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Long before you get to using fire to smelt metals, hammering is a much more fundamental requirement for basic technologies. Hammering lets you

  1. Flake stones to make stone tools
  2. Process food--grinding grain, cracking nuts, cracking shells, tenderizing meat, etc.
  3. Construct things--e.g., using thorns as nails (a thing which Colonial-era Americans actually did).
  4. Even work metals--e.g., cold-working native gold to make jewelry.

Lots of animals use hammering in air. Woodpeckers hammer their beaks into trees. Birds will smash snails on rocks, or lift things into the air and drop them. Otters actually use rocks as tools to smash seashells. And of course, humans are particularly good at precision hammering with our unique arm structure.

Underwater, though, the only animal I can think of that uses hammering is the mantis shrimp. And it's not too hard to see why: swinging things isn't particularly efficient with all the drag that water exerts, and dropping stuff to exploit gravity for enhancing your hammering runs into the same problem.

So, how might one design an effective hammer that could be used by, say, a particularly intelligent stone-age octopus, or mermaid? Or, alternately, how might you have to design a sea creature to be good at hammering, without hyper-specializing it just for that task like a mantis shrimp is?

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  • $\begingroup$ Mermaids in my setting go outside the water for several activities - from basic ones, like foraging on the beaches, attracting sailors, trading with other races, braiding their hairs, to more complex ones like manufacturing stuff. Their tails are a bit longer than the typical mermaid, enabling them to slither in land. Think about a crossover between a regular mermaid and a World of Warcraft Naga, or the Undine in Yggra Union. $\endgroup$ – T. Sar Nov 27 '19 at 15:43
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    $\begingroup$ Use a hammerhead shark, duh $\endgroup$ – Whelkaholism Nov 27 '19 at 17:03
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    $\begingroup$ The BluePlanetII series had an episode of a fish hammering open shells on a coral anvil. EDIT: radiotimes.com/news/2018-01-26/blue-planet-2-fish-clams-tools It takes the fish a LOT of time and hits!! You'd evolve hands for less! $\endgroup$ – user3445853 Nov 27 '19 at 17:48
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    $\begingroup$ I'm not sure what you were referring to with the mantis shrimp, but this phenomenon is worth mentioning because it relates to forces underwater en.wikipedia.org/wiki/Sonoluminescence $\endgroup$ – Jay Nov 28 '19 at 11:14
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    $\begingroup$ Some of those techniques need not use hammering (that is, sharply moving force). Pressure flaking for stone tools, grinding or rubbing for processing food or working metal, binding or wedging for construction, in short using leverage and pressure can get you a lot of the same things concussive force can. And slow and almost-gentle taps from a heavy object can serve nearly as well as fast sharp blows from a lighter one, for many things, and better for some (such as when needing precision or when dealing with brittle materials) $\endgroup$ – Megha Dec 4 '19 at 6:30
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That would be not a hammer, but an axe.

Stone age hammers and axes did not differ much, and in most cases were both - depending on the side you used (this is still the case for "civil" axes). An axe has a very aerodynamic (hydrodynamic) form and can be easily used underwater. And it was and is used underwater by divers where a knife is not enough (like cutting thick ropes)

Axe is the most universal tool developed by intelligent animals - you can utilize it for any "simple" technology (with less efficiency that special tools) - from killing (hunting) to artworking. It is easy to invent and construct.

For heaver hammering intelligent octopus would use a (metal) rod. It would allow the tool to have a lot of mass for its cross-section, so drag is not a problem.

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Since your setting seems to be pretty primitive, thus not making highly advanced technology an option, I think we can look to nature for possible alternatives to hammering.

An apt answer (similar to another answer by user V.Aggarwal here) that can be found in nature is the pistol shrimp.

The pistol shrimp has a special claw that lets it snap its claw so incredibly quickly that the sudden displacement of the water at the end of the claw causes that water to evaporate instantly, briefly creating a bubble made of water vapor. This bubble then collapses almost immediately due to the surrounding water pressure. This implosion in turn creates a (comparatively speaking) powerful shockwave capable of outright killing or knocking out the shrimp's prey.

An interesting fact is that pistol shrimps also use their ability to drill into solid basalt rock, little snap after little snap, in order to dig a burrow.

Assuming your creature knows about this phenomenon, it could try to develop a tool or natural appendage similar to the pistol shrimp's, or domesticate and then breed pistol shrimps (which are prettty tiny) in order to make them into larger and larger species over generations until they are big enough to be substantially useful.

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    $\begingroup$ I think this is called cavitation. Very bad for ships and propellers $\endgroup$ – Matt Hill Nov 28 '19 at 4:42
  • $\begingroup$ @MattHill Precisely. This is a rare (the only other than mantis shrimps ?) instance of cavitation cause by animal activity. Typically cavitation is associated with mechanical phenomenons (propellers, torpedos etc). $\endgroup$ – Kaloyan Nov 28 '19 at 13:31
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A Vacuum hammer

Not exactly a vacuum hammer, but one that uses its principles to operate.

There is a certain fish that preys-on using these principles (I forget its name). It simply opens up its mouth wide and quick enough that the surrounding water along with the smaller fishes in it, storms into its mouth with astonishing speed.

Your creature can use similar principles to create a hammering effect. Assuming there is a hammerhead and an object that needs to be hammered, create a vacuum between these two such that they collide.

How to create that vacuum is something I haven't figured out yet, but something like this may work: A long hollow tube, having the hammerhead tightly packed at one end like a bullet inside a gun barrel, and the object placed at the other end (This end should be watertight), The tube suddenly expands creating a pressure difference and the water above hammerhead rushes inside, pushing the hammer towards the object and smashing the two.

Hope this one gives you a slight idea.

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Use water at your advantage, with a focused shock wave.

Having water a very low compressibility, any pressure you give it will be transferred to the other end. If you focus the pressure wave you can deliver quite some energy, reaching the same result of a hammer hit.

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  • $\begingroup$ That's how Mantis Shrimp does it. $\endgroup$ – Morris The Cat Nov 27 '19 at 14:30

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