Chondrichthyes – Characteristics, Classification & Examples

Kingdom	Animalia
Phylum	Chordata
Subphylum	Vertebrata
Division	Gnathostomata
Superclass	Pisces
Class	Chondrichthyes

• Chondrichthyes, derived from the Greek words “chondr” meaning cartilage and “ichthys” meaning fish, refers to a class of jawed fishes characterized by their cartilaginous skeletons. Unlike the bony fishes known as Osteichthyes, which have skeletons primarily composed of bone tissue, Chondrichthyes have skeletons primarily composed of cartilage. This class encompasses a diverse group of fishes including sharks, rays, skates, and chimaeras, all of which are mostly found in marine environments.
• Members of the Chondrichthyes class share several common characteristics. They possess jaws and paired fins, allowing them to efficiently navigate their aquatic habitats and capture prey. Paired nares, or nostrils, help these fishes detect scents in the water, aiding in hunting and locating food sources. Their bodies are covered with placoid scales, which are small, tooth-like scales that provide protection and reduce drag in the water.
• One notable aspect of Chondrichthyes is their wide range of body sizes. The smallest known member is the finless sleeper ray, measuring around 10 centimeters (3.9 inches) in length, while the largest is the whale shark, reaching up to 10 meters (32 feet) in length. This vast size difference reflects the diverse adaptations and lifestyles within this class.
• Chondrichthyes are further classified into two subclasses: Elasmobranchii and Holocephali. The subclass Elasmobranchii consists of sharks, rays, skates, and sawfish. Sharks are perhaps the most well-known members of this subclass, characterized by their streamlined bodies and sharp teeth. Rays and skates have flattened bodies and are known for their wing-like pectoral fins, allowing them to glide gracefully through the water. Sawfish, on the other hand, possess a long, toothed snout resembling a saw.
• The subclass Holocephali includes chimaeras, also known as ghost sharks. Chimaeras have distinct features, such as a single gill opening covered by an operculum and a venomous spine located in front of their dorsal fin. Although they are sometimes separated into their own class, they are often grouped within Chondrichthyes due to their shared characteristics and evolutionary history.
• Chondrichthyes have adapted to various marine ecosystems, occupying different niches and playing crucial roles in the food chain. They exhibit a wide array of feeding habits, including carnivorous, herbivorous, and filter-feeding behaviors. Their diversity and ecological significance make them an intriguing and important group within the animal kingdom.
• In conclusion, Chondrichthyes is a class of jawed fishes with cartilaginous skeletons. These fishes, encompassing sharks, rays, skates, and chimaeras, have evolved unique adaptations for life in the ocean. With their diverse body sizes, feeding strategies, and habitats, Chondrichthyes contribute to the rich biodiversity of marine ecosystems worldwide.

Characteristics of Chondrichthyes

Chondrichthyes, also known as Sharks, Skates, and Rays, exhibit a range of fascinating characteristics. Here are some key features of these jawed fishes:

