Meristics
Encyclopedia
Meristics is an area of ichthyology
Ichthyology
Ichthyology is the branch of zoology devoted to the study of fish. This includes skeletal fish , cartilaginous fish , and jawless fish...

 which relates to counting quantitative features of fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...

, such as the number of fins or scales. A meristic (countable trait) can be used to describe a particular species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 of fish, or used to identify an unknown species. Meristic traits are often described in a shorthand notation called a meristic formula.

Meristic characters are the countable structures occurring in series (e.g. myomeres, vertebrae, fin rays) in fish. These characters are among the characters most commonly used for differentiation of species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 and populations. In the salmonids, scale counts have been most widely used for the differentiation of populations within species. In rainbow and steelhead trout
Trout
Trout is the name for a number of species of freshwater and saltwater fish belonging to the Salmoninae subfamily of the family Salmonidae. Salmon belong to the same family as trout. Most salmon species spend almost all their lives in salt water...

 the most notable differences among populations occur in counts of scales.

Meristic characters are used in many other fields, such as in botany
Botany
Botany, plant science, or plant biology is a branch of biology that involves the scientific study of plant life. Traditionally, botany also included the study of fungi, algae and viruses...

. Meristic comparison is used in phenetic and cladistic analysis.

Meristic analysis

A meristic study is an often difficult task, as counting the features of a fish is not as easy as it may appear. Much of meristic analyses are performed on dead fish that have been preserved
Fixation (histology)
In the fields of histology, pathology, and cell biology, fixation is a chemical process by which biological tissues are preserved from decay, thereby preventing autolysis or putrefaction...

 in alcohol. Meristic traits are less easily observed on living fish, though it is possible. On very small fish, a microscope may be required.

Ichthyologists follow a basic set of rules when performing a meristic analysis, to remove as much ambiguity as possible. The specific practice, however, may vary depending on the type of fish. The methodology for counting meristic traits should be described by the specialist who performs the analysis.

Meristic formula

This a shorthand method of describing the way the bones (rays) of a bony fish's fins are arranged, comparable to the floral formula for flowers.

The meristic formula of Siganus luridus (a rabbitfish with venom tipped bones in its fins found in the Eastern Mediterranean) is: D, XIV+10; A, VII+8-9; P, 16-17; V, I+3+I; GR, 18-22

This means the fish has 14 rays (bones) in the first part of its dorsal fin (D), followed by 10 in a second array. A means the Anal fin, P means Pectoral (fins near the gills and eyes where 'arms' would be), V are the ventral or belly fins (equivalent to 'legs') GR represents the tail fin and the number of rays in the top lobe (18) and the bottom lobe (22).

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