Difference between Bony Fish and Cartilaginous Fish

Hundreds of millions of years of evolution have shaped the incredible diversity of aquatic life on our planet. One of the most intriguing branches of this evolutionary tree involves two seemingly similar types of fish, bony and cartilaginous fish. While they may appear alike on the surface, a closer look reveals stark differences in their skeletal structures, adaptations, and habitats. Our planet hosts a wide range variety of fish species, with estimates hovering around 20,000 different types. To accommodate this diversity, these aquatic creatures are classified under the superclass Pisces within the Chordata class, primarily because fish make up over half of all chordates.

Within the Pisces superclass, fish are further categorized into two major groups: Osteichthyes and Chondrichthyes, representing bony fish and cartilaginous fish, respectively.

The Skeleton Divide: Bony vs. Cartilaginous

The most conspicuous distinction between bony and cartilaginous fish lies in their skeletons. Bony fish, as the name suggests, has a skeleton composed entirely of bones, much like the human skeletal system. In contrast, cartilaginous fish rely on a framework made entirely of cartilage, the same flexible tissue found in our noses and ears.

Chondrichthyes

1. Cartilaginous fish, belonging to the class Chondrichthyes, inhabit marine environments.
2. Their skeletons are primarily composed of cartilage, and a persistent notochord supports their bodies throughout their lifespans.
3. The skin of cartilaginous fish is covered in placoid scales, or dermal denticles, offering protection against external forces.
4. The caudal fin is heterocercal, with unequal lobes externally and internally.
5. They possess powerful jaws armed with teeth modified from placoid scales, which are backwardly directed.
6. They feature 5 to 7 pairs of gills that open directly outside the body through gill slits, as they lack opercula (gill covers). The gills are sheet-like (lamelliform)
7. The mouth is located ventrally on the head.
8. They are formidable predators with strong jaws, and their predaceous nature is well-established.
9. They excrete urea. They also retain urea within their body to maintain osmotic balance.
10. They lack air bladders and lungs, necessitating constant swimming to prevent sinking.
11. Their unique spiral valve or scroll valve in the intestine increases surface area for digestion, and a bilobed liver is a common characteristic.
12. Some cartilaginous fish possess specialized structures called “Ampullae of Lorenzini,” serving as thermoreceptors.
13. Reproduction in cartilaginous fish varies, but many are viviparous, with internal fertilization aided by male “claspers,” which are developed from the inner edges of pelvic fins.

Examples

1. Sharks: Examples include Scoliodon sorrakowah (Indian dogfish), Rhincodon typus (whale shark, the largest fish), Sphyrna zygaena (hammerhead shark), and Carcharodon (Great white shark).
2. Rays: Examples include Torpedo (electric ray), Trygon or Dasyatis (stingray), and Pristis (sawfish).

Osteichthyes

1. Osteichthyes, or bony fish, are highly diverse and are found in both freshwater and marine environments.
2. Their skeletons are made of bones, and their exoskeletons feature scales, which can be cycloid, ctenoid, or ganoid types, differing from the placoid scales of cartilaginous fish.
3. The caudal fin is homocercal, and the upper and lower lobes of the tail are about the same size.
4. They rely on four pairs of gills for respiration, covered by opercula on each side of their bodies. The gills are thread-like (filiform).
5. They have terminal or subterminal mouths with teeth found in their jaws.
6. The suspensorium of the jaw is autostylic.
7. They are generally ammonotelic, excreting ammonia as their primary nitrogenous waste. Ammonia is more toxic but easier to excrete in aquatic environments.
8. They use air bladders, which may also serve as hydrostatic organs for maintaining buoyancy.
9. They lack a spiracle and scroll valve in the intestine, and they possess an annus instead of a cloacal aperture. Bony fish do not feature the “Ampullae of Lorenzini” thermoreceptor.
10. Reproduction in bony fish includes oviparous, ovoviviparous, and viviparous species, depending on the type.

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Examples

Marine Fishes

• Exocoetus (Flying Fish): Flying fish are known for their ability to leap out of the water and glide through the air for short distances using their wing-like pectoral fins.
• Hippocampus (Seahorse): Seahorses are characterized by their unique appearance, with a horse-like head and a prehensile tail. The male seahorse has a brood pouch in which it carries and protects the developing eggs.

