Final answer:
The small fleshy fin near the end of some fish species is called a lobe fin, found on lobe-finned fish such as coelacanths and lungfish. These muscular, fleshy fins differentiate lobe-finned fish from the ray-finned fish, and they are structurally similar to the limbs of early tetrapods.
Step-by-step explanation:
The small fleshy fin near the end of some fish species is known as a lobe fin. This type of fin can be found on lobe-finned fish, which are part of a group of bony fish. Lobe-finned fish, such as coelacanths and lungfish, have fins that contain a stump-like appendage of bone and muscle.
In contrast to the ray-finned fish, which includes species like tuna and salmon where the fins are supported by bony rays or spines without muscles, the lobe fins of lobe-finned fish are muscular and fleshier.
The presence of these fleshy fins is a significant feature that distinguishes lobe-finned fish from the more common ray-finned fish. Ray-finned fish have fins with webs of skin over flexible bony rays, and unlike lobe-finned fish, their fin movements are controlled by muscles in the body wall rather than muscles within the fins themselves.
The lobe fins are considered evolutionarily significant as they are similar in structure to the limbs of early tetrapods, indicating a close relationship with the ancestors of amphibians, reptiles, birds, and mammals.