What are Fish Meristic Characteristics and the Basic Rules of Meristic Features?

June 14, 2023 Agric4Profits

These are internal and external features of a fish such as fins, gill, and mouth and teeth structure. Meristics are countable features that are useful to scientists and fish keepers in identifying their fish. Different species of fish have varying numbers of fins, scales, fin rays, barbells and other
features that can be used to help tell them apart.

These characteristics can be compared to those listed in books, descriptions and in dichotomous keys to help you work out which species you are looking at. Meristics are often combined with details on anatomy and measurements of certain features called morphometrics.

The importance of fish identification cannot be overemphasised in the study of fish and for taxonomic purposes. The basic rule must be followed in order to represent a desired class or species of fish correctly by practically use of the identification tools and not by mere assumption
on physical appearance or at sight.

This is because it is a known fact that more than one species of fish may have similar features in certain respects but which are still yet not in the same species and even not in the same class.

Basic Rules of Meristic Features

At their most basic, meristics are as simple as counting fins, fin rays or scales. However, there are lots of different features to count and there are slightly different ways of counting the same things, and expressing them in a way so other people can understand what you are going on about.

The first rule with meristic features is that the number is always whole (a discrete variable). Just as most people don’t usually have 2.25 arms or 9.2 fingers, most fish have whole numbers of fins, rays, scales and other anatomical features, not parts of them.

1. Internal Features

This is basically the use of gill raker. Gill raker is a comb-like structure usually found inside the gill arches of a fish. The number and shape of gill raker is the important tool in this respect as it varies from one species of fish to another.

The gill raker performs several functions ranging from protection of the fragile gill filaments to aiding filtering of minute food items from the water as it passes through them.

In which case, those fish species with numerous and fine gill rakers feed on fine food particles like planktons and are known to be microphagous while those with stronger and fewer gill rakers feed on large food items and are referred to as macrophagous.

However, the gill rakers are protected by the opercula plate, a hard disc-like plate that is a continuum of the rest of the fish body.

2. External Features

These are found and could be outside on the fish like the fins, mouth, teeth, scales, body shape and the colour of fish.

(i) Fins

These are very important for fish identification and requires taking into considerations their number, sizes, position on the body and a times colour. The number of spines or rays may be branched or not, but the number of spines on the dorsal fins and anal fin is the most relevant and consistent features in a species since two different fish species are most unlikely to have the same number of fins or rays.

For data recording purposes, Roman numerals are adopted for spine count. An example of this method is represented by the dorsal fin of Tilapia zillii D XIV-XVI, 11-13; A III,7-10. This means that T. zillii has dorsal with 14-16 spines and between 11-13 rays, while the anal fin has 3 spines and between 7-10 rays.

Meanwhile some fish species have two dorsal fins of which the first (anterior) is truly a fin with spines and rays while the second (posterior) is in the form of flesh called adipose fin. This adipose fin usually vary in sizes from one fish species to another even of the same genera and thus, used sometimes in fish identification.

The adipose fin is a soft, fleshy fin found on the back, behind the dorsal fin and just forward of the caudal fin. It is absent in many fish families, but is found in Salmonidae and catfishes.

Tail or caudal fin is another very important fin used in identifying fish species. The shape of caudal fin could be lobed, that is forked, having upper and lower lobes, it could be pointed, blunt or truncated and it could be rounded. Examples of species with the different types of caudal fin shapes are shown below.

(a) Why is counting features so difficult?

Some people interpret things differently, so most experts follow the same basic rules on what constitutes a particular type of fin ray and how scales are counted.

Some specialists on certain groups of fishes do their meristic analyses in slightly different ways. If you’re reading a description of a new species or a review of a family, you’ll often see in the methodology section of the paper detail on whose technique they are following.

For cichlids, for example, most people use Barel’s recommendations, which were written for those working on Lake Victoria haplochromine cichlids – a notoriously difficult group to work on.

(b) How do you actually do the counting?

Meristic studies are usually undertaken by taxonomists working on dead fish that have been pickled in alcohol. Counting the number of scales along the flank of a live Tinfoil barb is a rather trickier proposition.

If you are very patient and the fish you are looking at is fairly large and slow moving, you might be able to get reasonable results by watching the fish in the tank, but it might be easier to take some photographs instead. If you do this, it’s wise to use only pictures of the same
individual – some numbers might differ between members of the same species.

