Classification of Fish (pisces) – Systematic Classification and Based on feeding habit, habitat and manner of reproduction

A fish is an aquatic vertebrate which is mostly characterized by the presence of gills throughout life, a streamlined body, and paired as well as unpaired fins which helps in swimming. It is the group that comprises jawless forms like hagfish and lampreys, cartilaginous fishes like sharks and rays, and the bony fishes which is the largest group. It is the process where most fishes are cold-blooded (ectothermic) because the body temperature is controlled by the surrounding water. Some fishes like tuna show a higher internal temperature because of continuous activity.

Fish is traditionally placed under the Class Pisces, but this is not used in modern taxonomy because it is not a monophyletic group. This is the condition where the group includes the aquatic ancestors of vertebrates but excludes the land vertebrates (tetrapods), so the class becomes paraphyletic. Most living fishes are bony fishes under the group Actinopterygii and among these, teleosts form the major population.

It is the observation that fishes are distributed in almost all types of aquatic habitats. These are found in freshwater rivers, lakes, high altitudes, estuaries, seas, polar waters, and even in deep oceanic zones. The scientific study of fishes is referred to as ichthyology.

General Characteristics of fish

• Fish are aquatic vertebrates living in freshwater and marine habitats.
• The body is usually streamlined for easy swimming in water.
• It is covered by scales which can be cycloid, ctenoid, ganoid or placoid.
• Fins are present for movement and balance. These are pectoral, pelvic, dorsal, anal and caudal fins.
• Respiration is carried out by gills which is present on both sides of the pharynx.
• The heart is two chambered (one atrium and one ventricle).
• Most fish are cold-blooded, so the body temperature changes with the surrounding water.
• It is the process of buoyancy adjustment that is controlled by the swim bladder in bony fishes.
• Excretion is performed by mesonephric kidneys.
• Fertilization is mostly external, but in some fishes internal fertilization is also seen.
• The body shows bilateral symmetry.
• Fish possess a lateral line system which help in detecting vibrations and water movements.
• Reproduction usually produces a larval stage after hatching.
• These are highly diverse and found in almost all aquatic environments.

Position of Fishes in Phylum Chordata

Fish are placed under the Phylum Chordata. It is the phylum where organisms show notochord, dorsal tubular nerve cord, gill slits, post-anal tail and ventral heart. These basic characters are present in the early stage or throughout life.

Subphylum
The majority of fishes are included in the Subphylum Vertebrata. It is the group where the vertebral column is formed around the nerve cord.

Jawless fishes like lampreys are true vertebrates, but hagfishes are kept under Craniata because the vertebral column is absent and the notochord is persistent.

Infraphylum Gnathostomata

Most of the fishes are jawed forms. It is the group where jaws are developed from the modified visceral arches.

Class Chondrichthyes

These are cartilaginous fishes. The skeleton is made of cartilage. Sharks, rays and chimaeras are included.

Superclass Osteichthyes

These are the bony fishes. It is the group where endoskeleton is mostly bony.

– Class Actinopterygii
These are ray-finned fishes. The fins are supported by bony rays. It is the largest group among fishes and teleosts form the major portion.

– Class Sarcopterygii
These are lobe-finned fishes. The fins are fleshy and supported by internal bones. This group is important because tetrapods have originated from these fishes.

Paraphyletic Nature

The term “Pisces’’ is not accepted in modern classification. It is considered paraphyletic because tetrapods are excluded although they have originated from bony fishes. Hence the traditional group of fishes does not represent a complete lineage under cladistic rules.

History of Classification of Fishes

Early Classification

The study of fishes was first described in the writings of Aristotle. It is the period where aquatic animals were grouped mainly by external characters. Later the scientific base was given by Peter Artedi whose system was used in Linnaeus Systema Naturae. In this system all gill-breathing aquatic forms were included under Class Pisces. The grouping depended mainly on body covering, fins and skeletal nature.

Traditional System

It is the system where fishes were divided into three groups. These are Placodermi, Chondrichthyes and Osteichthyes. Placoderms were extinct armoured fishes. Cartilaginous fishes included sharks and rays. Bony fishes formed the largest group. Teleosts were first recognized as a separate group by Müller. But later studies showed that the characters used earlier were not sufficient for fossil forms so revisions were made.