1. Habitat: Chondrichthyes are primarily found in marine environments, inhabiting various oceans and seas around the world.
2. Powerful Jaws: These fishes possess a pair of jaws that are well-developed and equipped with sharp teeth. This allows them to efficiently capture and consume their prey.
3. Size: Among the Chondrichthyes, the whale shark holds the title of being the second-largest vertebrate and the largest fish species, reaching lengths of up to 15 meters.
4. Ventral Mouth: The mouth is positioned on the ventral side of the body, aiding in the intake of food.
5. Cartilaginous Endoskeleton: Chondrichthyes have skeletons primarily composed of cartilage. Although cartilage is flexible, the deposition of calcium salts provides strength to their skeletal structure.
6. Notochord: These fishes retain a notochord throughout their entire lives, which serves as a supportive structure.
7. Heterocercal Tail: Most Chondrichthyes possess a heterocercal tail, characterized by an elongated upper lobe and a shorter lower lobe. This tail structure aids in locomotion and provides stability while swimming.
8. Placoid Scales: The skin of Chondrichthyes is covered with small, tooth-like structures called placoid scales. These scales reduce drag in the water and provide protection to the body.
9. Teeth: The teeth of Chondrichthyes are modified placoid scales and are not attached to the jawbones. Instead, they are embedded in the tissue. As old teeth fall out, they are continuously replaced by new teeth formed behind them.
10. Gill Structure: Chondrichthyes typically possess 5-7 pairs of gills for respiration. Gaseous exchange occurs as water passes over the gills, enabling the extraction of oxygen.
11. Buoyancy Control: Unlike bony fishes, Chondrichthyes lack swim bladders. As a result, they actively swim to avoid sinking and maintain their buoyancy.
12. Poikilothermy: Chondrichthyes are poikilothermic or cold-blooded animals. They are unable to regulate their internal body temperature and rely on their surrounding environment to maintain their metabolic processes.
13. Predatory Feeding: Chondrichthyes are predominantly predatory fishes, feeding on other fishes, crustaceans, and mollusks. Some species also filter food particles like plankton from the water current that passes through their mouth, pharynx, and gills.
14. Circulatory System: Chondrichthyes have a two-chambered heart, consisting of one auricle and one ventricle. The circulatory system includes a well-developed renal portal system, and their red blood cells are oval-shaped and possess a nucleus.
15. Sensory Organs: These fishes have well-developed sense organs. Sharks, in particular, possess electroreceptors on their heads, allowing them to detect the electric currents generated by the movement of their prey. Additionally, they have sensory cells in the lateral line organ, which help detect vibrations, motion, and changes in water pressure.
16. Reproductive Systems: Chondrichthyes exhibit separate sexes, with males and females having internal fertilization. Some species are oviparous, laying eggs outside the body, while others are ovoviviparous, retaining the eggs within the body until they hatch. Viviparity, where embryos develop inside the female and receive nourishment directly from the mother, is also observed in certain species.
17. Brain and Cranial Nerves: Chondrichthyes possess a brain and spinal cord, which are protected by vertebrae. The brain exhibits distinctive features, including massive olfactory lobes and a large cerebellum. The cranial nerves are divided into ten pairs, serving various sensory and motor functions.

These characteristics contribute to the unique adaptations and biological traits of Chondrichthyes, making them a diverse and captivating group of fishes in the animal kingdom.

Chondrichthyes Examples

Chondrichthyes, the class of cartilaginous fishes, encompasses various fascinating examples. Here are some prominent representatives of this diverse group:

Sharks:

1. Scoliodon (Dogfish): A small shark species commonly found in coastal waters.
2. Rhincodon (Whale Shark): The largest fish and a filter feeder that primarily consumes plankton.
3. Charcarodon charcharias (The Great White Shark): Known for its large size and formidable predatory capabilities.

Rays:

1. Trygon (Stingray): Characterized by their flattened bodies and long, whip-like tails equipped with venomous spines.
2. Torpedo (Electric Ray): Possesses specialized electric organs that produce electric discharges for defense and prey capture.
3. Narcine bancroftii (Lesser Electric Ray): A smaller species of electric ray found in shallow coastal waters.
4. Manta (Manta Ray): Known for their large size and graceful swimming patterns. They are filter feeders.

Skates:

1. Leucoraja erinacea (Little Skate): Found in the western Atlantic Ocean, these skates have a small size and distinctive patterns on their skin.
2. Cruriraja andamanica (Andaman Leg Skate): A species of skate found in the Andaman Sea with unique leg-like extensions on their pectoral fins.
3. Gurgesiella (Small Deepwater Skate): Deepwater skates with a wide distribution across different oceans.

Sawfishes:

1. Pristis clavata (Dwarf Sawfish): A smaller species of sawfish with a distinctive saw-like rostrum adorned with teeth.
2. Anoxypristis cuspidata (Narrow Sawfish): Recognized for its elongated snout with teeth on the sides.

Chimaeras:

1. Hydrolagus alphus (Whitespot Ghost Shark): Found in the deep-sea regions, these chimaeras have a ghostly appearance and specialized sensory structures.
2. Callorhinchus milii (Australian Ghost Shark): Known for its elongated snout and distinctive appearance.
3. Chimaera argiloba (Whitefin Chimaera): A species of chimaera with white-colored fins.

These examples represent the remarkable diversity within the class Chondrichthyes, showcasing the unique characteristics and adaptations found in cartilaginous fishes.

