Question: What colours can a Catshark see?

Lesser-Spotted Catshark answered on 3 Nov 2017:

Catsharks don’t have very good colour vision. In fact we think that, like most sharks, they have ‘monochromatic’ vision meaning they have only one visual pigment and can’t distinguish colours. Instead, they see them as different shades and distinguish them by contrast. Perhaps to compensate for this, catsharks are only one of 3 groups of chondrichthyans which have evolved biofluorescence; they produce their own light. This appears green to us under UV light, but to other catsharks it increases the contrast between their spots and stripes making it easier to spot each other.