Fossil Evidence

The origin of fishes traced back to Cambrian time where early chordate-like forms appeared. Ostracoderms were jawless fishes with bony plates. The rise of jawed forms occurred in Silurian and Devonian period. Placoderms were the first jawed fishes and diversified widely. Early ray-finned fishes appeared in late Silurian. After extinction events the bony fishes became dominant. Actinopterygians invaded freshwater and later the sea. Teleost fishes showed major radiation in Cretaceous and continued to diversify.

Modern Changes

With the development of phylogenetic studies, the earlier Class Pisces was not accepted. It is the process where classification is based on ancestry. Fishes form a paraphyletic group because tetrapods have originated from lobe-finned fishes. Hence Pisces does not include all descendants.

Jawless Fishes

The classification of Agnatha showed controversy. Hagfishes were earlier kept outside vertebrates because vertebral column is absent. But molecular data indicated that hagfishes and lampreys form a single clade named Cyclostomata. The absence of some structures in hagfish is considered as secondary loss.

Present Clade System

Fishes are now placed in different monophyletic groups. Cyclostomata includes hagfish and lampreys. Gnathostomata includes all jawed fishes. Under this Chondrichthyes are cartilaginous forms. Osteichthyes includes the bony forms. Actinopterygii are ray-finned fishes and form the major living group. Sarcopterygii includes lobe-finned fishes from which tetrapods have evolved. This system explains the evolutionary relation among all fish groups.

Systematic Classification of fishes (pisces)

The systematic classification of fishes (Pisces) placed them under Kingdom Animalia, Phylum Chordata, and Subphylum Vertebrata. They are aquatic vertebrates that possess gills, fins, and a notochord at least in early stage, with a backbone replacing it in adults. The body shows bilateral symmetry and segmentation, though in few primitive forms this feature looks not so perfect. The nervous system organized with a dorsal hollow nerve cord, and the heart is ventrally located—two chambered mostly, showing single circulation. Reproduction usually external and fertilization in water; eggs are laid in large number, that part actually changes how population sustains.

Within Vertebrata, fishes are included under Superclass Gnathostomata, meaning “jawed vertebrates.” In this group, one pair of visceral arches has modified into jaws which help in feeding activity. The internal ear usually contain three semicircular canals for balance. Paired appendages, which may be fins or limbs, are characteristic of this superclass. Gnathostomata divided into two main groups: Pisces (fishes) and Tetrapoda (amphibians, reptiles, birds, mammals). The term Pisces traditionally used for all fishes, though modern classification treat it paraphyletic, still it used in teaching notes, because old habits die steady.

The Class Placodermi, now extinct, represented ancient jawed fishes with bony plates over the head and shoulder region; they were among the earliest gnathostomes. Skeleton mostly cartilaginous with partial ossification; paired fins present; these species prevailed (wrongly used for “appeared”) during Devonian period and became vanished later.

The Class Chondrichthyes, or cartilaginous fishes, includes Squalus, Raja, and similar forms. Skeleton made entirely of cartilage, scales are placoid type, and gills usually five pairs without an operculum. Fertilization internal, and both oviparous / viviparous species occur. Heart two-chambered, and respiration by gills only. These fishes mostly marine and carnivorous in habit, with keen sense organs and lateral line system for vibration detection.

The Class Osteichthyes, or bony fishes, forms the largest group. Skeleton fully ossified, and body covered with cycloid or ctenoid scales. Gills covered by an operculum, and air bladder (swim bladder) present for buoyancy regulation. Fertilization mostly external; development either direct or with larval stage. Members include both freshwater and marine forms, like Labeo rohita, Catla catla, Salmo trutta, and others. Subclasses are Actinopterygii (ray-finned fishes) and Sarcopterygii (lobe-finned fishes), the latter being ancestral to terrestrial vertebrates.

