Text by Chetana Purushotham & Images by Umeed Mistry

The ways of light in the ocean

  • The light spectrum comprises a rainbow of different wavelengths (400nm to 700nm) that are perceived as different colours.
  • Light operates in the ocean differently than on land.

  • Most of the light hitting the water surface is reflected back into the atmosphere. The rest is absorbed or scattered by the seawater and creatures living in it (such as plankton). Different wavelengths are affected to different degrees with increasing depth or horizontal distance.

  • Longer wavelengths of light are absorbed and scattered quickly and lost within tens of metres (like red and orange), while shorter wavelengths attenuate less and travel much further and deeper in the ocean (blue)
  • Despite being scattered easily, ultraviolet light accounts for a substantial number of the light particles near the water surface and sometimes up to 18% of the light available even as far as 100 metres deep in the ocean

  • Practically no light can reach beyond 400 to 700 metres deep, where fish produce their own source of light (bioluminescence) or utilise heightened senses other than vision.
Trumpet, or cornet, fish have incredibly good eyesight which assists them in capturing their prey

The wonders of UV

  • At least 100 species from 26 fish families can detect UV light. Several species are adorned with colours and patterns that reflect UV and can only be seen by fish equipped with eyes possessing the necessary light-sensitive pigments. Some examples are thread-fin butterflyfish, Chaetodon auriga (on its dorsal fin-edge) and coral grouper, Plectropomus leopardus (markings on the flank).
The eyes of the mud skipper are actually better adapted to see in air than water
  • Colour vision helps fish communicate with competitors, allies and mates, hide from predators, and search for prey. UV vision is thought to enhance these functions further:
    1. Detecting predators and prey, since objects appear in much higher contrast against their backgrounds when seen in UV

    2. Identifying others of the same species (two-stripe damselfish, Dascyllus reticulatus)

    3. Coordinating with neighbours while schooling (green chromis, Chromis virdis)

Some creatures, such as this splendour wrasse, may seem conspicuous, but to other fish their patterns appear to blend in with the background

For the rest of this article (Asian Diver 2016 Issue 1 No 140) and other stories, check out our past issues here or download a digital copy here.

The 25th anniversary of the largest and longest running dive show, Asia Dive Expo (ADEX) is set to occur on the 11-14th April 2019. Centred on the theme – Plastic free Future, ADEX is more than just a dive show with its commitment to the environment. Among an exciting lineup of programs, attendees can look forward to a Future Forward Series of Panel Discussion on the Single-Use Plastic Conundrum in Asia, on 13th April.

So join us at the event, get inspired and for all you know, you might just liberate the inner diver in you! More details of the event here

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