There are two major types of visual pigments-

1. Rhodopsins, which are found throughout the vertebrates, and

2. Porphyropsins, which are restricted mainly to some teleosts fish, amphibians and aquatic reptiles.

In addition to these, three other types of lens pigmentation occur in the visual systems of fish living in various habitats-

1. Kynurenine or 3-hydroxykynurenine, (also found in humans) or their derivatives, (absorption at 370 nm)

2. Carotenoids ( absorption at 425-480 nm), and

3. Mycosporine-like amino acid (MAAs) (300-360 nm).

Mesopelagic fish contain carotenoids and kynurenine or its derivatives while the lenses of many shallow water fish contain MAAs.

Scientists have long believed that the ability to perceive UV light is virtually non-existent in vertebrate life forms. However, ants, bees, flies, spiders and other insect-like creatures possess that ability. Now, researchers have discovered UV-absorbing cone cells in a vertebrate, the Japanese dace fish, that enables the fish to see wavelengths down to 360 nm. They have also determined that carp and common goldfish are able to perceive ultraviolet light too.

Reseachers have found mycosporine-like amino acids which absorb UV light (300-360 nm) in a variety of shallow water fish. In addition to these we have also found gadusol (structurally related to MAAs) in the fish-eye lenses.

We have investigated as to how these chemicals are "preserved" in the lenses of 5 species of tropical fish-

1. Coral trout

2. Bludger trevally

3. Sweetlip

4. Common bream and

5. Blue salmon

The mycosporine-like amino acid asterina 330 was found "bound" to beta crystallin and gadusol to gamma crystallin, two of the three major soluble crystallin proteins present in fish lenses.