Chondrichthyes And Osteichthyes: A Deep Dive Into Cartilaginous And Bony Fish

Fish come in all different shapes, sizes, and types. But when it comes down to it, most fish fall into one of two categories: chondrichthyes or osteichthyes. What’s the difference? Whether a fish has a cartilaginous or a bony skeleton.

In short: Chondrichthyes are cartilaginous fish like sharks and rays, while osteichthyes are bony fish like tuna and trout. Read on as we dive deeper into the anatomy, evolution, diversity, and more when it comes to these two major fish groups.

Defining Key Differences Between Chondrichthyes and Osteichthyes

Skeletal Structures: Cartilage vs. Bone

One of the most significant differences between chondrichthyans (sharks, rays, and skates) and osteichthyans (bony fish) is in their skeletal structure. Chondrichthyes have a cartilaginous skeleton made up of flexible, elastic cartilage while Osteichthyes possess a bony endoskeleton of calcified bone.

The cartilage skeleton of chondrichthyans is lighter, more flexible, and able to withstand greater pressure than bone. This aids in their hydrodynamic body shape and facilitates deep dives. However, cartilage grows slower than bone and lacks the calcium salts that provide rigidity.

Osteichthyans’ mineralized bones provide more robust structural support and protection.

Interestingly, both groups have jaws made from cellular cartilage rather than true bone. This delivered strength with some springiness. While the cartilage jaw evolved independently in the two lineages, it enabled more active predation. The innovation was key to their evolutionary success.

Habitats and Evolutionary History

The habitat and evolutionary history of chondrichthyans and osteichthyans also diverged significantly. Chondrichthyes originated in the oceans over 400 million years ago during the Devonian period. They are exclusively marine creatures.

In contrast, Osteichthyes emerged around 420 million years ago with ancestors that lived in freshwater environments. They later branched into saltwater species.

The move into freshwater was a major transition that required adaptations like gills able to extract oxygen from water with lower dissolved oxygen levels. Kidneys able to produce dilute urine were also key. This enabled osteichthyans to escape competition and predation from marine species.

It allowed them to radiate and occupy new ecological niches.

Today, Osteichthyes represent over 95% of extant fish species. They display staggering diversity and inhabit waters ranging from high mountain elevations to the abyssal zone. Chondrichthyans occupy a smaller niche focused on marine habitats. Nonetheless, over 1,000 species thrive around the world.

Chondrichthyes: The Cartilaginous Fish

Sharks

Sharks are some of the most iconic cartilaginous fish. There are over 500 known species of sharks living in oceans all over the world. Sharks have a skeleton made of cartilage rather than bone, giving them a very flexible, streamlined body shape.

Their skin is covered in tooth-like scales called placoid scales, and most species have very rough skin that reduces drag while swimming. One of the most distinguishing features of sharks are their powerful jaws filled with serrated, triangular teeth that are constantly replaced throughout their lives.

Sharks are apex predators that play an important role in maintaining balance in ocean ecosystems. Some interesting shark species include:

• The great white shark, made famous by the movie Jaws, that can grow over 20 feet long.
• The megamouth shark, which can grow to 18 feet but eats mostly plankton through filtration.
• The goblin shark, which looks prehistoric with its unusual snout and protractible jaws for grabbing prey.

While attacks on humans are extremely rare, sharks are still greatly feared by many. However, sharks are now vulnerable and their populations are declining worldwide. According to the IUCN Red List, over 30% of open ocean shark species are threatened with extinction, primarily due to overfishing.

Conservation efforts to protect sharks and restore their populations are crucial for maintaining balance in ocean ecosystems.

Rays and Skates

Rays and skates are flattened, disc-shaped cartilaginous fish that are closely related to sharks. There are over 600 species of rays and skates, including stingrays, manta rays, electric rays, and guitarfish.

Like sharks, rays and skates have skeletons made of cartilage and tooth-like placoid scales covering their skin. One of their most unique features is having their pectoral fins expanded into large, flat, wing-like shapes that allow them to gracefully glide through the water.

While some rays like manta rays filter feed on plankton, most are active predators that hunt fish and invertebrates on the seafloor. Some interesting species include:

• Manta rays, which are the largest rays with wingspans up to 23 feet wide.
• Electric rays like the torpedo ray that can generate powerful electric shocks to stun prey.
• Sawfish, which are guitarfish with long, saw-like snouts lined with sharp teeth for slashing through schools of fish.

Rays and skates often lay their spiral-shaped egg cases on the seafloor, sometimes in very large groups called ray nurseries. Many species live in shallow coastal waters and estuaries, making them vulnerable to habitat loss and degradation.

According to the IUCN Red List, 25% of ray and skate species are now threatened with extinction primarily due to overfishing and bycatch.

Chimaeras

Chimaeras, also known as ghost sharks or ratfish, represent the oldest living lineage of cartilaginous fish. There are around 50 known species of chimaeras found in oceans worldwide, typically in deep water habitats. They have elongated, tapered bodies and large eyes.

Their most distinctive feature is their large forehead topped with a rabbit-ear like projection. Chimaeras have venomous spines in front of their dorsal fins and males also have a retractable sexual organ on their forehead used for internal fertilization.

