The Twilight Mesopelagic Zone
Diel Vertical Migration, Luciferin Bioluminescence, and the Oxygen Minimum Layer
Between 200 and 1,000 meters depth lies the mesopelagic twilight zone, where downwelling blue sunlight dims tenfold every 75 meters. At 800 meters, midday light is 100 billion times fainter than on land. No alga can photosynthesize here, yet enough photons trickle straight down for big-eyed predators to spot dark silhouettes overhead.
Testing anti-submarine sonar off San Diego in 1942, U.S. Navy physicists found a phantom seafloor at 400 meters. The layer rose to the surface at dusk and sank at dawn. Scripps biologist Martin Johnson proved in 1945 that this Deep Scattering Layer was a cloud of fish swimbladders and siphonophores executing Diel Vertical Migration.
The most abundant vertebrate on Earth is the finger-sized mesopelagic bristlemouth fish (Cyclothone), numbering in the quadrillions alongside lanternfish. Acoustic and trawl surveys led by Xabier Irigoien in 2014 revealed that mesopelagic fish total 10 billion metric tons of biomass. That weight equals ten times prior estimates and one hundred times the annual global wild fish catch.
Because twilight-zone light comes strictly from overhead, any opaque fish casts a dark shadow when viewed from below. To erase their silhouettes, hatchetfish (Argyropelecus) and lanternfish line their bellies with downward-pointing bioluminescent photophores. Wired to dorsal light sensors, these lamps match the exact intensity and 475-nanometer blue color of overhead skylight.
Whereas bioluminescence is rare on land, 75 to 80 percent of individual animals living between 200 and 1,000 meters manufacture their own light. Across nine marine phyla—from comb jellies to squid and fish—evolution converged on the same imidazolopyrazinone fuel molecule called coelenterazine. It oxidizes inside luciferase enzymes to emit cold blue photons.
Because red light is absorbed by water and deep-sea eyes are blue-tuned, red light is invisible in the twilight zone. The stoplight loosejaw dragonfish (Malacosteus niger) mounts suborbital photophores under its eyes that project a 700-nanometer far-red beam like a sniper scope. It packs its own retina with dietary chlorophyll derivatives to see the red reflection.
To catch the faintest trickle of overhead photons, the barreleye fish (Macropinna microstoma) builds two emerald-green tubular eyes pointing straight upward. Remotely operated vehicle video filmed by Bruce Robison in 2009 showed that these tubular eyes sit inside a fluid-filled, transparent dome skull. They rotate 90 degrees forward like periscopes when the fish tilts up to feed.
Growing two giant eyes in the food-scarce twilight zone is costly, so the cock-eyed squid (Histioteuthis heteropsis) builds two mismatched eyes. Its small, blue-tinted right eye scans the dark abyss below for bioluminescent flashes. Meanwhile, its giant yellow-lensed left eye grows twice as wide and points upward to spot camouflaged fish against blue skylight.
Between 400 and 1,000 meters, bacterial decay of sinking detritus strips 95 percent of dissolved oxygen from the water, creating vast Oxygen Minimum Zones where sharks suffocate. Here thrives the vampire squid (Vampyroteuthis infernalis) at 3 percent oxygen saturation ($0.2\text{ mL O}_2/\text{L}$). It combines the lowest metabolic rate of any cephalopod with high-affinity blue copper hemocyanin blood.
Two-meter Humboldt jumbo squids (Dosidicus gigas) hunt voraciously in oxygenated surface waters at night, then plunge into the anoxic core of the eastern Pacific Oxygen Minimum Zone at dawn. Physiologist Brad Seibel proved how they survive twelve daytime hours in near-zero oxygen. The squids suppress their metabolism by 80 percent until dusk.
When attacked in the dark water column, the transparent pelagic polychaete worm Tomopteris is one of the very few marine animals that flashes golden-yellow bioluminescence (570 nanometers) instead of blue. Rather than lighting up its body trunk, the worm fires glowing yellow mucus sparks from its paddle feet as decoys. It then swims silently away.
Flapping through the upper mesopelagic zone on wing-lobes evolved from a snail's foot, shelled pteropods (Limacina helicina) build paper-thin shells of metastable aragonite—50 percent more soluble than calcite. Cold mesopelagic water absorbs carbon dioxide rapidly. Consequently, pteropod shells develop dissolution pits within forty-eight hours whenever aragonite saturation drops below 1.0.