Sunday, April 4, 2010

Hide and Seek in the Open Sea

Duke biologist Sonke Johnson has made many trips underwater to study aquatic organisms. The direct observations enable his research, but he laments that any observation - whether via submersible, diving, or otherwise - causes some disturbance that impedes observation of normal behavior. At worst, with a schoolbus-sized submersible, “you film responses of stark terror,” Johnsen said.

Johnsen’s lecture, April 2, was given in memoriam of Dr. Knut Schmidt-Nielsen, the pioneering physiologist who discovered, among other things, the function of a camel’s hump for storing water and energy. (You may have seen the statue of him with a camel near Bio Sci.)

Johnsen equated trawling, another research method, to “flying over London with a grappling hook and trying to determine the behavior of a London gentleman from what you catch. You can get some information from this, but it’s very limited.”

Blue water diving, sans submersible, is less disruptive, but “you’re left studying the slow and the dumb.”

Johnsen has also spent time researching the ecological pressures (namely predators) that cause ocean organisms to look one way or another. These organisms live in an environment where there is nowhere to hide, and where successful camouflage ensures survival.

“Predation in the open ocean is, even by predation standards, pretty brutal,” Johnsen said.

Johnsen described four modes of camouflage used by ocean organisms:
  1. transparency

  2. cryptic coloration

  3. mirroring

  4. counterillumination (“hiding yourself with lights”)

Transparency is a very common adaptation in the ocean, Johnsen said, but there are strings attached. “The fundamental trick these animals have to solve is how not to scatter light.” Transparent organisms must be extremely flat, because if their bodies give light a chance to scatter (even at a cellular level), they will appear opaque-- and thus vulnerable to predators.

Unfortunately, a flat body is not the only problem transparent animals have to solve. In order for eyes to work, they must have pigment to absorb light. Thus, they must be visible-- unless the organism can find another solution. Some organisms spread out the eye to present less of a target to predators and others compact their retinas (at a great cost to function).

Transparent stomachs also can be troublesome, Johnsen said. Even transparent animals will become opaque when chewed, and perhaps colorful. “You’ll need an opaque gut, otherwise you’ll light up like a Christmas tree when you’re digesting your food.” Some organisms address this problem by making their gut as small as possible, or by resorting to a liquid diet.

Cryptic Coloration
According to Johnsen, “something that is ridiculously colorful on land could actually be ridiculously cryptic underwater.” Organisms’ coloring also depends greatly on where they live in the water column. “As you go deeper, some of the blue light is actually converted into red light,” due to Raman scattering. For that reason, many abyssal creatures are either transparent, red, or both.

“The oceanic light field is fairly symmetrical. Turn around, and you’ll see the same amount of light.” Mirroring takes advantage of this, and works by effectively showing a representation of what should be at a particular spot (if the organism were not). “It’s not lost on nature that this works really well.”

Johnsen tried to evaluate whether mirroring or coloration was a better protection from predators, depending on the effectiveness of each in different environments (coastal vs. oceanic water, noon vs. sunset, different depths). He found that in general, mirroring was more successful because it was more robust, but that coloring was fine if the organism tended to stay in the same spot.

Some deep-sea organisms hide themselves from predators below with bioluminescence (creating their own light). These organisms can replicate light that is the same intensity of daylight, and thus trick predators into thinking they aren’t there.

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