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Difference Between Chondrichthyes and Osteichthyes (NEET)

Biology · Animal Kingdom · NEET

Chondrichthyes are cartilaginous fishes (like sharks). Their skeleton is made of soft cartilage, they have no gill cover (operculum) and no air bladder, so they must keep swimming to avoid sinking. Osteichthyes are bony fishes; they have a hard bony skeleton, an operculum covering the gills, and an air bladder that keeps them floating. Memory hook: "Chondro = Cartilage, Osteo = Bone" — same as your bones (osteo-porosis) being bony.

At a glance

EndoskeletonCartilage (soft, flexible)Bone (hard)
Operculum (gill cover)Absent; gill slits separatePresent; covers gills
Air bladderAbsent (must swim to avoid sinking)Present (regulates buoyancy)
ScalesPlacoid scalesCycloid or ctenoid scales
Mouth positionVentral (lower side)Mostly terminal (front tip)
ExamplesScoliodon, Pristis, Trygon, TorpedoExocoetus, Pterophyllum, Betta, Labeo, Hippocampus
Chondrichthyes vs OsteichthyesChondrichthyes (Shark)Cartilage skeletonNo operculum, No air bladderPlacoid scales, ventral mouthMust swim to avoid sinkingOsteichthyes (Rohu)Bony skeletonOperculum + Air bladderCycloid/ctenoid scales, terminal mouthAir bladder keeps it afloat
Left: Chondrichthyes (shark) with cartilage skeleton, exposed gill slits (no operculum) and no air bladder, so it must swim constantly. Right: Osteichthyes (bony fish) with a bony skeleton, operculum covering the gills, and an air bladder (shown as the oval sac) that controls buoyancy.

Your doubts, answered

What is the main difference between Chondrichthyes and Osteichthyes?

The main difference is the skeleton. Chondrichthyes (like sharks and rays) have a skeleton made of CARTILAGE, which is soft and flexible. Osteichthyes (like Rohu, Catla) have a skeleton made of BONE, which is hard. This is why the names help you: 'Chondro' means cartilage and 'Osteo' means bone. For NEET, this cartilage-vs-bone point is the most tested fact.

Which fish has an air bladder, Chondrichthyes or Osteichthyes?

Only Osteichthyes (bony fishes) have an air bladder. Chondrichthyes do NOT have an air bladder. The air bladder is a gas-filled sac that controls buoyancy, meaning it helps the fish stay floating without effort. Because sharks (Chondrichthyes) have no air bladder, they would sink if they stopped, so they must swim all the time. NEET loves this exact point.

Why do sharks (Chondrichthyes) have to swim constantly?

Sharks belong to Chondrichthyes and they lack an air bladder. The air bladder is what keeps bony fishes floating at rest. Without it, a shark is heavier than water and slowly sinks. So it must keep swimming to push water and stay up. NCERT states this directly: 'Due to the absence of air bladder, they have to swim constantly to avoid sinking.'

Does Chondrichthyes have an operculum (gill cover)?

No. In Chondrichthyes the gill slits are separate and open directly to the outside, without any operculum (gill cover). In Osteichthyes the gills ARE covered by an operculum. This is a common NEET trap: the statement 'All pisces have gills covered by an operculum' is WRONG, because cartilaginous fishes have no operculum.

What type of scales and mouth position does each class have?

Chondrichthyes have tough skin with tiny placoid scales, and their mouth is located ventrally (on the lower side). Osteichthyes have cycloid or ctenoid scales, and their mouth is mostly terminal (at the front tip). Also, teeth in Chondrichthyes are modified placoid scales pointing backward, which is a favourite NEET detail.

Are the examples of Chondrichthyes and Osteichthyes different?

Yes, memorising examples wins matching questions. Chondrichthyes: Scoliodon (dogfish), Pristis (sawfish), Trygon (stingray, has a poison sting), Torpedo (electric ray). Osteichthyes: Exocoetus (flying fish), Pterophyllum (angel fish), Betta (fighting fish), Hippocampus (sea horse), Labeo (Rohu), Catla. NEET repeatedly asks you to sort these names into the correct class.

⚠️ The NEET trap
✗ All fishes (pisces) have gills covered by an operculum.
✓ Only Osteichthyes (bony fishes) have an operculum. In Chondrichthyes the gill slits are separate and have NO operculum.
🧠 Operculum = only in Osteichthyes (both start with 'O'). If you see 'ALL pisces have operculum', mark it FALSE.