Some fish are too small to study, unless you are really skillful and have access to a microscope. In the lab, most scientists working on typical aquarium-sized fish use a dissecting microscope and manipulate the fins with a pointed needle so they can count accurately.

If the fish is small, you will have your work cut out, or you will have to wait until a fish dies and you can use the carcass.

(c) How do I count fin rays?

Fin rays fall into three main types: spinous (spines), soft and hard rays, and each is counted separately. This means that you need to be able to tell the difference between the two, and fin rays can differ a bit in their overall structure.

Soft rays which are thin, flimsy and usually branched at the top end. The group includes branched rays, caudal fin rays (apart from the principal rays at the top and bottom of the tail).

Hard rays are made from a group of soft rays, are rigid and sometimes pointed. Hard rays are seen in the fins of Perciform fishes, Sisorid catfishes and in the principal rays of the caudal fin.

Spinous rays are harder than hard rays and are made of bony tissue, so they’re stronger and often sharper. These aren’t usually covered with skin tissue and often have serrated edges, such as in many Doradid catfishes.

All of the rays in all of the fins are counted, including the tiny ones on the inside of the pelvic fins, which is not easy on a live fish. For instance D XII, 10-12 is a fin formula for the dorsal fin. To save repetition, ichthyologists use this special shorthand language for describing the fins.

The first letter stands for the fin, so D here stands for dorsal. Other letters, such as A for anal and P for pelvic, are also used. Roman numerals, such as XII, refer to the number of spinous rays in the fin, while the numerals, 10-12 in this example, describe the number of soft
segmented rays.

The range covered, 10-12 here, shows the typical counts seen across the species, something called intraspecific variation. Importantly, not all members of the same species are identical.

Sometimes you might see lower-case Roman numerals used in the same formula alongside Arabic numerals, for example, Pii, 5. These differentiate between branched and unbranched rays.

The lower-case Roman numerals refer to the unbranched rays and the Arabic numerals refer to the branched rays. The rays of the caudal fin or tail are counted by adding up the number of branched rays and adding two, one for the upper and lower principal rays.

Sometimes the count is split into two bunches of rays, the dorsal group on the upper half and the ventral group on the lower half. So principal caudal rays 6 + 7 states that there are 13 rays, 6 on the dorsal portion and 7 on the ventral portion.

Some fish, like Perch or Polypterids, may have more than one dorsal fin, while others have a dorsal which is divided.

If the fin is divided, a slash shows the position of the split e.g. DX/I; 8-10 shows a dorsal fin that’s split into two after the tenth spine. If there are several dorsal fins, each gets a separate number and formula.

(ii) Scales

Some fish are scaly while others are without scales, but the scaly fish are in the majority. Presence of scale in fish is a form of mechanical or external support in fish and also a form of beautification to the species having it usually in different patterns.

Scales are of different types and these are cycloid scale (rounded shape), ctenoid (comb-like), ganoid (rhomboidal), placoid (tooth-like) and scute.

(a) Cycloid Scale

They are small oval-shaped scales with growth rings cyclic in shape at the exposed margins and this gives fish species having it smooth body surface. Hepsetus odoe, Heterotis niloticus and the cichlid family such as Oreochromis niloticus have cycloid type of scale.

(b) Ctenoid Scale

They are similar to the Cycloid scales, with growth rings. They are distinguished by the spines that cover one edge at the posterior surface of the scale making it to be rough to touch. This type of scale is found in Ctenopoma kingsleyae.

(c) Ganoid Scale

This is rhombic or diamond-shaped. They are flat, basal-looking scales that cover a fish body with little overlapping. Example of fish species with ganoid scale is Polypterus spp.

(d) Placoid Scale

It is also called dermal denticles. The shape of a placoid scale is tooth-like with the posterior margin carrying a small cusp. Shark belongs to fish species having such type of scale.

(e) Scute

Another, less common, type of scale is the scute, which is an external shield-like bony plate, or a modified, thickened scale that often is keeled or spiny, or a projecting, modified (rough and strongly ridged) scale, usually associated with the lateral line, or on the caudal peduncle
forming caudal keels, or along the ventral profile. Some fish (e.g. pinecone fish) are completely or partially covered in scutes.

The various types of scale in fish are presented in this unit. However in some instances the number of scales on the caudal peduncle and on lateral lines of fish is used in identifying fish.

(a) How do I count the scales?

Fortunately, you would not have to count all of the scales, only those in specific places.

