(In which his lordship talks at people about deep sea creatures, and is probably just looking for an excuse to post pictures of horrible deep sea mermaids)
I figure it's been long enough since the last time I've done one (about three or four months, unless I'm mistaken), so here we go again. Let's talk about freaky deep sea creatures!
What is the deep sea? Well, that should be pretty obvious. It's the part of the sea that's...deep. More precicely, deep sea is officially considered to start at around 1000 fathoms (1,8 km), below the thermocline. However, I'll also be covering the area below 200 m, as that's the point where light becomes so dim photosynthesis is no longer possible and the fish start to become weird. Up to sometime in the 1800s, it was generally assumed no complex life could live at such depth, what's with the complete lack of sunlight and immense pressure. However, life has a tendency to find a way to survive just about anywhere, and nowadays we know that while the deep sea is very sparcely populated due to lack of resources, its sheer size (it is by far the largest habitat on the planet) means it hold an enormous biodiversity. Also, significant part of this biodiversity is composed of really scary looking fish.
Also, suggested reading for people intrested in deep sea exploration, the book written by the first man to use the bathysphere to dive to the depth of half a mile and observe the life there is available for free online: archive.org/stream/halfmiledown00beeb#page/n0/mode/2up (I'm fortunate enough to own a physical copy of this book, and consider it one of my most prized possessions).
We'll start with the midwater zone, above the deep sea proper. Also known as mesopelagic, or the twilight zone (that actually is an accepted term for it among marine scientists), this is the region where there's still light, but not very much. Not enough for plants to grow but enough to see by, at least dimly. Many fish here have very large and sensitive eyes, to better see in the perpetual twilight. Many also have light organs on their underbelly, to break up their outlines when viewed againt the light filtering from above, thus hiding them from predators lurking below. Some of them migrate up and down the thermocline, following microscopic animals that rise closer to surface at night, while others remain at these depths their whole adult lives (larvae of midwater and deepwater fish live closer to surface).
James Brooks
A good example of an odd midwater fish is the barreleyy. The fish's eyes are the green orbs in the picture, while the black dots are actually its nostrils. While the fish has been known for a long time, it was only when modern submarines allowed us to observe it in its natural habitat that we learned the top of its head is covered with a transparent dome (the structure is very fragile and always broke when the fish was caught in nets). It's assume this protects the eyes from stinging cells of jellyfish and siphonophorae, allowing this fish to steal food from their tentacles. The eyes of the barreleye are tubular in shape, and point directly upward, allowing the fish to spot prey above it. The green colour of the eyes is though to tint the vision in such a way to make bioluminescent light stand out better from sunlight, negating the counter-illumination of its prey. One mystery that was also solved when observing the fish in its natural habitat has to do with the fact that its mouth is not within its field of vision. So how does it know when it's within biting range of prey? Well, it turns out the eyes just rotate in their sockets 90 degrees, letting it see where it's going!
William Peterson
Another fish with tubular eyes, this time forward-pointing, is the telescopefish (genus Gigantura). The large eyes can see even with little light and position of the eyes give it binocular vision, letting the fish gauge distance to its prey. It has very little ability to not see anything directly in front of it, though, but this is probably a worthy tradeoff. Like many deep sea fish, the telescope fish has extensible jaws and an elastic stomach, letting it swallow prey bigger than itself (the one in the picture has swallowed something quite big, as seen from the balloon-like stomach).
Samuel Green
The dragon- and viperfish are one of the many genuses in the very varied order Stomiiformes, which includes deep sea horrofish like dragonfish, stareaters and stolight loosejaw, and also bristlemouth and those perpetually terrified-looking marine hatchetfish.
Dragonfish in particular are pretty much the archeotypical serpentine sea-monsters, except tiny. They, along with viperfish, are normally found in mid- to deep waters, but can rise closer to surface during the night to feed. Both are primarily ambush predators, although capable of bursts of speed to chase prey or escape predators if necessary. Both also posess impressive jaws, as do many Stomiiformes (in fact, the name of the order comes from the Greek word for mouth, due to the huge mouths many of them have). Viperfish has teeth long enough it has specific "sheathes" for the lower jaw's teeth to slide into when it closes its mouth to keep it from accidentally stabbing its own brain with its teeth, while the dragonfish can open its jaws to almost 180 degrees, like some kind of living beartrap. This is possible to many of these fish missing one of their neck vertebrae. Not as in they have one less vertebrae in their neck than expected (well, that too), but that immediately after the skull the notochord is actually without the bony vertebra, letting the fish open its jaws in a way that would break the neck of most animals.
Jaxson Walker
A gif demonstrating the dragonfish's jaw movement. Note how the neck can bend due to lacking the bony vertebra at the base of the skull.
On an unrelated note, all but one member of Stomiiformed known produce light, and intrestingly some have at least some light-producing organs that don't function using the normal way of housing bioluminescent bacteria, but instead produce light through an enzymatic reaction.
Parker Long
The common bristlemouth is an unremarkable-looking fish that, along with the lanternfish, is by far the most populous vertebrate on the planet, with population estimated to be in at least hundreds of billions, possibly trillions. By comparison the most populous land vertebrate, the domestic chicken, has a population of about 20 billion. These small fish migrate up and down the water column to feed, and are thought to play a major role in oceanic carbon sequesteting due to the sheer amount of organic matter they move from the surface to deep waters. They are so numerous, in fact, that early sonar operators used to be baffled by what appeared to be a "false bottom" on their sonar scans, hovering several miles above the actual ocean floor and changing depth with the time of day. This illusionary bottom is caused by sound reflecting from the swimming bladders of enormous schools of bristlemouth and lanternfish.
Christian King
Moving on to deep sea proper, the bathypelagic zone, aka. midnight zone or underwater vore hell (the latter is not an actually accepted scientific term...yet) starts at the point where no light from the surface ever reaches, around 1 km down. Creatures at this depth typically have very small eyes, as there's not much for them to see, and non-streamlined bodies as they generally just float in place, waiting for food to come to them. Most are pitch-black, brown, or blood-red, to better blend in with their surroundings (red is the first spectrum of light to be absorbed by water so most deep sea fish can't even see it). Bioluminescence is common, but rather than being used for counter-illumination (no light from the surface reaches this deep, so counter-illumination is useless) it is used to attract prey.
Tyler Sullivan
Probably the best known deep sea fish fish are the deep sea anglerfish (suborder Ceratitoidei), which are actually a very diverse group of fish. Aside from the common spherical with a big toothy mouth and glowing lure at the end of a rod look, anglerfish come in various shapes and sizes. Some lack a lure entirely, or have one growing directly from the roof of their mouth. Some have long spines used to detect movement, or "beards" with bioluminescent organs in addition to the lure. In some cases the lure is actually evolved to look like a small fish (or in one species, the wolftrap angler, bizarrely enough like an actual fishing rod complete with a hook on the end). As with many dep sea fish, they can often swallow prey their own size or bigger. This all applies to the female anglerfish only, though. The male is much smaller, and in many species lacks the ability to even feed itself when it reaches adulthood. It has a very good sense of sight or small, though, allowing it to hopefully track down a female and mate before dying. Such extreme sexual dimorphism is very common with deep sea fish. Famously, certain genuses of anglerfish have what is called parasitic mating, where the male bites into the female and generates an enzyme that causes his body to fuse together with her. The attached male gains nutrition from the female's bloodstream, and over time will atrophy to the point of becoming little more than a pair of testicles attached to the female. Some species may have multiple males attached to one female, while some have never been observed with more than one.
Blake Johnson
Yes, deep sea horror fish! I'm monitoring this thread.
