Pisces

Pisces are permanently aquatic vertebrates that has gills for respiration. They are devoid of the true spine. Fishes may be present in fresh, marine, and brackish water. The organs for locomotion in fishes are fins. However, fins are sometimes paired or unpaired. The study of fishes is ichthyology. Meanwhile, it is believed that they evolved from sea-squirt which is like a primitive fish.

Classification of Pisces

Pisces consist of fish. The classification includes –

• Kingdom- Animalia
• Phylum- Chordata
• Super-class- Gnathostomata
• Class- Pisces

Characteristics

The characteristics of this class are-

• They have a streamlined body.
• The circulatory system is of close type.
• The body has three regions- head, trunk, and tail.
• The skeleton can be bony or cartilaginous.
• The sexes are separate.
• A well-developed digestive system is present.
• No extraembryonic membranes.
• Fertilization either internal or external.
• The body consists of scales, which acts as an external covering.
• They can be herbivorous, carnivorous, or omnivorous.

Classification of Pisces

They have divided into further three classes-

Placodermi

This class includes fishes that are extinct now. They consist of pre-historic fishes. The knowledge of these fishes is through fossils. Placoderms are the first fishes with jaws.

The characteristic of placoderms are-

• They survived up to a Permian period of the Paleozoic era.
• The endoskeleton is bony
• Heterocercal caudal fin were present in placodermi
• They consist of two orders- firstly Arthrodiriformes, and secondly, Antiarchiformes.
• For example- Climatius, Bothriolepis.

Chondrichthyes

Another name for Chondrichthyes is elasmobranch. The characteristics of class Chondrichthyes are-

• Operculum (covering of the gills) is absent.
• They are present only in marine water.
• The mouth is present on the ventral side of the body.
• Cartilage composes the endoskeleton of the Chondrichthyes.
• The skin consists of placoid scales. Placoid scales are sometimes referred to as denticles.
• The caudal fin is heterocercal. Sharks possess heterocercal tails.
• The air bladder is completely absent in these fishes.
• Ampulla of lorenzini is present.
• They have an amphystylic or hyostylic jaw suspension. Further, in amphistylic jaw suspension, the upper jaw is braced against the cranium and gets its support by hyomandibular. Thus, this type of jaw suspension is present in some sharks.
• Males possess a pair of claspers. These claspers act as the reproductive part.
• They are further divided into two subclasses- Selachi and Bradyodonti.
• For example- Scoliodon, Pristis ( saw-fish), Chimaera, Stingray, etc.

Osteichthyes

These are often called bony fishes. Moreover, this is the largest class of vertebrates. Moreover, it consists of about 29,000 species. They possess bony internal skeletons.

The characteristics of Osteichthyes are-

• They are present in marine water, freshwater, or brackish water.
• The claspers for male reproduction are absent.
• The exoskeleton comprises placoid scales, ctenoid scales or, ganoid scales.
• The jaw suspension of Osteichthyes is autostylic. In this type of jaw, the upper jaw is directly connected to the cranium.
• The air bladder is usually present.
• In addition, the covering of gills i.e. operculum is present in this class.
• The Osteichthyes have three subclasses, firstly, Dipnoi, secondly, Crossopterygii, and last Actinopterygii.
• Dipnoi consists of lungfishes, for example- South American lungfish
• Crossopterygii have fins in pairs that are for support in the water.

For example- Coelacanthimorpha, and Rhipidista.

• Actinopterygii is a thin and elongated fish. Accessory respiratory organs are absent.

For example- Anglerfish, Mullet, Snakehead, Smelt, etc.

Respiration

Respiration in fishes occurs through gills. Gills are present on either side of the pharynx. The gaseous exchange takes place here. Oxygen is taken in and carbon dioxide is released. For instance, sharks and lampreys have many gill openings. On the other hand, bony fishes have only one opening.

Moreover, there are some fishes, which possess lungs for respiration. For example, West African lungfish, spotted lungfish, marbled lungfish.

