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If you live in the upper ocean, it pays to be transparent to avoid the gaze of Things Bigger and Hungrier Than You, since sunlight will pass right through. But if you live deep in the ocean, where predators often come equipped standard with searchlights, being transparent means lighting up like a Christmas tree under their voracious gaze. Glassy transparency scatters, refracts, and reflects their blue high beams. Far better to be a light-absorbing red or black.
But what if you live somewhere between, in a zone where, dependent on the weather, the time of day, or the soupiness of the water, you may be better off being transparent or colored? Well, if you're an octopus or squid, the shape and color shifters of the aquatic world, this is one solution:
https://www.youtube.com/watch?feature=player_embedded&v=f0-_tSgtQsA
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These octopuses are expanding and contracting pigmented chromatophores to produce the effect. According to new research in Current Biology by scientists at Duke University, the octopus Japatella heathi and squid Onychoteuthis banksii will produce the effect when a blue light is shined on them or they're physically touched, but not when an object passes in front of or overhead of the animal. The scientists could see that the octopus could see the objects (and were not amused), however, because they followed them with their eyes.
They also used spectrophotometers and probes to test reflectance; they found that octopuses with chromatophores deployed reflected half as much light as in transparent mode, and similar amounts of light to other deep sea fish and invertebrates.
This species shows a depth preference change as they age; the young prefer shallower, daylight waters, and adults prefer deeper, darker waters. So being able to shift between evasive strategies would make a lot of sense for these creatures. Furthermore, most deep sea predators have blue light searchlights. When the scientists tested these octopus with red light, they got no reaction either. I think there's a joke about a Red Light District wandering around here somewhere . . .
In any case, whether your own chromatophores were deployed or contracted this past year, or a mixture of both, I wish you a sunny, tranquil 2012. Happy New Year!
Zylinski, S., & Johnsen, S. (2011). Mesopelagic Cephalopods Switch between Transparency and Pigmentation to Optimize Camouflage in the Deep Current Biology, 21 (22), 1937-1941 DOI: 10.1016/j.cub.2011.10.014