They feed on worms, mollusks, crustaceans and small fish by grinding prey with their tooth plates. Some interesting species include:

• The giant chimaera which can grow over 5 feet long and has a vicious spine capable of inflicting painful stings.
• The ghost shark which has a retractable sex organ on its forehead used to internally fertilize eggs.
• The spookfish which has an upward looking eye that allows it to search for prey overhead.

Chimaeras are rarely encountered by humans due to their deep ocean habitats. However, they are vulnerable to deep sea fishing activities. According to the IUCN Red List, at least 15 species of chimaeras are considered threatened, endangered or critically endangered.

Protecting deep ocean environments will be key for preserving these fascinating living fossils.

Osteichthyes: Fish With Bony Skeletons

Ray-Finned Fish

Ray-finned fish, or Actinopterygii, make up over 99% of living fish species today. As their name implies, their fins contain bony support rays or “spines”, rather than being fleshy like other fish. Well-known species of ray-finned fish include tuna, bass, salmon, and trout.

Ray-finned fish come in an incredibly diverse array of shapes, sizes, colors and adaptations for habitats ranging from the deep sea to high mountain streams.

There are two major groups of the stunning diversity of ray-finned fish: teleosts and non-teleosts. Modern teleosts represent about 96% of all ray-finned fish species. They are characterized by a homocercal tail (equal sized top and bottom lobes), flexible jaw connections, and a more efficient gas bladder to maintain buoyancy (🎉).

Some of the most extreme adaptations are seen in families like deep-sea anglerfish and flatfish.

Lobe-Finned Fish

Unlike other fish, the fins of lobe-finned fish contain bones connected to the skeletal system rather than just rays. This gives them greater ability for walking and crawling along the seafloor. Ancient lobe-fins were the first vertebrates to evolve true limbs for terrestrial movement.

There are only eight living species today in two families: lungfish and coelacanths.

• Lungfish can breath air using primitive lungs (hence the name). They are found in freshwater habitats in South America, Africa and Australia.
• Coelacanths were thought to be extinct until discovered in 1938 off the east coast of South Africa. Subsequent studies revealed two coelacanth species living in deep tropical waters of the Western Indian Ocean.

These small populations of lobe-finned fish are literally living fossils (😊). But over 385 million years ago, ancient lobe-fins underwent dramatic changes to adapt to permanent life on land…eventually evolving into the first tetrapods including amphibians, reptiles, dinosaurs and mammals like us!

The Diversity of Bony Fish

The astonishing diversity of bony fish includes over 30,000 species inhabiting our oceans, lakes and rivers (👏). As humans study this amazing taxonomic class more closely, we continue discovering surprising new species on a regular basis.

Just in 2021, researchers identified 216 new species of fish across the world.

This incredible variety of evolutionary adaptations makes bony fish one of the most successful vertebrate groups for thriving across aquatic realms. Sadly overfishing and habitat destruction now threaten many species.

Much remains to be learned about these diverse fish that could hold medical wonders and secrets of survival! (🤔💡)

Similarities and Remaining Questions

Shared Characteristics and Behaviors

Chondrichthyes (sharks, rays, skates) and Osteichthyes (bony fish) share many common characteristics and behaviors as jawed aquatic vertebrates. Both groups are equipped with jaws, fins, gills, and a lateral line system for sensing vibrations in the water.

They also share basic organ systems like a circulatory system with a heart to pump blood, a digestive system, and a nervous system.

When it comes to behavior, most species are mobile swimmers that breathe through their gills and hunt for food. Parental care of young is seen in some species of both groups. Additionally, many cartilaginous and bony fish demonstrate schooling behavior for protection from predators.

However, there are also some key differences between the two groups. Cartilaginous fish have skeletons made of cartilage while bony fish have skeletons made of bone. Bony fish also have scales covering their skin and a swim bladder to control buoyancy, features lacking in sharks and rays.

The method of reproduction differs as well – cartilaginous fish utilize internal fertilization while most bony fish use external fertilization.

Outstanding Areas for Future Research

While much has been discovered about cartilaginous and bony fish, there are still many unknowns that warrant further scientific investigation. Some outstanding questions include:

• What are the genetic factors that regulate skeletal development in each group, leading to differences in cartilage versus bone?
• How exactly does schooling behavior evolve and what are the neural mechanisms that allow large coordinated movements?
• What enables some deep sea species to survive at extreme depths with high water pressure?
• How do the electrosensory systems of chondrichthyes and osteichthyes compare in structure and function?
• What can comparisons of their circulatory systems reveal about the evolution of terrestrial vertebrates?

Studying the physiology, genetics, behavior, and evolution of cartilaginous and bony fish still has much to teach us about vertebrates, including humans. Comparative research examining the similarities and differences between the two groups will shed light on how vertebrates adapt to their environments.

Discoveries could also inform conservation efforts for vulnerable or endangered aquatic species. More investigation is needed to fully understand these diverse jawed fish.

Conclusion

While chondrichthyes and osteichthyes may seem quite different, they have co-existed for hundreds of millions of years across Earth’s waters. As studies advance, we continue unveiling new insights into cartilaginous and bony fish alike.

But no matter how much we discover, the diversity and wonder of aquatic life never ceases to amaze.