Along Tra Sakinig's coast sit large boulders with drag marks behind them, though no one has seen them move, which is why the patrol that found them called them sliding rocks. They are obsidian crabs, multi-legged, crab-like animals that grow a hard, rock-like shell from the minerals in the plants they eat. They keep to the coast to feed on plankton and washed-up plant debris, hiding in the shell and only coming out to drag themselves closer to or further from the water as the tides come in and out.

They are passive and rely on camouflage and the shell to keep predators off, cryptlings above all, but fight fiercely when attacked, and can spit a corrosive compound to drive attackers away. Their shells are a possible source of armour.