An actinopterygian is a ray-finned fish of the class Actinypterygii.
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The class Actinopterygii, which includes most bony fish, is primarily ectothermic, meaning they rely on external environmental temperatures to regulate their body heat. However, some species within this class, such as certain tunas and billfish, exhibit regional endothermy, allowing them to maintain higher body temperatures in specific tissues, which enhances their swimming performance and metabolic efficiency. Overall, the majority of actinopterygian fish do not possess true endothermic capabilities.
The Actinopterygii, or ray-finned fishes, constitute a class or subclass of the bony fishes. The ray-finned fishes are so called because they possess lepidotrichia or "fin rays", their fins being webs of skin supported by bony or horny spines "rays", as opposed to the fleshy, lobed fins that characterize the class Sarcopterygii which also, however, possess lepidotrichia. 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.