Freshwater Fishes

• Labeo rohita (Rohu): Rohu is a popular freshwater fish in South Asia and is known for its culinary importance.
• Catla (Katla): Katla is another widely consumed freshwater fish in South Asia.
• Clarias (Magur): Clarias, also known as walking catfish, can survive out of water for a limited time and are capable of moving short distances on land.

Brackish Water Fish

Lates calcarifer (Bhetki): Lates calcarifer, also known as barramundi or Asian sea bass, is a popular brackish water and marine fish known for its delicious taste.

Aquarium Fishes

• Betta Splendens (Siamese Fighting Fish): Betta splendens are known for their vibrant colors and elaborate fins. They are kept as ornamental fish and are often aggressive, especially towards other males.
• Pterophyllum (Angel Fish): Angelfish are known for their graceful appearance, with tall, triangular bodies and long fins. They are popular in aquariums for their beauty.

Summary

In the realm of aquatic life, two major categories of fish, bony (Osteichthyes) and cartilaginous (Chondrichthyes), showcase distinctive characteristics and adaptations. The primary divergence lies in their skeletal compositions, with bony fish possessing bone-based skeletons and cartilaginous fish relying on cartilage. While bony fish inhabit various aquatic environments, including freshwater and marine habitats, cartilaginous fish predominantly reside in marine waters. The differences extend to their caudal fins, mouths, gills, the presence of a cloaca, scales, and reproductive methods. Notably, cartilaginous fish feature heterocercal tails, ventral mouths, and lamelliform gills, while bony fish exhibit homocercal or diphycercal tails, terminal mouths, and filiform gills. Additionally, cartilaginous fish employ internal fertilization, often involving specialized male claspers, and are mostly viviparous, while bony fish generally reproduce externally and are predominantly oviparous. Understanding these disparities sheds light on the fascinating diversity within the aquatic world, showcasing the remarkable adaptations that have evolved over hundreds of millions of years.

FAQs on the Difference Between Bony Fish and Cartilaginous Fish

What is the main difference between bony fish and cartilaginous fish?

The primary difference lies in their skeletal composition. Bony fish have skeletons made of bones, similar to humans, while cartilaginous fish have skeletons made of cartilage, a flexible tissue found in our noses and ears.

Where do bony fish and cartilaginous fish typically live?

Bony fish can be found in a wide range of environments, including marine, freshwater, and brackish water. Cartilaginous fish are primarily marine, inhabiting saltwater environments.

What are the key characteristics of bony fish?

Bony fish have scales (cycloid, ctenoid, or ganoid), homocercal or diphycercal caudal fins, terminal mouths, filiform gills, and a swim bladder for buoyancy. They typically reproduce externally, with oviparous, ovoviviparous, and viviparous species.

How do cartilaginous fish differ from bony fish in terms of gills and mouths?

Cartilaginous fish have lamelliform gills (sheet-like) and ventral mouths, while bony fish have filiform gills (thread-like) and terminal or subterminal mouths.

What is the significance of the presence or absence of a cloaca in these fish?

Cartilaginous fish have a cloaca, while bony fish lack this structure. The cloaca is a common chamber for excretion and reproduction in cartilaginous fish.

Do bony fish and cartilaginous fish have air bladders?

Cartilaginous fish lack air bladders and must constantly swim to avoid sinking. Bony fish, on the other hand, have air bladders, which help them maintain buoyancy.

How do cartilaginous fish reproduce, and what are claspers?

Cartilaginous fish typically reproduce internally. Male cartilaginous fish have specialised structures called claspers that aid in internal fertilization. These claspers are developed from the inner edges of pelvic fins.

Can you provide examples of bony and cartilaginous fish?

Bony fish examples include Rohu, Katla, Magur, and Betta splendens. Cartilaginous fish examples include Indian dogfish, whale shark, hammerhead shark, and stingray.

What are some unique adaptations of cartilaginous fish?

Cartilaginous fish have unique adaptations such as placoid scales, backward-directed teeth, thermoreceptors called Ampullae of Lorenzini, and a spiral valve in their intestine for digestion.

Why are these distinctions between bony and cartilaginous fish important?

Understanding these distinctions is vital for marine biology, ecology, and conservation efforts. It helps researchers and enthusiasts appreciate the incredible diversity and adaptations within the aquatic world, contributing to the preservation of these species and their ecosystems.