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Actinopterygii vs. Sarcopterygii — What's the Difference?

By Urooj Arif & Maham Liaqat — Updated on March 7, 2024
Actinopterygii, or ray-finned fishes, have fins supported by long, bony rays, and represent the largest group of fishes. Sarcopterygii, or lobe-finned fishes, have fleshy, lobed, paired fins, which are precursors to the limbs of land vertebrates.
Actinopterygii vs. Sarcopterygii — What's the Difference?

Difference Between Actinopterygii and Sarcopterygii


Key Differences

Actinopterygii are characterized by their fins, which are supported by bony spines or rays, facilitating a wide range of movement and flexibility in water. This group includes familiar species such as salmon, trout, and goldfish. Sarcopterygii, in contrast, possess fleshy, lobed fins that are connected to the body by a single bone, resembling the structure of a limb and enabling a different mode of movement. This group includes coelacanths and lungfishes, as well as the ancestors of all terrestrial vertebrates.
The diversity within Actinopterygii is vast, comprising nearly 99% of all fish species. This group has adapted to a wide variety of aquatic habitats, from deep oceans to freshwater streams. Sarcopterygii, however, are far less diverse in today's ecosystems, with only a few species remaining, highlighting their significant evolutionary transition from water to land.
The evolutionary significance of Sarcopterygii lies in their unique fin structure, which provided the foundation for the evolution of legs and the eventual move of vertebrates onto land. Actinopterygii, while remaining aquatic, have evolved an incredible variety of forms and ecological niches, showcasing the adaptability and versatility of ray-finned fishes.
Actinopterygii typically reproduce by external fertilization, where eggs and sperm are released into the water. Sarcopterygii’s reproduction can vary, but lungfish, for example, have developed more complex reproductive strategies, including the building of nests and exhibiting parental care, indicative of their closer relationship to terrestrial vertebrates.
The study of both groups provides valuable insights into the evolutionary history of vertebrates. Actinopterygii shed light on the adaptability and diversification of aquatic life forms, whereas Sarcopterygii are crucial for understanding the evolutionary steps that led to the colonization of land by vertebrates.

Comparison Chart

Fin Structure

Supported by bony rays
Fleshy, lobed fins with a central bone


High, nearly 99% of all fish species
Low, includes few living species


Salmon, trout, goldfish
Coelacanths, lungfish


Diverse: freshwater to deep sea
Primarily freshwater, limited marine

Evolutionary Significance

Showcases adaptability in water
Link to terrestrial vertebrates

Reproductive Strategy

Mainly external fertilization
Varied, some with complex behaviors

Anatomical Adaptations

Wide variety, adapted to various niches
Primitive limbs, precursor to terrestrial movement

Ecological Role

Extensive, filling many aquatic niches
Limited, important for evolutionary study

Compare with Definitions


Ray-finned fishes, fins supported by bony rays.
The goldfish in your pond is an example of Actinopterygii.


Lobe-finned fishes, with fins that resemble limbs.
The ancient coelacanth is a living example of Sarcopterygii.


Adapted to various aquatic environments.
The deep-sea anglerfish is an Actinopterygii adapted to dark, high-pressure environments.


Precursors to amphibians and all terrestrial vertebrates.
Fossil evidence of Sarcopterygii shows the early development of limbs.


Characterized by significant evolutionary adaptability.
The adaptability of Actinopterygii is evident in their colonization of freshwater from marine environments.


Adaptations for a semi-terrestrial lifestyle in some species.
The African lungfish can survive dry seasons by burrowing and entering a state of estivation.


Exhibits a wide range of reproductive strategies.
Salmon migrate upstream for spawning, demonstrating the reproductive diversity of Actinopterygii.


Few surviving species, but crucial for understanding vertebrate evolution.
Lungfish, capable of breathing air, highlight the evolutionary bridge to land vertebrates.


Represents the largest group of fishes.
The diverse species found in coral reefs are mostly Actinopterygii.


Important for studying the transition from aquatic to terrestrial life.
The study of Sarcopterygii provides insights into the evolution of terrestrial locomotion.


Actinopterygii (New Latin actino- ('having rays') + Greek πτέρυξ (ptérux 'wing, fins')), members of which are known as ray-finned fishes, is a clade (traditionally class or subclass) of the bony fishes.The ray-finned fishes are so-called because their fins are webs of skin supported by bony or horny spines (rays), as opposed to the fleshy, lobed fins that characterize the class Sarcopterygii (lobe-finned fish). These actinopterygian fin rays attach directly to the proximal or basal skeletal elements, the radials, which represent the link or connection between these fins and the internal skeleton (e.g., pelvic and pectoral girdles).


