113 — Bioluminescence and camouflage in the deep-sea

Once you get down to 1000m in the ocean sunlight is gone, absorbed by the water above.

One thing I noticed during my doctoral research was that many of the fishes at this depth were drab shades of brown, some darker than others. If you live in a world of complete darkness why be brown? For that matter why be any colour at all, what evolutionary advantage could being brown give you?

Well the thing is—it’s not completely dark at these depths—far from it. There may no longer be any sunlight, but there is light from other sources. The world at 1000m is full of point-source bioluminescence, created by the animals that live there. Watch this great TED talk by marine scientist Edith Widder for a more detailed look at deep-sea bioluminescence.

If you’ve watched that talk, you may have noticed that most of the bioluminescence was light-bluish. Now the interesting thing here is that if you invert light blue: you show the colour not being reflected by that object, and that is brown.

What does this mean: If you are a brown fish in a world where the only light is bluish, then you are camouflaged against it, since brown absorbs blue.

Big-scaled slickhead (Alepocephalus australis),
northestern Chatham Rise, 1196m.

Big-scaled slickhead - inverted colour.

Baxter's dogfish (Etmopterus baxteri),
Puysegur Seamount, 1050m.

Baxter's dogfish - inverted colour.

Warty oreo, (Allocyttus verrucosus),
northeastern Chatham Rise, 901m.
~30cm total length (TL).

Warty oreo - inverted colour

The photos show a few examples of brown-coloured fishes and what they look like when the colour is inverted; that is, the colour they absorb.

Apologies for my dodgy 2004 photography. Thanks again to NIWA for providing the fishes.