin formula of this fin can be written as D: I, 8 or D 9 (1/8).
- C. If a fish have two dorsal fins, it is denoted by D1 and D2.
2. Hard rays or spines are generally written using Roman numerals
and soft rays are written using Arabic numbers. For example,
D: V, 8 - is a fish that has 5 dorsal spines followed by 8 soft raysP : 7 - is a fish which has 7 pectoral rays and no spineSometimes an oblique stroke (/) is used to separate two types offin rays instead of Roman numerals. For example,D 16 (3/13) - is a fish that has 3 dorsal spines followed by 13 softraysA 7 (2/5) - is a fish that has 2 anal fin spines and 5 soft raysSometimes a vertical stroke (|) is used toseparates different fins,such as rayed dorsal from the adipose dorsal or the spinyfinfromthnonspiny one. For example, the fin formula for dorsal fin inthe Silurid fish may be D 1/7|1, i.e., it has 1 spine, 7 soft rays and1 adipose fin.3. A complete fin formula of a fish (e.g. Labeo rohita) is written as:D: III, 13; P: 17; V: 9; A: II, 5; C: 19orD 16 (3/13); P 17; V 9; A 7 (2/5); C 19This means theLabeorohitahas a dorsal fin with 3 spines and 13soft rays, pectoral fin with 17 soft rays, pelvic fin with 9 soft rays,
anal fin with 2 spines and 5 soft rays and caudal fin with 19 softrays.4. Since there areslightvariations in fin rays between members ofthe same species, the numbers of fin rays are generally provided inarange. For example, D: 10-13, A: 10-14, P: 12-15.
b. Counting lateral line scales
Lateral line is a row of perforated scales between the angle of gillopening and the base of caudal fin. Lateral line scales are countedalong the lateral line (distinguished by pores on scales) startingfrom the scale just behind the upper end of the gill opening to thebase of caudal fin and their number is shown after the abbreviationL1 (Figure 4.20). However, not all fish have a lateral line, and evenif present, it is often only partially complete. In these cases, we cansimply count the scales along an imaginary line where the lateral
line would be normally present.Other scale counts include counting the lateral line transverse
scales, counting the number of pre-dorsal or post-dorsal scalesand counting the number of scales around the narrowest part ofthe caudal peduncle. Lateral line transverse scales are generallydivided into two parts, scales above lateral line and scales belowlateral line. The former includes counting the scales from the
anterior base of the first dorsal fin to the lateral line and the laterincludes counting the scales below the lateral line to the anteriorbase of the pelvic fins (or anal fins) (Figure 4.20). Their number isshown after the abbreviation L.tr. The scales above and below thelateral line are generally separated by an oblique (/). For example,if a species have a lateral line scales count of 40, scales abovelateral line are 8 and scales below lateral line are 9, the lateral lineformula is written as L1 40; L. tr. 8/9. The total count of transverse
scales can be equal to “count of scales above lateral lines” + “countof scales below lateral line” + 1 (lateral line scale), i.e. 8+9+1 = 18.Thus, the lateral line formula can also be written as L1 40; L. tr. 18.Conducting counts of scales can be quite easy or very difficultdepending on the species being examined. Some fish have
relatively large and firmly attached scales, which make countingvery easy. Others have very small scales that may be covered with mucous, or have scales that are very deciduous.
c. Counting vertebrae
For systematic studies, it is often necessary to collect data onvertebral counts, which can vary between and within species.These counts are often made from radiographs (X-rays), butcounts are sometimes made from cleared and stained specimensand dried skeletons. The typical hypural plate of most teleosts iscounted as a single vertebra. However, definite suture along thevertebral axis are regarded as separating vertebrae, even thoughthe sutures lie within the hypural complex.
d. Counting branchiostegal rays
Branchiostegals are the bony rays supporting the gill membranesbehind the lower jaw. They are mostly found in Elasmobranchii,but also in some teleosts such as salmonids. When makingcounts, it is often desirable to separate by a plus sign (+) thosebranchiostegal rays which lie on the outer side of the hyoid arch
from those that are inserted more interiorly and more ventrally onthe inner face of the arch. The anterior rays may be small and,especially in laterally compressed fishes, several of them may be
bound tightly together, necessitating the use of a sharp probeto locate each one.
e. Counting pyloric caeca
Pyloric caeca are the finger like projections located near the junction of the stomach and the intestine. In counting pyloric caeca, all tips are enumerated unless the condition of branching isspecifically described.
f. Counting gill rackers
Gill rackers are cartilaginous structures situated along the anterior side of the gill arch in the opposite direction of gill filaments, which is covered by operculum. Gill rackers are generally counted
on the first gill arch. The gill arch is divided into the upper and lower arms (hemibranch), and gill rackers are counted from top to bottom. There is frequently one gill racker at the junction between
the two arms; this gill racker is included in the count of gill rackers on the lower arm.
where is that figure 4.19
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