Okay, so I was working on making a sapient creature that lives underwater ended up with what was basically a two-hundred pound crab.

They have human level language skills, and I decided to justify this social skill by making them build structures to protect themselves and their young from predators. Over time, these structures would grow larger and branch into storehouses, temples, castles, and other specialized buildings. To teach new generations to build these things, or to keep building the things their progenitors started, this species learned complex language and writing. Basically, its the old tool-use-leads-to-civilizations idea, except with architecture instead of handheld tools.

The problem is what kinds of construction methods could a crab use underwater? All the methods of building I know of require some form of mortar or specially cut stone, neither of which seem possible for a underwater creature without fingers. Just making rock igloos wouldn't work either, as large predators could just knock them over.

Two important details: In addition to their crushing claws, this species has a "beard" of sensitive, octopus-like tentacles for more dexterous work, but are only little over a foot long. Secondly, they can go on land, but need to be submerged or sprayed with water in order to avoid drying out and risking health problems (other than their legs and arms, they have soft shells covered in skin).

They can comb beaches and travel a little inland for useful materials and farm undersea plants to turn into ropes, but they can't really use tools other than their claws and tentacle-beard. Metallurgy and building mortar are right out, but they can find and transport stone easily enough. I'm not sure if they would be able to mine, as that would quickly were down their claws.

Any ideas or suggestions?

  • 1
    $\begingroup$ "what kinds of construction methods could a crab use underwater?" Unclear what you're asking. $\endgroup$
    – RonJohn
    May 12, 2018 at 22:32
  • $\begingroup$ I require a crude, hand-drawn image of these crabs in order to answer this question. $\endgroup$ May 12, 2018 at 22:56
  • $\begingroup$ @RonJohn I don't think it's unclear. The question is basically asking "how can giant crabs use their crab-claws to build underwater buildings when they don't have any fingers?" $\endgroup$ May 14, 2018 at 0:06
  • $\begingroup$ Not sure why you'd go for tentacle beard/arms. There are plenty of invertebrates that possess armlike appendages (called pedipalps) or otherwise antennae or frontlegs that can serve a similar purpose. These can serve a variety of purposes including defense(claw style pedipalps) or eating. And while there are no known examples(no creatures with these are smart enough) they could be used for tool use in your case. Having this is evolutionary more likely than your tentacles but I suppose you can still go with them. In the end it depends on your preference, each option has its own dis/advantages $\endgroup$
    – ArcWraith
    May 14, 2018 at 0:12
  • $\begingroup$ @SydneySleeper it's a crab, underwater. What else can it do besides pile stones on top of one another? $\endgroup$
    – RonJohn
    May 14, 2018 at 0:16

2 Answers 2


You don't need mortar to build in stone. Check out dry stone construction for examples of things that a mere predator would never be able to knock over. (Waves are another matter - you'd be wanting to build below the depth of wave action).

They can wield a chisel to shape the stone (remember a lot of the things in the above article were built by people without metal tools), or maybe they live somewhere that the local rocks splits naturally into slabs.

Use biological mortars.

  • Barnacles secrete a glue to stick themselves to rocks. article on the science
  • Mussels spin byssus threads to anchor themselves to rocks (and to pin down enemies like dogwhelks). Wikipedia summary
  • There are a whole bunch of encrusting organisms like bryozoans, coralline algae, corals and things which you can encourage to grow over your structure and hold the stones in place. In fact stopping them from growing on everything you build will be pretty much impossible! Google 'shipwreck' and 'encrusting organisms' and see what happens even to things which were painted with anti-fouling chemicals to prevent the encrusters growing.
  • 1
    $\begingroup$ Slow growing corals could also be used as mortar. $\endgroup$
    – John
    May 13, 2018 at 21:26
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    $\begingroup$ Oh man, this is perfect! Heck, I may have to revise some details because the crabs would have it easier than I originally thought. As for stopping the growth, culling the buildings of excess organisms might be a frequent chore. $\endgroup$ May 15, 2018 at 5:42
  • $\begingroup$ @PinionMinion Some of the organisms will be edible to a crab. Mussels for instance. $\endgroup$
    – DrBob
    May 16, 2018 at 9:12


enter image description here

This is a monastic "beehive hut" on Skellig Michael, an island (More of a large rock, to be honest.) off the coast of Ireland. For over a millennium, it has withstood winds of over 20mph on average and the kind of waves that can, and do, claim lives.

enter image description here

Waves just off Skellig

So how did hermits build such resilient, yet crude-looking, structures? They used a technique called corbelling. Flattish stones are stacked on top of each other, gradually getting closer and closer to eachother, until the slabs meet in the middle. The gaps between the stones are so small that you couldn't fit a knife edge between them. Stupid tourists have thrown rocks at the hut and tried to kick the walls in, yet the structures remain 100% stable.

All the crabs really need to do is pick up stones, so I wouldn't put it past them to build structures like these.

  • $\begingroup$ So the scene in The Last Jedi where she shoots a hole in the wall of one of those huts wouldn't actually happen? $\endgroup$
    – ArcWraith
    May 13, 2018 at 23:50
  • $\begingroup$ @ArcWraith Well, an energy blaster might do it... but they're still pretty strong. $\endgroup$
    – SealBoi
    May 14, 2018 at 7:13
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    $\begingroup$ And, just like that, you have radically changed the aesthetic of these creatures for the better. An underwater city of "hive-huts" would look glorious. $\endgroup$ May 15, 2018 at 5:45

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