Real NEET questions

2020

Match the following columns and select the correct option. Column-I: (a) 6-15 pairs of gill slits, (b) Heterocercal caudal fin, (c) Air bladder, (d) Poison sting Column-II: (i) Trygon, (ii) Cyclostomes, (iii) Chondrichthyes, (iv) Osteichthyes

A · a-iv, b-ii, c-iii, d-i
B · a-i, b-iv, c-iii, d-ii
C · a-ii, b-iii, c-iv, d-i ✓
D · a-iii, b-iv, c-i, d-ii
Solution: Cyclostomes have 6-15 pairs of gill slits, so a-ii. The unequal (heterocercal) tail fin is a feature of cartilaginous fishes, so b-iii (Chondrichthyes). The air bladder is present only in bony fishes, so c-iv (Osteichthyes). The poison sting is borne by Trygon (sting ray), so d-i. Answer: a-ii, b-iii, c-iv, d-i = option C. Remember: air bladder = Osteichthyes, never Chondrichthyes.
2016

Which of the following characteristic features always holds true for the corresponding group of animals?

A · Cartilaginous endoskeleton - Chondrichthyes ✓
B · Viviparous - Mammalia
C · Possess a mouth with an upper and a lower jaw - Chordata
D · 3-chambered heart with one incompletely divided ventricle - Reptilia
Solution: A cartilaginous endoskeleton is ALWAYS true for Chondrichthyes, so option A is correct. The others fail: some mammals lay eggs (Ornithorhynchus), so not all are viviparous; jawless cyclostome chordates have no jaws; and crocodiles (reptiles) have a 4-chambered heart. This confirms the defining feature of Chondrichthyes is its cartilage skeleton.
2026

Select the set of fishes which belong to the class Osteichthyes:

A · Devil fish, Cuttlefish and Hagfish
B · Saw fish, Fighting fish and Dog fish
C · Starfish, Hagfish and Cuttlefish
D · Flying fish, Angel fish and Fighting fish ✓
Solution: Osteichthyes are bony fishes. Flying fish (Exocoetus), Angel fish (Pterophyllum) and Fighting fish (Betta) are all Osteichthyes, so option D. The wrong options mix in Chondrichthyes (Saw fish = Pristis, Dog fish = Scoliodon), a cyclostome (Hagfish = Myxine), molluscs (Cuttlefish, Devil fish) and an echinoderm (Starfish).

Solved Animal Kingdom NEET PYQs

Try the real previous-year questions from this chapter — each with the answer and a full solution.

See all 41 Animal Kingdom NEET PYQs ›
Next concept: What Is the Air Bladder and Why Bony Fishes Have It →Keep learning — 2 minFeeling ready? Solve the Animal Kingdom NEET PYQs ›Or practice on your phone — get the free MedicNEET app ›

Frequently asked

Is a shark a Chondrichthyes or Osteichthyes?

A shark (Scoliodon) is a Chondrichthyes, a cartilaginous fish. Its skeleton is cartilage, it has no operculum and no air bladder, so it must swim constantly to avoid sinking.

Do both classes have gills?

Yes, both are aquatic and breathe using gills. The difference is that Osteichthyes have their gills covered by an operculum, while Chondrichthyes have separate gill slits with no operculum.

What is the heart structure in these fishes?

Both Chondrichthyes and Osteichthyes have a two-chambered heart (one auricle and one ventricle) and both are cold-blooded (poikilothermic). The heart chamber count is the same, so do not use it to tell them apart.

Are they oviparous or viviparous?

Osteichthyes are mostly oviparous (egg-laying) with external fertilisation and direct development. Many Chondrichthyes show internal fertilisation, and some are viviparous, which is a point NEET sometimes tests.

Which class is more common in NEET matching questions?

Both appear often together. NEET usually gives a mixed list of fish names and asks you to sort them, or pairs a feature (air bladder, operculum, placoid scales, cartilage) with the correct class. Learn the examples and the 4 key features to score every time.

Related concepts:
Class Chondrichthyes: Cartilaginous FishesClass Osteichthyes: Bony Fishes and the Air BladderWhat Is the Air Bladder and Why Bony Fishes Have ItClass Cyclostomata: Jawless Ectoparasitic FishesHow Many Heart Chambers in Fish, Amphibians, Reptiles, Birds and Mammals?
Practise Biology for NEET:
Biology MCQs (free, chapter-wise)Animal Kingdom PYQs