Thus, the classification of fishes underlines gradual transition from primitive armored forms to advanced bony ones. The position of Pisces inside Gnathostomata clearly marks the evolutionary stage before appearance of Tetrapods. Despite changes in terminology, this system still used for teaching and understanding diversity of aquatic vertebrates, serving as the base for ichthyological studies today.

Class 1: Placodermi (Aphstohyoids)

• It is the class of extinct fishes.
• The body was covered by heavy bony plates forming armour.
• The endoskeleton was bony giving support to soft parts.
• Jaws were primitive and the hyoid arch not involved in jaw support.
• Autodiastylic jaw suspension was present.
• The gill-slits including hyoidean openings was complete and not reduced.
• The caudal fin was heterocercal.
• Paired fins were present but small in size.
• The body usually become flattened and many lived near bottom.
• Dental plates present instead of true teeth.
• They appeared in Silurian period and flourished in Devonian.
• They survived up to Permian period of Paleozoic era.
• Many forms were important predators in ancient waters.
• Their extinction allowed later bony fishes to become dominant.
• Climatius and Bothriolepis are examples.

Class 2: Chondrichthyes or Elasmobranchi

• These are cartilaginous fishes found only in marine habitat.
• The body is adapted for fast movement in water and active feeding.
• The endoskeleton is completely cartilaginous which makes the body light and flexible.
• It is the reason these fishes can swim quickly and change direction very easily.
• The exoskeleton is made of placoid scales.
• These placoid scales are tooth-like and give a rough surface to the skin.
• The scales help in reducing drag and act as protective armour.
• The air-bladder is absent in this group.
• Buoyancy is maintained mainly by a large liver filled with oily substances.
• The caudal fin is heterocercal, where the upper lobe is bigger than the lower one.
• The heterocercal tail provides upward thrust and stability during swimming.
• They have 5 to 7 pairs of gill-slits.
• The gill openings are separate and not covered by any operculum.
• Respiration occurs through continuous flow of water over these gills.
• Jaws are arranged in amphistylic or hyostylic type of suspension.
• This jaw arrangement makes the mouth opening large and mobile.
• It helps them to catch prey strongly and quickly.
• The mouth is generally ventral in position.
• Ampullae of Lorenzini present around head region.
• These are special sensory organs that detect weak electric signals produced by other animals.
• Such electrical sensing helps in locating prey even in dark or deep waters.
• Fertilization is internal.
• Males possess claspers which are modifications of pelvic fins.
• The claspers help in copulation and direct transfer of sperms.
• Many species show ovoviviparous or viviparous conditions.
• The group includes sharks, skates, rays and chimaeras.
• Subclass Elasmobranchii includes sharks, skates and rays.
• Members of Elasmobranchii show 5–7 gill-slits and lack operculum.
• Subclass Holocephali includes chimaeras, also called ghost sharks.
• Holocephali possess 4 pairs of gills covered by a soft operculum-like fold.
• Jaw suspension in Holocephali is holostylic (autostylic) where the upper jaw is fused to skull.
• Males of Holocephali show an additional frontal clasper on head.
• Chimaera is the common example under this subclass.
• These fishes are important members of marine food chain.
• They also represent one of the ancient surviving lineages of vertebrates.

Subclasses of Chondrichthyes

1. Subclass Elasmobranchii

• It includes sharks, skates and rays.
• These fishes have 5–7 pairs of gill-slits on the sides.
• The gill openings are separate and not covered by operculum.
• Because of this, the gills remain exposed and this is referred to as naked gill condition.
• Respiration occurs by direct passage of water over these gill-slits.
• Jaw suspension is amphistylic or hyostylic type.
• In this type the upper jaw is not completely fused with cranium.
• It allows a wide mouth opening and flexible movement during feeding.
• The subclass is divided into Selachi and Bradyodonti.
• Selachi includes sharks and rays.
• Bradyodonti includes fossil forms and some chimaeras, sometimes placed under Holocephali.