Classification of Chondrichthyes

The class Chondrichthyes is divided into two sub-classes, viz:

Sub-class-1: Elasmobranchii

Sub-class-1: Elasmobranchii, which includes sharks, rays, skates, and sawfish, exhibits several distinguishing characteristics:

1. Skin and Scales: Elasmobranchii have a unique skin structure. Instead of superficial scales, their skin is covered in tiny tooth-like structures called denticles or placoid scales. However, electric rays and eagle rays have smooth skin.
2. Nostrils: They possess a single nostril on each side of the head. These nostrils play a crucial role in detecting scents and tracking prey.
3. Gill Openings: Most elasmobranchs have 5 pairs of gill openings, although in some cases, there may be 6 or 7 pairs. These individual gill openings allow water to pass through the gills for respiration. Unlike bony fishes, the gill arches are not covered by a bony operculum.
4. Caudal Fin: The caudal or tail fins of Elasmobranchii are asymmetrical, meaning the upper part is longer than the lower part. This type of caudal fin is known as a heterocercal caudal fin and aids in maintaining stability and maneuverability while swimming.

Sub-class-1: Elasmobranchii is further divided into eight orders, each with its own distinctive features and examples:

Order-1: Orectolobiformes (Carpet sharks)

Order-1: Orectolobiformes, commonly known as Carpet sharks, is characterized by the following features:

1. Nostrils and Naso-Oral Groove: Carpet sharks have specialized nostrils that aid in detecting scents and tracking prey. Additionally, their mouths contain a naso-oral groove, which allows for efficient water flow during feeding.
2. Nariel Barbell and Lateral Gill Openings: These sharks possess a sensory organ called the nariel barbell located near the nostrils. It helps in detecting prey by sensing electrical fields. The gill openings are positioned laterally on the body.
3. Ventral Mouth: The mouth of carpet sharks is located on the ventral side of their body. This positioning enables them to feed on prey located on the ocean floor.
4. Gill Openings and Pectoral Fins: The fourth and fifth gill openings of carpet sharks originate from the base of their pectoral fins and extend over the fins’ base. This arrangement is a distinguishing characteristic of this order.
5. Labial Fold: Carpet sharks have a labial fold located at the lateral corner of their mouths. This fold, found in front of the jaw, aids in feeding and manipulating prey.
6. Dorsal Fins and Spiracles: They possess two spiny dorsal fins, which provide stability while swimming. Additionally, carpet sharks have spiracles, which are small openings located behind their eyes. These spiracles enable the intake of water for respiration when the sharks are resting on the seafloor.

Examples of carpet sharks include:

1. Rhincodon typus (Whale shark): The whale shark is the largest fish species and a filter feeder. It is known for its enormous size and distinctive pattern of spots and stripes.
2. Stegostoma fasciatum (Zebra shark): The zebra shark, also known as the leopard shark, features a unique pattern of dark spots and ridges. It is a bottom-dwelling species that primarily feeds on mollusks and crustaceans.
3. Chiloscyllium punctatum (Brownbanded bamboo shark): The brownbanded bamboo shark has a slender body with distinct brown bands. It is a nocturnal species that inhabits coral reefs and feeds on small fishes, crustaceans, and mollusks.

These examples represent the fascinating diversity within the Order Orectolobiformes and highlight the unique adaptations of carpet sharks.

Order-2: Carcharhiniformes (Ground sharks)

Order-2: Carcharhiniformes, also known as Ground sharks, represents a diverse group of sharks with several distinguishing characteristics:

1. Nostrils: Ground sharks possess nostrils similar to those found in other shark species. They lack the specialized naso-oral groove, perinasal fold, or barbells seen in some other shark orders.
2. Labial Fold: The labial fold of Ground sharks is situated at the back of the jaw, aiding in capturing and manipulating prey during feeding.
3. Dorsal and Anal Fins: These sharks typically have two non-spiny dorsal fins and anal fins. These fins contribute to their stability and maneuverability in the water.
4. Spiracles: In most cases, Ground sharks possess spiracles. These small openings, located behind their eyes, assist in the intake of water for respiration while the sharks are stationary or resting on the seafloor.
5. Caudal Fins: The caudal fins of Ground sharks feature a long dorsal segment. This characteristic gives the caudal fin a distinct shape and contributes to efficient swimming and maneuvering.
6. Habitat Preference: Ground sharks are commonly found near the continental shelf, inhabiting coastal and nearshore waters. Some species are known to enter river mouths, adapting to both marine and freshwater environments.

