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Apparently hell is populated by squid. The first of my #DeepSN  Comic Con coverage. #DeepSN  
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Miriam Goldstein's profile photoNobilis Reed's profile photoBrian Turner's profile photoDanna Staaf's profile photo
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I'm pretty sure that most things that have tentacles...at least most large things that have tentacles...live in deep water.

The mythic and legendary associations between deep water and the infernal realms isn't new...
 
Maybe it's a the plaintive look in their eyes or the supplicative gesture of their arms, but those look more like lost souls than devils to me. So now I'm wondering, do we all have cephalopods for souls, or merely the wicked?

Miriam, I am so excited for your ongoing coverage. =)
 
+Nobilis Reed I think there is a simple mechanical reason why watery beings have tentacles and landmonsters have sticks and joints:  On land, you have to permanently expend energy at every mobile place to keep the limb in its position against gravity, therefore we have as little mobile places with as few degrees of freedom as possible (and we put the degrees of freedom as far as possible to the tips of the limbs to minimize the weight that has to be kept upright).  Under water, gravity doesn't matter but resistance does.  Stiff limbs suck there - you have to move them transversally which is hard.  By using tentacles, you just bend the tip and then let the place where the U-turn sits walk along the tentacle, so only the width of the 'U' moves transversally and the rest of the tentacle slides longitudinally (which gives a lower drag by factor of two).
 
It's a bit tangential to my point, but I'll engage you anyways, +Ralf Muschall because it's fun.

Counterevidence: there are lots of big arthropods in the deep ocean as well.
 
+Nobilis Reed Do they have to move quickly?  My idea was that they slowly crawl along the seafloor, and the feeding appendages are short so that their drag doesn't matter.  Also maybe my idea isn't that important - the factor 2 is not that large (but it gets bigger for tentacles/legs where the ratio of thickness to length becomes notably different from zero - see http://www2.imperial.ac.uk/~ajm8/BioFluids/lec1103.pdf).  The swimming ones depend on their legs for propulsion - I don't really know them but I think that the use their legs like ciliates use the cilia, so drag is good for them (for tiny animals, drag is the only means of propulsion - using the more efficient inertia requires either some body size or rocket science (+Danna Staaf - is your rocket propulsion paper already available?)).
 
+Ralf Muschall There are plenty of tentacular things close to the surface, but they're not (generally) big.  Hydra, for example.  Most of the larger creatures that live (or can live) closer to the surface are vertebrates.

Maybe that's because tentacles (and feelers, such as antennae and multiple legs) work better in low-light conditions, and arthropods and molluscs are better at those. Whereas eyes and ears (which are specializations with greater refinement in vertebrates) work better in shallower layers.
 
Sorry, guys, I gotta step in. The most abundant multicellular animal ANYWHERE in the ocean are copepods, which are (mostly) swimming arthropods.
 
Yes, but they don't get BIG.  And by that I mean something you would notice if it bit you.
 
+Miriam Goldstein How do copepods move?  I watched some crustaceans on youtube, the small ones swam like ciliates; larger ones rowed by propelling water backwards with a single large pair of legs (or antennae or whatever - but on arthropods, almost everything is a leg anyway), possibly reaching Re>1 and making use of the water's inertia.

In my first comment I had been thinking more about walking and grabbing things than about swimming (for swimming, one wants drag, at least sometimes), I should have said so.

My general idea about swimming thins goes like this: Tiny beings (Re<<1) must rely on drag - there is nothing else.  This begins with bacterial flagella, gets replaced with cilia (or similar things) for bigger ones.  Another way to do this is being a flagellum - I believe eels swim like this.  This gets better when the body is vertically flattened, as this improves the ratio of transversal to longitudinal drag.  Big things can take a bunch of water and throw it backwards, with tuna being the most visible example.
Cephs are exempt from the rules since they know rocket science (i.e. they can accelerate water backwards to reach useful Reynolds numbers even when they are small themselves (Bartol gives Re=1..100 for paralarvae)).
 
Copepods are about Re=10 when "jumping" for their escape response, but their normal swimming is just about Re=1. Most copepods swim up (using "scooping" motions of their appendanges) and then float gently down. Interestingly, their mouthparts are Re=0.1 or so and exist in a very viscous environment. 
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