Thursday, April 9, 2015

The light stuff: anglerfish brighten up the depths

goldfish showing off its tremendous fish-ness
photo: LoveToKnow
Think of the fish you know best.  They may look like this orange guy:

Having a certain hydro-dynamic shape facilitates movement in the ocean, so faster-moving fish (and marine mammals - think dolphins) tend to taper on both ends.

In the ocean's depths, however, speed is less critical for survival, and bizarre and ancient forms become the norm. Survival in the cold deep water, where there's little oxygen, light, or nutrients, requires exceptional adaptation and so brings out some of our world's crazier-shaped life forms.

The jumbo squid, we learned in a previous post, is able to move fast and hunt more successfully than other large predators in the deep by suppressing non-essential bodily functions and extracting oxygen from even anoxic waters. And to find each other, they flash and flicker light in patterns we don't yet understand.

Nemo and Dora meet The Queen of Anglerfish, with
her exceptional life form
But deeper still, in depths below 1000 m (3000 ft), where there really is no light, there are no plants.

Without plants, alternative food chains form, based primarily on marine snow, the organic material (dead fish, plankton, fecal matter, and other good stuff) that sinks from the shallower waters.

Tempting as these morsels sound, the animals in the dark still have to find them – and each other.

With few nutrients coming their way, anglerfish, like many predatory fish down below, maintain three basic diet rules:

1. it's dark, so save energy rather than search around in vain.
2. give yourself half a chance and attract whatever other living things are down there.
3. if you do find anything, eat it.

While some deep-sea fish have lost their sight altogether, since they don't have much to see anyway, others have gone the other way and evolved highly sensitive eyes that can detect faint light far better than we can.

And them about half of these ---- including the anglerfish, the heroine of this post --- carry around their own lights.

Anglerfish are built for ambush, not speed. They have their own fishing pole with a light on the end to attract and mesmerize potential prey. They shine light to attract food, find food, and blind food, as you can see:

OK, the light isn't really a light - it's actually a modified spine tipped with glowing tissue that it can dangle in front of its mouth. The tissue is filled with millions of bioluminescent bacteria that live inside the anglerfish and glow blue-ish white by moving around molecules of a pigment call luciferin.

a well-lit anglerfish  with big eyes and pressure sensors
photo: Larry Madin, WHOI
The lure resembles the lights produced by various tiny deep-sea creatures so it's an attractive sight to other fish. The anglerfish can control the light, and with its long, sometimes wavy. fishing pole, it can confuse fish like Nemo and Dora.

In other words, light can be pretty – or pretty creepy – depending on what it's attached to.

Yet deep-sea anglerfish tend to be small. This lovely lady, collected by Woods Hole oceanographers from a submersible down about 800 meters, is only 3 inches (7.6 cm) long.

The bumps that look like rivets on the fish's sides help it detect pressure, which can be immense at these depths.

In addition to the tell-tale spine with the shiny tissue on the end, you might notice the teeth on this wee predator of the ocean's "midnight zone".

female anglerfish flashing her famous smile and
showing off her tiny mate, attached, at bottom.
photo: UCalif. Berkely Evolution
They're huge and really sharp (see them better in the NatGeo video at the bottom).

So are the teeth of other "ambush" predators of the deep oceans (from roughly 600 m (nearly 2000 feet) deep down to a mile below the surface).

Their tiny size and relatively weak muscles mean they are no threat to people, but still. I wouldn't want to meet this fierce-looking beast anywhere, let alone in the darkness of the deep ocean.

Where's the beef?

As nature would have it, the male anglerfish does want to meet her!

The tiny male anglerfish don't have the fishing pole and light combo to hunt in the dark. They literally hook up with the much larger females and are stuck on her for life, feeding off the nutrients she finds.

Please check out one more excellent video, this one from National Geographic, stressing the wonderful weirdness of this group of fishes!

In this charming video (one more!) from Monterey Bay Aquarium, I found the peaceful flow of the female angler's relaxed swimming motion oddly juxtaposed with those crazy teeth. Do you agree?

Also, have a look at her surroundings - she and her cousins do find enough to eat among all that floating material around her!

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