Some examples of Ground sharks include:

1. Carcharhinus limbatus (Blacktip shark): The Blacktip shark is known for its distinctive black markings on the tips of its fins. It inhabits tropical and subtropical coastal waters and is often observed leaping out of the water.
2. Carcharhinus melanopterus (Blacktip Reef shark): The Blacktip Reef shark is a common species found in coral reef ecosystems. It features a black fin tip and prefers shallow, warm coastal waters.
3. Scoliodon laticaudus (Spadenose shark, Dog shark): The Spadenose shark, also known as the Dog shark, has a flattened snout and a slender body. It inhabits both coastal and freshwater environments and is known for its adaptability.

These examples highlight the diversity of Ground sharks and their distribution in various coastal habitats, showcasing their ecological significance in marine and freshwater ecosystems.

Order-3: Rhinobatiformes (Guiter fishes)

Order-3: Rhinobatiformes, commonly known as Guitar fishes or Shovelnose Rays, exhibit several distinctive characteristics:

1. Body Shape: Guitar fishes have an elongated body with a flat head and short ray-like fins. The body resembles a combination of sharks and skates, with a streamlined shape adapted for efficient movement in the water.
2. Fins: They possess two distinct dorsal fins and a caudal fin. The pectoral fins are of medium size and originate from the back of the top of the snout, contributing to their unique appearance.
3. Dermal Denticles: The body of Guitar fishes is covered by tiny dermal denticles or placoid scales. These scales provide protection and reduce friction while swimming.
4. Heterocercal Caudal Fin: Guitar fishes have large heterocercal caudal fins. The upper lobe of the caudal fin is longer than the lower lobe, which enhances their swimming and maneuverability.
5. Reproduction: They are ovo-viviparous, meaning the embryos develop inside the female’s body, and the young are born as fully formed individuals. This adaptation allows them to give birth to live offspring.
6. Bottom Dwellers: Guitar fishes are primarily bottom dwellers, inhabiting shallow coastal waters and sandy or muddy seabeds. They are well-adapted to benthic environments and spend a significant amount of time foraging on the seafloor.
7. Diet: Their diet consists mainly of mollusks and tiny crustaceans. They use their specialized dental plates and jaws to crush and consume their prey.

Some examples of Guitar fishes include:

1. Rhina ancylostoma (Shovelnose Ray, Brown-mouthed Guitarfish): This species is characterized by its shovelnose-shaped head and brown mouth. It inhabits coastal waters and is known for its bottom-dwelling behavior.
2. Rhinobatos annandalei (Annandale’s Shovelnose Ray): Annandale’s Shovelnose Ray is found in the Indo-Pacific region. It has a distinct shovel-like snout and is well-adapted to sandy or muddy substrates.
3. Rhynchobatus djiddensis (White-spotted Guitarfish): This species is known for its white spots and is found in the Indian Ocean and the western Pacific Ocean. It exhibits the typical characteristics of Guitar fishes, including its unique body shape and feeding habits.

These examples showcase the diversity within the Rhinobatiformes order and highlight the adaptations of Guitar fishes for their benthic lifestyle and specialized feeding habits.

Order-4: Myliobatiformes (Rays)

Order-4: Myliobatiformes, commonly known as Rays, exhibit several distinctive characteristics:

1. Disc-shaped Body: Rays have an ideally disc-shaped body that is typically more compressed in the vertical plane. Their bodies are wide and flattened, allowing them to glide gracefully through the water.
2. Eye and Spiracle: The eyes and spiracles, which function as modified gill slits, are located on the dorsal surface of the ray’s body. The spiracles enable them to draw in water for respiration even when partially buried in the substrate.
3. Ventral Gill Openings: The gill openings are located on the ventral side of the ray’s body. Typically, there are five pairs of gill openings that are perforated to allow water to flow over the gills for respiration.
4. Single Dorsal Fin: Rays possess a single small dorsal fin of medium size, which is located at the base of the tail. This dorsal fin helps stabilize their movement and assists in steering.
5. Caudal Fin Absent: Unlike many other fish species, rays lack a separate caudal fin. Instead, their tail is typically whip-like or elongated, and a dorsal spine may be present on the tail’s upper surface.
6. Wide and Compressed Trunk: The trunk of rays is wide, compressed, and thick. This anatomy contributes to their characteristic disc-shaped body and provides support for their pectoral fins.
7. Head and Tail Variations: In some ray species, such as the devil ray and eagle ray, the head is convex and separate from the disc-shaped body. Additionally, the tail may vary in length, with some species having short tails, like the butterfly ray.
8. Benthic Lifestyle: Rays are predominantly benthic, meaning they inhabit the ocean floor and are well-adapted to life on the seabed. They often bury themselves in sand or sediment to camouflage and forage for food.

