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So I've got these large crab-like creatures living on the Abyssal Plain under the ocean (way way down 9800-20,000 feet down). At this depth the main food source is the marine snow, "a slow drift of mucus, fecal pellets, and body parts—that sinks down from the surface waters". And, of course, your neighbors.

Let's call our creatures Crabites for now, because that's their name, and besides, it's fun to say. Crabites.

The Crabites have evolved intelligence and would like to build a civilization. The problem I'm seeing for them is that for civilization you need surplus, which is mighty thin down there on the Abyssal Plain.

The marine snow is not constant; there is a constant rate, but there are huge irregular windfalls (good article: http://www.mbari.org/feast-and-famine-on-the-abyssal-plain/ ). I'm planning to have their lives revolve around gathering and accumulating the regular snowfall, and then snap into frenzied action to gather and hoard as much of the windfalls as possible.

Problem is ... I'm not sure this is enough. I'd like to have some kind of agriculture because of its insane ROI (100 kernels of wheat from one seed, minus slippage). But what can they grow? The best I can think of is shallow-sea hunting, where they float themselves up to the rich zones and kill everything in sight, bringing it down. But -- and here's the main question -- is there anything down that far which can substitute for agriculture?

Update ... I'm seeing some great ideas in answers, in particular harvesting from hydrothermal vents. I am thinking this is promising, as are methane "cold seeps" (https://en.wikipedia.org/wiki/Cold_seep) which are less extreme, can be found in shallower waters, and can support chemotrophic-based ecosystems.

I may have to bump up my planet's volcanism and overall methane production...

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – Monica Cellio Nov 9 '17 at 17:48
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Oh boy, that gives me a nasty idea...

Who says it's the ground below them which they farm?

So, these creatures live mainly off "huge irregular windfalls" that basically happen when for some reason there's a lot of dead biomass coming down from the surface...

What if they found a way to engineer those "windfalls"? The article you link to mentions algae blooms as the cause of the windfalls they observed. Well, algae bloom when they get more nutrients than normal, mainly phosphates. So if your Crabites somehow find a way to propel large amounts of phosphates towards the surface, they could do some pretty productive farming! Maybe this is something that occasionally happens naturally in a way they can observe, understand and later influence, e.g. hot water vents creating an updraft in the middle of a phosphate deposit on the seafloor.

Or, in a rather less pastoral and more terrifying vein: the Crabites could also release toxins that float up and cause a large-scale dieoff of surface life in an area...

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    $\begingroup$ This also gives a reason for them to evolve intelligence, for cooperation and planning in the engineering of the "windfalls" +1 $\endgroup$ – Gryphon Nov 6 '17 at 15:10
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    $\begingroup$ Michael this is lovely! It's sneaky, naughty, and takes advantage of existing resources. This might work both ways... I'm reminded of a Far Side cartoon about fish; caption was Encased in styrofoam "shoes", Carl is sent to "sleep with the humans" $\endgroup$ – akaioi Nov 7 '17 at 2:14
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I see three main solutions to this problem.

1) Hydrothermal vents

Hydrothermal vents are locations of enormous biodiversity relative to the rest of the deep sea. They're places where chemosynthesis can substitute for photosynthesis and allow the conversion of chemical and heat energy into sugars. This is where most of the life in the deep sea has decided to live, so they may have a point about its likely habitability. However, this does limit your Crabites to the areas near mid-ocean ridges and they wouldn't be able to survive on the abyssal plain.

2) Living off the detritus from above

This seems to be the most like what you're looking for- a rain of energy from above that sustains life. Unfortunately, it's a very small amount of energy that actually reaches the seafloor- of the 100 petagrams of carbon that pass through the surface ocean each year, 0.1% of this actually reaches the seafloor. In the below diagram, we're interested in the Particulate Organic Carbon (POC) because that's the carbon that's still in edible form.

PICPOCDICDOC Diagram

To put this number in perspective, that's approximately equivalent to one tree every 50 square kilometers, or the grass in the American West growing at about 1% of its normal capacity. There's a reason these areas are referred to as deserts!

I envision this scenario less like a pastoral scene with a happy rancher surrounded by grass/trees and more like a Mad Max style wasteland, with intense competition for the moments when food is actually found.

3) Abandon the agriculture and return to hunter ways

This is probably the most realistic scenario. Your Crabites would be able to survive most easily by hunting deep-sea pelagic fish. These fish undergo a daily vertical migration from the surface to about 1500m, which is the largest migration on Earth by biomass. The fish do this to escape predation at the surface during the day, when predators could easily see them and they'd be eaten. Your Crabites, on the other hand, would be doing the opposite- ascending from the depths each day to meet the fish as they descend, hunting vigorously during the daytime, perhaps with the help of some electroreception or "night-vision" capabilities, and descending back to the depths during the night to do whatever Crabites do when they aren't hunting. The good news with this strategy is that fish are fairly easy to find in the deep sea- at times, they're so dense they look like the seafloor via sonar, which gave them the name of the deep scattering layer.

