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?