Examples of rays include:

1. Gymnura poecilura (Longtail Butterfly Ray): This ray species is known for its long-tailed appearance and is found in tropical waters. It has a disc-shaped body and displays a range of colorful patterns.
2. Dasyatis zugei (Pale-edged Stingray): The pale-edged stingray is characterized by its distinctive pale edges and is found in the Indo-Pacific region. It has a flattened body with a wide disc shape.
3. Himantura imbricata (Scaly Stingray): The scaly stingray is recognized by its rough and scaly skin. It inhabits shallow coastal waters and has a unique body shape with a separate head and disc.
4. Aetomylaeus nichofii (Banded Eagle Ray): Banded eagle rays are known for their banded coloration and can be found in various oceanic regions. They possess a disc-shaped body with a distinct head and are well-adapted to life near the seabed.

These examples demonstrate the diversity within the Myliobatiformes order and highlight the unique adaptations of rays for their benthic lifestyle and locomotion in the water.

Order-5: Pristiophoriformes (Sawfishes)

Order-5: Pristiophoriformes, commonly known as Sawfishes, possess several distinctive features:

1. Compressed Head: Sawfishes have a compressed head that is flattened in the vertical plane. This adaptation allows them to move efficiently through the water and navigate their habitat.
2. Elongated Snout: One of the most striking characteristics of sawfishes is their long snout, which has evolved into a flat, narrow blade-shaped rostrum. The rostrum is lined with long tooth-like structures on each side, resembling a saw.
3. Jaws and Spiracle: Sawfishes have well-defined jaws and a spiracle, which is a respiratory opening located on the surface of their head. The spiracle helps them draw in water for respiration even when they are buried in sediment.
4. Ventral Gill Openings: The gill openings of sawfishes are located on the ventral side of their body. This positioning allows them to extract oxygen from the water efficiently.
5. Two Large Dorsal Fins: Sawfishes possess two large and separate dorsal fins. These fins provide stability and control during swimming.
6. Small Pectoral Fins: The pectoral fins of sawfishes are relatively small compared to their body size. They assist in maneuvering and maintaining balance.
7. Large and Strong Caudal Fins: Sawfishes have robust caudal fins, which are important for generating powerful propulsion and steering while swimming.
8. Weak Electrical Organ: Sawfishes have a relatively weak electrical organ that develops from the tail muscles. This organ is involved in generating weak electric fields for sensory purposes.

Examples of sawfishes include:

1. Anoxypristis cuspidata (Knife-tooth Sawfish): This species is characterized by its elongated snout and distinctive tooth-like structures on the rostrum. It is found in tropical and subtropical coastal waters.
2. Pristis microdon (Large-tooth Sawfish): The large-tooth sawfish has a long snout with large tooth-like structures. It inhabits coastal and estuarine environments in tropical regions.
3. Pristis zijsron (Green Sawfish): The green sawfish is named after its greenish hue and possesses a saw-like rostrum. It is distributed in coastal waters of the Indo-Pacific region.

These examples highlight the unique adaptations of sawfishes, particularly their elongated snouts with saw-like structures. Sawfishes are fascinating creatures that demonstrate remarkable specialization for their specific habitats and feeding strategies.