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    $\begingroup$ After a while as the intelligent crabites become exceedingly good at catching these pelagic fish, perhaps the fish will evolve or otherwise adapt to not swim so deep. That would make for an interesting crisis. $\endgroup$ – kojiro Nov 6 '17 at 11:56
  • $\begingroup$ @kojiro if they end up using cold seeps (methane leaks) and their chemotrophic bacteria ecosystems as part of their food supply, they'll have an ecological crisis if hydrocarbons stop pouring into the environment... ;D $\endgroup$ – akaioi Nov 6 '17 at 16:34
  • $\begingroup$ Option 3 seems like it would only really work in shallow oceans. Putting aside the pressure differential of several kilometers (or more) of water, that's a long way to travel every day for food, and I'm struggling to think of a way for an organism to get a net energy gain from such large amounts of travel. $\endgroup$ – HopelessN00b Nov 7 '17 at 6:34
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    $\begingroup$ petagrams is an odd measure of weight $\endgroup$ – Journeyman Geek Nov 7 '17 at 7:04
  • $\begingroup$ @HopelessN00b actually, it's fairly energy free to move vertically in the ocean because it's done via buoyancy changes rather than swimming. Many plankton can control their exact depth with microscopic swim bladders which, when filled with air, give an easy boost to the surface. $\endgroup$ – Dubukay Nov 7 '17 at 7:33
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I would suggest animal husbandry as a substitute to agriculture. Plants, of course, don't grow down in the deep, though it might be possible to farm bacteria or other microorganisms.

What's most interesting, and I didn't know this before, is that crabs of the abyssal plains are already engaging in something very like husbandry!

It turns out that crabs already use sea pigs as crab nurseries. Your sentient crabs, I think, could certainly be herding sea pigs and using them not only as nurseries, but also as surplus food source.

I can easily imagine a semi-nomadic civilisation of wandering tribes of Crabites managing their vast flocks of sea pigs!

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  • $\begingroup$ Crabs are weird, they als use other creatures as weapons and armor. See pom pom crabs. $\endgroup$ – John Nov 6 '17 at 6:05
  • $\begingroup$ Oddly enough, that's what humans sometimes do in some very sparse environments, such as Mongolia. $\endgroup$ – Andrew Grimm Nov 6 '17 at 7:19
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They live under a Whale Graveyard, similar to an Elephant Graveyard.

Some whale-like aliens in the surface always go to die of old age at the same spot. Their corpses fall rather than float.

The whale population blooms for some reason, so suddenly the Crabites have plenty of food.

Optional: feasts of food descending from above irregularly has quite an effect on religion.

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  • $\begingroup$ Yes, I'm hoping to use the sporadic huge windfalls as a driver for intelligence; they need to scramble to gather, store, and protect the occasional whale corpse or whatnot. The "blessed rain" will feature in some of their religions, though things might get a little ... awkward ... once they figure out what the rain is made of! $\endgroup$ – akaioi Nov 6 '17 at 16:31
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If the energy flow is 1/nth that of the prairie, then they live at 1/nth the rate. In that sense problem solved.

The organic snow is constant, but accumulates steadily. So there could be periodic harvests. This constant fall would encourage extreme territoriality.

Some areas would be more valuable than others. Places like the equator where there is no sharp upwelling would be even more deserty. Areas where there were algae blooms on uprising currents would have a 'food plume' descending down current from them. A huge win would be if they could create such plumes. If they could establish communication with us surface dwellers, then they could help us make OTEC engines.

https://en.wikipedia.org/wiki/Ocean_thermal_energy_conversion

This lifts quantities of nutrient rich abyssal water to the surface where it would foster a surface bloom.

The economics of OTEC are marginal -- you're running a heat engine on about a 20 degree thermal gradient, so at best you are going to get 20/270 or about 6-7% thermal energy. A cubic meter of seawater with a 20 C temp diff has 20 kCal potentially or about 80 kJ. If you can get 1/4 of this out net, then 20 kJ/m3 or 180 m3 of water moved per kWh. This requires really big pipes to be efficient.

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They don't need more energy to have a surplus if they can be more efficient.

The pre-civilized Crabites compete with hundreds of other species in dozens of tribes. If a group dedicated itself to monopolizing a patch they might be able to harvest it with less energy. Instead of scouring the patch ready to defend against attackers living in the area, a small well armed response force protects all the dedicated harvesters.

If feast and famine is the norm there is an advantage to developing food preservation and granaries. This incentivises cities to defend and live off those stockpiles.

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Abyssal Agriculture

Can be tough without the sun but there's one alternative:

Microbies can thrive in that thermal vents sustained by the minerals and heat it provides. Maybe the crabies builds their "huts" around a vent and use it's heat and harvest it's microorganism in addition to hunting and collecting. Maybe there are vast thermal vents fields and they can tend the "crops" developing thecniques to best harvest ang grow colonies of microbies they can consume.

For example they can build "parachute" like structures anchored in the sea floor over the vents to better use the expeled materials from it and grow colonies of microorganims in its canvas. Once the canvas is "mature" (covered with enough microorganism colonies) they can harvest it.

They can also resort in some kind of bivalve farms, much like we do today. And don't forget "cattle", large fish grazing in plancton.

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Murder pits.

Food comes from above and is gathered on the bottom - but is that deep enough? On a universally flat Abyssal Plain, food is distributed more or less equally on the ocean bed, thereby making it necessary for the Crabites to go and hunt for their food. However, by teaming up the Crabites can dig deeper, creating larger pits with gradually increasing slopes on each side, in which the food naturally gathers.

Right above it, the Crabites create a toxic cloud which kills everything several meters above it.

In order to do this, the Crabites secrete a saliva rich in certain bacteria that produce methane gas when exposed to decaying corpses in an oxygen-poor environment (i.e., the process of anaerobic digestion). Basically, they spit digestive bacteria into the pit.

Once the pit is sufficiently filled with corpses, body parts and other "ocean snow", they proceed to consume it.

Then, they lie in wait until the next "batch" is finished...

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