Sarcopterygii (; from Greek: σάρξ sarx 'flesh' and πτέρυξ pteryx 'fin') — sometimes considered synonymous with Crossopterygii ("fringe-finned fish", from Greek κροσσός krossos 'fringe') — is a taxon (traditionally a class or subclass) of the bony fishes whose members are known as lobe-finned fishes. The group Tetrapoda, a superclass including amphibians, reptiles (including dinosaurs and therefore birds), and mammals, evolved from certain sarcopterygians; under a cladistic view, tetrapods are themselves considered a group within Sarcopterygii.

Common Curiosities

Why are Actinopterygii more diverse than Sarcopterygii?

Actinopterygii have adapted to a wide range of aquatic environments, leading to their extensive diversity. Sarcopterygii, though less diverse today, represent an important evolutionary step towards terrestrial life.

What role do Actinopterygii play in aquatic ecosystems?

Actinopterygii fill numerous ecological niches in aquatic ecosystems, ranging from predators to filter feeders, playing crucial roles in maintaining the balance of these environments.

How did Sarcopterygii contribute to the evolution of terrestrial vertebrates?

Their lobed fins, resembling primitive limbs, were crucial in the evolution from aquatic to terrestrial environments, leading to the development of legs and the first amphibians.

Are there any living examples of Sarcopterygii?

Yes, living examples include the coelacanths and lungfish, which are studied for their evolutionary connection to terrestrial vertebrates.

Can Sarcopterygii live out of water?

Some Sarcopterygii, like lungfish, have adaptations allowing them to survive out of water for extended periods, particularly in environments subject to seasonal drying.

How does the reproductive behavior of Actinopterygii differ from that of Sarcopterygii?

Actinopterygii generally reproduce through external fertilization, whereas Sarcopterygii can exhibit more varied reproductive strategies, including internal fertilization and parental care, similar to terrestrial vertebrates.

How do the habitats of Actinopterygii and Sarcopterygii compare?

Actinopterygii are found in a wide range of aquatic habitats, from the deep sea to freshwater streams. Sarcopterygii are primarily found in freshwater environments, with a few species in marine settings.

What is the significance of the coelacanth in understanding evolutionary biology?

The coelacanth, a Sarcopterygii, was once thought extinct but its discovery in modern times provided invaluable insights into the evolution of vertebrates from water to land, due to its limb-like fin structure.

How do scientists study the evolutionary transition from Sarcopterygii to terrestrial vertebrates?

Scientists study this transition through the fossil record, comparative anatomy, and molecular biology, tracing the morphological and genetic changes that facilitated the move from aquatic to terrestrial environments.

What role does genetic research play in understanding the differences between Actinopterygii and Sarcopterygii?

Genetic research helps clarify the evolutionary relationships between these groups, revealing how genetic variations have led to their distinct anatomical features and adaptive strategies, further illuminating the path of vertebrate evolution.

What are the main anatomical differences between Actinopterygii and Sarcopterygii?

The key anatomical difference lies in their fins: Actinopterygii have fins supported by bony rays, while Sarcopterygii have fleshy, lobed fins with a central bone, resembling the structure of limbs.

Why are Sarcopterygii less diverse today?

Sarcopterygii's decline in diversity is attributed to their evolutionary transition from water to land, leading to the emergence of terrestrial vertebrates. Their aquatic descendants are fewer in number due to this pivotal shift in their evolutionary history.

What challenges do Sarcopterygii face in the modern world?

Modern Sarcopterygii, like the coelacanth and lungfish, face challenges such as habitat loss, pollution, and climate change, which threaten their survival and limit our understanding of vertebrate evolution.

How do Actinopterygii adapt to their environments?

Actinopterygii exhibit a wide range of adaptations, including body shapes, coloration for camouflage, and specialized feeding mechanisms, allowing them to thrive in diverse aquatic environments.

Can Sarcopterygii be considered a link between fish and amphibians?

Yes, Sarcopterygii are considered an evolutionary link between aquatic fish and amphibians, as their anatomy showcases early adaptations for life on land, such as their limb-like fins.

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Author Spotlight

Written by
Urooj Arif
Urooj is a skilled content writer at Ask Difference, known for her exceptional ability to simplify complex topics into engaging and informative content. With a passion for research and a flair for clear, concise writing, she consistently delivers articles that resonate with our diverse audience.
Co-written by
Maham Liaqat

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