Order-6:Torpediniformes (Electric Rays)

Order-6: Torpediniformes, commonly known as Electric Rays, exhibit several distinctive characteristics:

1. Ray-like Body: Electric rays have a body structure similar to other rays, with a flattened disc-shaped body and broad pectoral fins that resemble wings. They are adapted for a benthic lifestyle, dwelling on the seafloor.
2. Strong Electrical Organs: One of the defining features of electric rays is their powerful electrical organs. These organs are derived from the branchial muscles located in the head region. They generate and discharge electric shocks for various purposes, including defense and prey detection.
3. Soft Skin and Small Eyes: Electric rays have soft and loose skin, which lacks the typical rough denticles found in many other cartilaginous fishes. Their eyes are relatively small in size, reflecting their reliance on other sensory adaptations, such as electroreception.
4. Well-Developed Caudal Fin: Electric rays possess a well-developed caudal fin that lacks spines. This fin aids in locomotion and helps them navigate their aquatic environment.
5. Large and Round Chest Disc: The chest disc, which forms the main body of electric rays, is large and round to shovel-shaped. It is thick and soft, providing support and protection for the internal organs.
6. Scale-less Body: Unlike some other cartilaginous fishes, electric rays lack scales on their body. This contributes to their soft and smooth skin texture.
7. Kidney-Shaped Electrical Organs: At the base of the pectoral fin, electric rays have large kidney-shaped electrical organs. These specialized structures generate and store electrical charges.

Examples of electric rays include:

1. Narcine brunnea (Brown Electric Ray): This species of electric ray is characterized by its brown coloration and electric organ discharge capabilities. It is found in certain coastal regions.
2. Narcine timlei (Black Spotted Electric Ray): The black spotted electric ray is known for its distinctive black spots on its body. It possesses strong electrical organs and is distributed in specific marine habitats.
3. Narke dipterygia (Spot-tail Electric Ray): The spot-tail electric ray is named after the spot pattern on its tail. It is capable of producing electric shocks and inhabits certain coastal areas.

These examples highlight the unique adaptation of electric rays to generate electric shocks, which they use for self-defense and hunting. Electric rays are fascinating creatures with specialized sensory abilities that set them apart from other cartilaginous fishes.

Order-7:Heterodontiformes (Horn sharks)

Order-7: Heterodontiformes, commonly known as Horn Sharks, possess several distinguishing characteristics:

1. Unique Teeth: Horn sharks are notable for having different types of teeth, which vary in shape and function within their jaws. They typically have strong, sharp teeth at the front for capturing and gripping prey, while their back teeth are flattened for crushing and grinding shells.
2. Sub-terminal Mouth: The mouth of horn sharks is positioned ventrally, closer to the underside of the head. This placement allows them to feed on benthic organisms found on the ocean floor.
3. Two Dorsal Fins with Spines: Heterodontiformes have two dorsal fins, both of which are supported by strong spines. These spines provide stability and serve as a defense mechanism against potential predators.
4. Varied Diet: Horn sharks have a diverse diet, which includes various mollusks (such as clams and snails), crustaceans (like crabs and shrimp), and echinoderms (such as sea urchins and starfish). Their specialized teeth enable them to effectively consume these different prey items.

Example: Heterodontus galeatus, commonly referred to as the Crested Horn Shark, is found in the Pacific Ocean. It showcases the characteristic features of the Heterodontiformes order, including its distinct teeth, sub-terminal mouth, and two dorsal fins with spines. This species is known for its unique appearance, with a series of prominent ridges and spines on its dorsal surface.

Horn sharks, with their specialized dentition and feeding habits, play a vital role in maintaining the balance of marine ecosystems. They are fascinating creatures that have adapted to their environment in remarkable ways, allowing them to thrive in their respective habitats.

Order-8:Hexanchiformes (Cow sharks, Frill sharks)

Order-8: Hexanchiformes, known as Cow Sharks or Frill Sharks, exhibit distinct characteristics that set them apart:

1. Cow Sharks: The fish belonging to this order are commonly referred to as Cow Sharks due to their resemblance to cows, particularly in their body shape and size.
2. Single Dorsal Fin: Hexanchiformes have only one dorsal fin located on their backs. Unlike some other shark species, they lack spines in their dorsal fin.
3. Sub-terminal Mouth: The mouth of Cow Sharks is positioned ventrally, similar to Horn Sharks. This adaptation enables them to feed on prey near the ocean floor.
4. Multiple Gill Openings: Unlike many other shark species that typically have five pairs of gill openings, Hexanchiformes have more than five gill openings.
5. Absence of Nictitating Membrane: The eye of Cow Sharks lacks a nictitating membrane, which is a translucent protective eyelid found in some other shark species.
6. Ovo-viviparous Reproduction: Cow Sharks are ovo-viviparous, meaning their eggs develop and hatch inside the female’s body, and the young are born live.

