1. There are two kinds of photoreceptor cells , cone cell and rod cell, in the retina.

2. Human eyes have three cone cell types, with each type dedicated to receiving either blue, green or red light. This allows most people to tell the difference between different colored objects.

3. Human eyes have three cone cell types, with each type dedicated to receiving either blue, green or red light.

4. But no cone cells were observed in 10 of the 17 species, while only one type of cone cell appeared to be present in the other 7.

5. The scientists, who examined retinas of 17 different species of shark, discovered that the creatures had only one type of colour-sensitive cell, known as a cone cell, in their eyes.

6. This was the clue that the high oxygen levels that occur naturally in the retina after rods die was the suspect regarding cone cell death.

7. In turn lead to vascular stenosis, microcirculation, depending on the rod cell death, depending on the cone cell death.