Excretion

The main excretion in fishes is urea. Therefore, they possess ureotelic excretion. However, gills can act as organs for excretion as well. The freshwater fishes excrete more water. In some of them, special organs like kidneys may be present.

Circulation

In fishes, the flow of blood is in one direction only. It has a closed circulatory system. Moreover, it consists of a heart with two chambers only. The heart comprises four distinct parts- sinus venosus, the atrium, ventricle, and bulbous arteriosis. The blood is pumped from the heart to the gills. In addition, the blood gets oxygenated in the gills and then flows all over the body.

Digestion

Food is taken through the mouth. Breaking of food occurs in the oesophagus and digestion in the stomach. Pyloric caeca are present in some fishes. It is the finger-like pouches that secrete the digestive enzymes. Absorption of nutrients also occurs through it. The intestine is present for the absorption of nutrients.

 Nervous System

The nervous system can be divided into two parts. Firstly the central nervous system (CNS) and secondly, the peripheral nervous system (PNS). The central nervous system consists of the brain and the spinal cord. Also, the nervous system is the derivative of ectoderm.

Further, the peripheral nervous system connects nerves which connect the muscles and the sensory organs to CNS. The function of the nervous system is control and coordination during swimming. In some fishes like Scoliodon, there are ten cranial nerves present.

Sensory Organs

The sensory organs in Pisces include-

1. Eyes

The eyes of fishes are adapted to see in the water. Moreover, they do not have lids in their eyes. The eye consists of many layers. They possess a spherical lens. Moreover, some fishes have the ability to see ultraviolet (UV) or polarized lights.

Some fishes have primitive eyes. On the other hand, some have well-developed eyes.

2. Hearing

For hearing they have, ears and lateral lines

Reproductive system

The sexual organs are internal. Female fish consists of ovaries and male fished testes. Moreover, the female fish produces eggs and males sperms. The eggs and sperms fuse i.e. fertilization.

However, in most cases, they do not directly do mating. On the other hand, the female fish scatters her eggs in the water and the male ejects its sperms. Therefore, external fertilization happens.

Moreover, in the fishes with internal fertilization, the eggs either hatch internally or are released before hatching.

FAQs on Pisces

Question 1- What are the characteristics of Pisces?

Answer 1- The characteristics of this class are-

• They have a streamlined body.
• The circulatory system is of close type.
• The body has three regions- head, trunk, and tail.
• The skeleton can be bony or cartilaginous.
• The sexes are separate.
• A well-developed digestive system is present.
• Fertilization either internal or external.
• The body consists of scales, which acts as an external covering.

Question 2- What are the three sub-classes of Pisces?

Answer 2- They have divided into three classes-

• Placodermi

This class includes fishes that are extinct now. They consist of pre-historic fishes. Also, the knowledge of these fishes is through fossils. Placoderms are the first fishes with jaws. Further, heterocercal caudal fin were present in placodermi. They consist of two orders- firstly Arthrodiriformes, and secondly, Antiarchiformes.

For example- Climatius, Bothriolepis.

• Chondrichthyes

Another name for Chondrichthyes is elasmobranch. The characteristics of class Chondrichthyes are, operculum (covering of the gills) is absent. They are present only in marine water. Moreover, the mouth is present on the ventral side of the body. Cartilage composes the endoskeleton of the Chondrichthyes.

For example- Scoliodon, Pristis (saw-fish), Chimaera

• Osteichthyes

These are often called bony fishes. Moreover, this is the largest class of vertebrates. It consists of about 29,000 species. They possess a bony internal skeleton. They are present in marine water, freshwater, or brackish water. For example- Mullet, Anglerfish, etc.

 Question 3- How do Pisces respire?

Answer 3- Respiration in fishes occurs through gills. Gills are present on either side of the pharynx. Moreover, there are some fishes, which possess lungs for respiration. For instance, West African lungfish, spotted lungfish, marbled lungfish.