Examples of Cow Sharks include:

• Hexanchus griseus, commonly known as the Bluntnose Sixgill Shark or Cow Shark, is a large species found in various oceans. It is characterized by its six gill slits and broad head.
• Heptranchias perlo, also called the Sharpnose Sevengill Shark, is another member of the Hexanchiformes order. It has seven gill slits and is typically found in deep waters.

Cow Sharks play a crucial role in marine ecosystems, maintaining the balance of predator-prey relationships and contributing to the overall biodiversity of the oceans. Their unique adaptations and behaviors make them fascinating subjects of study for marine biologists and enthusiasts alike.

Sub-class-2: Holocephali

Sub-class-2: Holocephali, commonly known as Chimaeras, exhibit several distinctive characteristics:

1. They possess four pairs of gills, each protected by a single operculum. These gills enable them to extract oxygen from the water.
2. Unlike some other fish species, chimaeras lack a spiracle, which is an opening behind the eye that aids in respiration.
3. As they mature, holocephalians do not develop scales, resulting in a smooth skin texture.
4. Chimaeras do not possess a cloaca, which is a common opening for excretion and reproduction in many aquatic animals.
5. In male chimaeras, a unique reproductive feature called the pelvic intromittent organ is present. It contains the prepelvic tentacula and is located in the anterior head region. This organ aids in mating by utilizing a clasping structure called the tentaculum.
6. The teeth of holocephalians are plate-shaped and permanent. These teeth are adapted for crushing and grinding prey rather than tearing or cutting.
7. Chimaeras have relatively large eyes, which contribute to their well-developed vision in their deep-sea habitats.
8. They reproduce through an oviparous method, meaning that females lay eggs that hatch outside the body.

Examples of holocephalians include species such as:

• Hydrolagus alphus (Whitespot ghost shark)
• Callorhinchus milii (Australian ghost shark)
• Chimaera argiloba (Whitefin chimaera)

These fascinating creatures, belonging to the sub-class Holocephali, showcase unique adaptations and characteristics that distinguish them from other fish species.

Order: Chimaeriformes

Order: Chimaeriformes comprises a group of fish commonly known as ratfishes or chimaeras. They possess several distinctive features that set them apart:

1. The gill opening in chimaeriformes is covered by an operculum, a bony flap that helps protect the delicate gill structures.
2. Chimaeriformes lack both a cloaca, which is a common opening for excretion and reproduction, and a spiracle, which aids in respiration in some other fish species.
3. One notable characteristic of chimaeriformes is the presence of strong spines located in front of the first dorsal fin. These spines serve as a defensive mechanism against potential predators.
4. The tail of chimaeriformes is slender and whip-shaped, contributing to their graceful swimming motion.
5. In chimaeriformes, the upper jaw is directly attached to the skull, providing additional stability and strength during feeding.
6. Both the upper and lower jaws of chimaeriformes are equipped with large, prominent teeth. These teeth are adapted for capturing and consuming a variety of prey.
7. Chimaeriformes reproduce through an oviparous method, meaning that females lay eggs that hatch externally.

An example of a species belonging to the order Chimaeriformes is Chimaera monstrosa, commonly known as the rat fish. It showcases the unique characteristics and adaptations seen in this order.

Chimaeriformes, or ratfishes, represent an intriguing group of fish that demonstrate specialized morphological features and reproductive strategies. Their distinct traits contribute to their successful survival in their respective marine habitats.

Anatomy of Chondrichthyes

The anatomy of Chondrichthyes, which includes sharks, rays, and chimaeras, exhibits several interesting characteristics:

• Skeleton: Chondrichthyans have a cartilaginous skeleton. During development, the notochord, a flexible rod-like structure, is gradually replaced by a vertebral column, except in Holocephali (chimaeras), where the notochord remains intact. Some deepwater sharks have a reduced vertebral column.
• Appendages: With the exception of Holocephali, chondrichthyans have tough skin covered with dermal teeth called placoid scales or dermal denticles. These scales give their skin a sandpaper-like texture. The pectoral and pelvic fins of most species are ventrally connected, providing flexibility and allowing for graceful swimming. Sharks typically possess a heterocercal tail, which aids in locomotion.
• Body covering: Chondrichthyans have tooth-like scales called dermal denticles or placoid scales, which provide protection and streamline their bodies. Some species also possess mucous glands.
• Respiratory system: Chondrichthyans respire through five to seven pairs of gills, depending on the species. Pelagic species require continuous swimming to maintain a constant flow of oxygenated water over their gills. Demersal species, on the other hand, can actively pump water in through their spiracles (small holes located behind each eye) and out through their gills. However, exceptions exist within different species.
• Nervous system: Chondrichthyans have a relatively small brain, 8-10 pairs of cranial nerves, and a spinal cord with spinal nerves. They possess various sensory organs that provide valuable information for processing. These include the ampullae of Lorenzini, which are electroreceptors that sense electric fields in water, aiding in prey detection and navigation. The lateral line system consists of modified epithelial cells that sense motion, vibration, and pressure in the surrounding water. Chondrichthyans generally have well-developed eyes, powerful nostrils, and olfactory organs. Their inner ears, composed of three large semicircular canals, help with balance and orientation. They also have sound-detecting apparatus, although its range is limited. Some species possess electric organs used for defense and predation. Overall, their nervous system is relatively simple, with the forebrain not greatly enlarged.
• Immune system: Chondrichthyans, like other jawed vertebrates, possess an adaptive immune system, which allows them to recognize and respond to specific pathogens and antigens.

The anatomy of Chondrichthyes exhibits remarkable adaptations that enable them to thrive in various aquatic environments. Their unique skeletal structure, body covering, respiratory system, nervous system, and immune system contribute to their success as a diverse group of cartilaginous fish.

Reproduction of Chondrichthyes

Reproduction in Chondrichthyes, which includes sharks, rays, and chimaeras, exhibits various reproductive strategies:

• Fertilization: Chondrichthyans practice internal fertilization, where the male deposits sperm directly into the female’s reproductive tract. This process ensures a higher chance of successful fertilization compared to external fertilization seen in many other fish species.
• Development: Chondrichthyans display different modes of development. The majority of species are ovoviviparous, meaning the embryos develop inside eggs that remain within the female’s body until they hatch. This live birth strategy allows for increased protection and nourishment of the developing embryos. Some chondrichthyans, however, are oviparous, laying eggs that hatch outside the female’s body. A few rare species are viviparous, where the embryos receive nourishment directly from the mother through a placental connection.
• Parental Care: After birth or hatching, there is typically no parental care in Chondrichthyes. The newborn or hatched offspring are self-sufficient and must fend for themselves. However, some species of chondrichthyans do exhibit limited parental care, particularly in guarding their eggs to protect them from predators.
• Capture-Induced Parturition: A phenomenon known as capture-induced parturition has been observed in sharks and rays. This occurs when individuals are caught in fishing gear, leading to premature birth or abortion of their offspring. It is often mistaken for natural birth by recreational fishers and is not commonly considered in commercial fisheries management. Capture-induced parturition has been documented in a significant portion of live-bearing shark and ray species, with at least 12% of species affected.

The reproductive strategies of Chondrichthyes reflect their adaptation to different aquatic environments and the need to ensure the survival of their offspring. From internal fertilization to live birth or egg-laying, these reproductive strategies contribute to the success and diversity of chondrichthyan species.

FAQ

References

• https://biologyeducare.com/chondrichthyes/
• https://collegedunia.com/exams/chondrichthyes-biology-articleid-3169
• https://www.notesonzoology.com/vertebrates/pisces/classification-of-chondrichthyes-vertebrates-chordata-zoology/7830
• https://static1.squarespace.com/static/6454d2c5a7800f4a6e1218d6/t/64703a5bae7b2e6871d75ed8/1685076572211/pomudajetuwix.pdf
• https://rajusbiology.com/chondrichthyes-general-characteristics/
• https://ask.learncbse.in/t/write-the-main-characteristics-of-chondrichthyes/1480
• https://zyrnh.umbebagbag.com/characters-of-chondrichthyes/5336294
• https://ugcmoocs.inflibnet.ac.in/assets/uploads/1/225/6915/et/14_Academic%20Script200319070703032727.pdf