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The unique characteristics of diurnal vertebrates—those that are active during the day—are primarily associated with their visual adaptations that allow them to see in bright light. Higher cone density is the key adaptation that distinguishes diurnal vertebrates, as cones are photoreceptor cells responsible for color vision and work best in well-lit conditions.

In contrast, nocturnal vertebrates, which are active during the night, possess a greater concentration of rods, another type of photoreceptor. Rods are highly sensitive to low light, making them more effective in dim environments but less effective for color differentiation. This distinction is crucial, as it highlights how vertebrates have evolved their visual systems to thrive in their respective activity periods—day and night—by optimizing either rod or cone photoreceptors depending on their lifestyle.

The other options, such as wider pupils or larger lenses, are adaptations that can influence how much light enters the eye or how well it is focused, but do not primarily define the difference in visual adaptations related to the time of activity.