2. Subclass Holocephali

• It includes chimaeras, also called ghost sharks or rat fishes.
• These fishes have only four pairs of gills.
• The gills are covered by a soft operculum-like fold of skin.
• Because of this covering only one external gill opening is present on each side.
• Jaw suspension is holostylic or autostylic.
• Here the upper jaw is completely fused with the skull.
• This rigid structure helps in strong crushing action while feeding.
• Males show a special frontal clasper present on the head.
• Chimaera is the common example of this subclass.

Class 3: Osteichthyes (Telostomi)

• These are called bony fishes.
• They live in marine, freshwater and brackish water habitats.
• This wide habitat range shows high adaptability in these fishes.
• The endoskeleton is completely bony.
• The hard skeleton gives strong support and controlled movement.
• The body is covered with cycloid, ctenoid or ganoid scales.
• These scales help in smooth swimming and also give protection.
• A bony operculum is present over the gills.
• The operculum helps in better flow of water across gills.
• Jaw suspension is autostylic type with upper jaw fixed to skull.
• It allows different feeding actions like biting or sucking.
• An air bladder is present in most species.
• It helps in buoyancy and sometimes in sound or hearing adjustments.
• Fertilization is mainly external because males do not have claspers.
• These fishes form the largest and most diverse group of vertebrates.

Classification within Osteichthyes

1. Sub-class Sarcopterygii (Lobe-finned fishes)

• Fins are lobed and fleshy with internal skeletal support.
• These forms represent ancestral stage leading towards tetrapods.
• Order Rhipidistia included many extinct fishes showing characters between fishes and early amphibians.
• Order Dipnoi includes lungfishes which can breathe air by lungs.
• Dipnoi mostly live in freshwater with stagnant or low-oxygen condition.
• Examples are Neoceratodus, Protopterus and Lepidosiren.
• Actinistia includes coelacanths.
• Coelacanth (Latimeria) called living fossil because form survived unchanged.

2. Sub-class Actinopterygii (Ray-finned fishes)

• This is the major group containing almost all living bony fishes.
• Fins supported by thin flexible bony rays called lepidotrichia.
• Super order Chondrostei includes sturgeons and paddle fishes.
• These forms may show ganoid scales and somewhat symmetrical tail.
• Skeleton may be partly cartilaginous in sturgeons.
• Example is Polypterus.
• Super order Holostei includes gars and bowfin.
• Skeleton is completely bony but several primitive characters still present.
• Example is Amia and Lepidosteus.
• Super order Teleostei is most successful group.
• Teleosts show protrusible jaw and highly movable bones around mouth.
• Scales are cycloid or ctenoid type.
• Tail is mostly homocercal.
• Many orders present like Clupeiformes, Cypriniformes, Anguilliformes and Perciformes.

Classification based on feeding habit

1. Herbivores
These fishes feed mainly on plant parts, algae or other vegetation. It is the process where the digestive tract is long and coiled because fibrous plant materials need more time for digestion. Tilapia and grass carp are examples, and these species feed on aquatic weeds. It is observed that they help in keeping the water body clean and balanced by removing excess vegetation.

2. Carnivores
These fishes depend on animal matter for food and many of them prey on other fishes. The teeth are sharp caniniform type which help in catching slippery prey. Lates niloticus (Nile perch) is one example.
Some of the main types are–
• Visual hunters – These are active mostly at day time as they detect prey by sight.
• Nocturnal feeders – In this type, fishes like eels depend on smell and the lateral line system for finding prey in dark conditions.

3. Omnivores
These fishes feed on both plant and animal materials. It is the process where their adaptability allows them to survive in different water bodies. Cyprinus carpio (common carp) is an example which feed on detritus, insects, algae and many other materials.

4. Planktivores
These species depend on plankton (tiny plants or animals). The gill rakers are modified for filtering plankton from water. Sardina pilchardus is one example. These are important because they help in energy transfer in aquatic food chains.

5. Detritivores
These fishes feed on decaying organic matter or mud rich in organic debris. This process occurs when they take in bottom mud and extract nutrients from it. Mugil cephalus (grey mullet) is an important example. They recycle nutrients in water bodies.

6. Grazers
Grazing fishes scrape algae or other bottom-dwelling organisms from rock or coral surfaces. The teeth are incisor like or sometimes fused into a beak-like form as seen in parrotfish. They keep biting small pieces continuously while moving.

