Final answer:
Rods are for low-light vision and do not see color, while cones provide color vision and are concentrated in the fovea centralis. Humans have three types of cones. The mantis shrimp has one of the most advanced color vision systems, with up to 16 types of photoreceptor cells.
Step-by-step explanation:
Rods and cones are types of photoreceptor cells found in the retina of the eye. Rods are responsible for vision at low light levels (scotopic vision), and they do not mediate color vision. On the other hand, cones are active at higher light levels (photopic vision) and are capable of color vision as well as fine spatial acuity. The highest concentration of cones is located in the fovea centralis, a small central pit composed of closely packed cones in the eye.
Humans typically have three types of cones that correspond to different wavelengths of light: S-cones (blue), M-cones (green), and L-cones (red). When it comes to color differentiation, the question of 'color crab' seems to be a misunderstanding, as crabs typically have a different visual system from humans. However, among the most sophisticated visual systems in the animal kingdom is that of the mantis shrimp, which possesses up to 16 types of photoreceptor cells for color analysis, more than any other creature, including humans.