Original Text
In the Eastern Sea there exists a type of clam that, when hungry, floats to the shore, opens its two shells, and from within crawls a small crab, tethered by a red thread; the little crab ventures several feet from the clam's shell to hunt for food, and only returns to the shell when sated, whereupon the shell closes again. If someone secretly severs the red thread between them, both the clam and the small crab perish. Truly, this is a marvel among the wonders of creation.
Commentary
This is likely a tale about hermit crabs or similar creatures. Small crabs and clams do indeed exist in a symbiotic relationship, but the notion that "if the connecting thread is secretly severed, both creatures will die" is probably an invention of Pu Songling's imagination.