7. Strainers
These fishes collect very small organisms such as diatoms and crustaceans by filtering water. Many fine gill rakers are present for efficient filtration. Clupea harengus (herring) is an example.

8. Suckers
These fishes are adapted for sucking up mud and separating food from it. They possess papillose or folded lips and sometimes pharyngeal teeth. In some cases teeth may be absent. Catla catla show partial sucking behaviour.

9. Parasites
These species attach to hosts and obtain body fluids. Petromyzon marinus (lamprey) is the example. They have plicate lips for firm suction and gland secretion that prevent clotting of host blood. This is referred to as a highly specialized feeding habit.

Classification of fish based on habitat

1. Freshwater Fish

These are fishes living in rivers, ponds, lakes and streams where salinity is less than 0.5 ppt. It is observed that nearly 40% of total fish species occur in this habitat. The water conditions are mostly stable but seasonal changes can affect the environment. Freshwater species have body features that help in preventing excess water intake because osmosis pressure is high in such environments.

Some of the main categories are–

• Coldwater Fish – These are found in cooler waters around 5–20°C. Mahseer and trout are examples. It is the process where they prefer high oxygen and are mostly present in fast flowing streams.

• Warmwater Fish – These are present in water having 25–35°C temperature. Common fishes are Cyprinus carpio (carp), Clarias batrachus (catfish), snakeheads and featherbacks. These fishes can survive low oxygen concentration also.

2. Brackish Water Fish

These fishes tolerate both fresh and saline waters and are found in estuaries, coastal lagoons and backwaters. The salinity range is from 0.5–30 ppt. It is the process where daily fluctuations in salinity occur, so their osmoregulatory system is flexible. This habitat works as a transition zone for many juvenile marine fishes.

Examples are Mugil cephalus (mullet), Chanos chanos (milkfish), Lates calcarifer (seabass), Etroplus suratensis (pearlspot) and mudskippers. Some of these can move freely between different water types.

3. Marine Fish

These fishes occur in seawater where salinity is more than 30 ppt. This is the largest group and they occupy shallow coastal areas to deep ocean regions. Their body is adapted to avoid dehydration by constant ion regulation. It is known that marine fisheries include around 240 species which supply a major part of food fish.

Some examples are Sardinella longiceps (sardine), Rastrelliger kanagurta (mackerel), Trichiurus lepturus (ribbonfish), Thunnus albacares (tuna), anchovy, grouper and cobia. Many are pelagic and swim near the surface, while others stay near the seabed as demersal types. Some species show schooling or long-distance migration for breeding.

Classification of fish based on manner of reproduction

1. Oviparous Fish

These fishes lay eggs outside the female body. It is the process where fertilization mostly occurs externally, but in some species of sharks internal fertilization can happen also. They generally produce a very large number of eggs because larval mortality in natural water is high. The eggs may float freely or they may adhere to aquatic weeds depending on the species habit.
In some cases, the parents guard the eggs or protect the fry for few days. This is referred to as mouth-brooding behaviour in some fishes.

Examples are Salmo trutta (trout), Salmo salar (salmon), Carassius auratus (goldfish). Most of the bony fishes are oviparous.

2. Viviparous Fish

In this type, fertilization is internal and the young ones are born alive. The embryos are nourished by maternal tissues or a placenta-like connection. It is observed that these fishes produce fewer offspring but the survival rate is higher. The female body system is modified to carry the embryos for a long period, and this reduces loss of eggs to the environment.

Examples include Poecilia reticulata (guppy), Squalus acanthias (dogfish shark) and some rays. In some species, nutrient supply is through yolk sac along with uterine milk type secretion.

3. Ovoviviparous Fish

This group shows a combination pattern. The eggs are fertilized internally, but the embryos remain inside the female until hatching. It is the process where nourishment is obtained only from the yolk and not from maternal tissues directly. When development is complete, the young come out alive which provide protection from predators during the early stage. The number of offspring is moderate in such fishes.

Examples are some sharks, Scomber scombrus (Atlantic mackerel) and certain rockfish species.