The most common scale count looks at the number of scales along the lateral line-the sensory line on the flank of the fish. This normally looks at the number of scales on the lateral line which have a pore in them. The scales on the caudal peduncle (wrist of the tail) after the lateral line do not get included in the count.

But not all fish have a lateral line, and even if present, it is often only partially complete. In these cases, you simply count along an imaginary line.

Other common scale counts include counting the scales below the lateral line to the base of the pelvic, counting the scales from the insertion of the first dorsal fin to the scale above the first lateral line scale; recording the number of pre-dorsal scales and the number of scales around the narrowest part of the caudal peduncle-here (the circumpeduncular count) you’d also indicate how many lie above and below the lateral line

(iii) Mouth

Mouth of fish can also be used in their identification and this is based on the variations in their structures and position. However, this form of variations is associated with the feeding habit of fish and the mouth could be regarded as being superior, inferior or terminal. Certain fish species also have a flesh-like structure attached to the mouth making it to be protrusible which may be protracted or retracted.

(a) Superior Mouth

Mouth of fish is said to be superior if it is positioned above the snout or situated directly upward. Such fish species like Epiplatys sexfasciatus and Cyprinodonts spp. feeds on insect that fall onto water or other surface water organisms such as mosquito larvae and pupae. They are said to be insectivorous or larvivorous.

(b) Inferior Mouth

The mouth of fish is inferior if it is positioned below the tip of the snout or situated underneath. Fish species with this type of mouthpart are usually bottom dwellers and therefore, feed on detritus, worms and algae attached to the bottom platforms. Clarias gariepinus, Synodontis spp. and Petrocephalus bane ansorgei possess inferior mouth and are known to be omnivorous.

A terminal mouth in fish is the one located at the tip or at the extreme end of the snout. This type is seen present in Hydrocynus brevis, Hepsetus odoe and Tilapia spp. The feeding habits of species with terminal mouth type may be carnivorous (predatory) or planktivorous.

(d) Protrusible Mouth

This is a structural characteristic of any particular mouth type of a fish. It is such that it can be extended or pushed forward and outward or fold backward and inward especially during feeding apart from opening and closing the mouth during ingestion. Fish species with such trait are Lates niloticus and Tilapia spp.

(iv) Lateral Line

The lateral line is a set of continuous mark or simple dots that run along the flank of a fish. These dotted lines are used in sensing moving objects in water such as predators or prey through vibrations detection since they are connected to the brain.

The lateral line may be the type that runs through the flank of fish in a single line (continuous) or may break along the flank only for another line to begin at another level and run to the end of the flank in an assumed continuation.

This point of line termination (synapse) is however taken into consideration by a little
overlap by the second line towards ensuring continuous message transmission. Fish species such as Protopterus annectens has continuous lateral line while Barbus callipterus has discontinuous lateral line.

Situations can also arise whereby fish possess other forms of lateral lines apart from the major ones and these are known as secondary lateral lines. This type is common to the Clarias spp. and they may be regular or irregular in distribution.

(v) Teeth

It is another very important tool in fish classification. Generally, fish are said to be homodont in dentition (having the same type or shape of teeth all through) on individual fish species basis.

However, they may differ in the number of cusps they have which may be unicuspid or bicuspid
e.t.c. or they may be of numerous granular structures forming a rough surface in the mouth of the fish. The most common forms of teeth in fish are canine, incisor, molar and villi or granule.

Apart from differences in the shape of fish teeth, teeth of fish are also positioned in different ways
such as;

(a) Premaxillary

This refers to the presence of teeth in front margin of the upper jaw.

(b) Maxillary

This means that teeth are present on the sides of the upper jaws of the fish.

This means that teeth are present on the sides of the lower jaw of the fish.

(d) Vomerine

This refers to the presence of teeth on the pharyngeal bones at the back of the mouth on the ventral and dorsal plates.

3. Biometrics Characteristics

This aspect involves taken measurements of certain parts of the body of fish for species identification purpose. This may involve counting the number of certain parts such as fins, which may be dorsal fin or anal fin and the total length. It may be the distance between one fin or the other, or distance between one parts of the fish in relation to another.

It may be depth of the body of the fish or percentage size or ratio of the head of a fish to the entire body and so on. This is used in some cases as such variations have been known to be species-specific. Terms adopted in this regards include total length, fork length, standard length, length of the head e.t.c.