Pressure is another challenge in the deep, though less than lacking oxygen. Many invertebrates and some fish can tolerate trips between the surface and a kilometer down.
In the deep sea, life’s diversity is high, but density is quite low. Marine snow can only feed so many. The food pyramid is small.
Those creatures that live in deep waters have adapted in various ways. They tend to be sluggish, and often gelatinous.
Calcium carbonate is hard to come by, and so skeletons are lightweight if they exist at all. Fish that live midwater tend to be small; typically, no more than 20 centimeters.
Invertebrates are not so restricted. Comb jellies become the size of basketballs. Giant squid may reach 20 meters. Siphonophores can be twice that. With a body length of up to 50 meters, the giant siphonophore is the longest sea life.
These marine invertebrates that resemble jellyfish are colonial. Each is comprised of innumerable, tiny, connected individuals – zooids – each with a specific function (feeding, defense, et cetera), energetically and communicatively coordinated into an acting organism.
◊ ◊ ◊
The dark is the final frontier of the deep, and so 90% of midwater creatures provide their own light. Bioluminescence offers several advantages beyond being able to see, including communicating to one’s own kind, luring prey, and startling predators. Some can even put an “eat me” sign on an advancing predator by releasing a sticky, glowing tissue that coats the attacker, making it more vulnerable to its predator.
At a few hundred meters depth, dim sunlight still penetrates. A bit of bioluminescence lets a creature blend in so as to be invisible from below.
Although able to flash light, fishes tend to be black, crustaceans red. Large comb jellies and jellyfish prefer purple or red as well. Long wavelength red light does not penetrate the deep, and so reddish hues are effective camouflage.
Top carnivores have no need to hide, and so seldom take such solemn tones of appearance. Tuna, seals, sea lions, dolphins, and whales come and go as they please.
Almost nothing is known of predation in the deep ocean – such as how a sperm whale can dive a kilometer and snag a giant squid.