Aristotle [ca. 350 BCE] (De animalibus, Book 4, chapter 4.14-17): [Book 4, chapter 4.14] The creature called carcinium [hermit crab] resembles both the malacostraca and the testacea, for this in its nature is similar to the animals that are like carabi, and it is born naked (not covered with a shell). But because it makes its way into a shell, and lives in it, it resembles the testacea, and for these reasons it partakes of the character of both classes. Its shape, to speak plainly, is that of a spider, except that the lower part of the head and thorax is larger. [Book 4, chapter 4.15] It has two thin red horns, and two large eyes below these, not within nor turned on one side, like those of the crab, but straight forwards. Below these is the mouth, and round it many hair-like appendages; next to these, two divided feet with which it seizes its prey, and two besides these on each side, and a third pair smaller. Below the thorax the whole creature is soft, and when laid open is yellow within. [Book 4, chapter 4.16] From the mouth is a passage as far as the stomach; but the anus is indistinct; the feet and the thorax are hard, but less so than those of the cancri; it is not united with the shell like the purpura and ceryx, but is easily liberated from it. The individuals which inhabit the shells of the strombus are longer than those in the shells of the nerita. [Book 4, chapter 4.17] The kind which inhabits the nerita is different, though very like in other respects, for the right divided foot is small, and the left one large, and it walks more upon this than the other; and a similar animal is found in the conchae, though they are united to their shells very firmly; this animal is called cyllarus. The nerita has a smooth, large, round shell, in form resembling that of the ceryx, but the mecon is not black, but red; it is strongly united in the middle. - [Cresswell translation, 1887]

Isidore of Seville [7th century CE] (Etymologies, Book 12, 6:51): Crabs are animals hostile to oysters. With marvelous ingenuity they live on oyster flesh, for, because the oyster’s strong shell cannot be opened, the crab spies out when the oyster opens the closed barricade of its shell, and then stealthily puts a pebble inside, and with the closing thus impeded, eats the oyster’s flesh. Some people say that when ten crabs are bound with a bundle of basil, all the nearby scorpion-fish come to that place to mate. There are two kinds of crabs, river crabs and sea crabs. - [Barney, Lewis, et. al. translation]

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Marine monsters 6.10; Fish 7.20): [Thomas describes the crab under the names cricos and cancris.] [Marine monsters 6.10] [Thomas has misunderstood Aristotle's account of the crab, and turned it into a marine monster called cricos.] A cricos is a marine animal, as Aristotle says. It has two slits on the tip of the foot, which make three toes with three claws. Its right foot is small, and its left foot is large; and for this reason, when it walks, it carries its whole body on its left foot. Its skin is leathery, light, black and red in some parts. Whatever it adheres to, it adheres to it with strong application. When the sky is clear, this animal is freed and will walk; and when the winds blow, it will cling to the stones and rest, and will not be moved; and this seems very strange, that in clear weather this animal has power over itself, but in stormy weather it is weakened in body. Cricus, the monster of the sea, marks the dissolute young men of the age, who have little or no right foot, that is, the love they ought to have for God or for their parents; and they want to offend their neighbors and carry out their deceptions. The skin, that is, the cover of the illusion, which they call love, is testified, that is, light, clay, loose, and black, that is, ugly, drawing the red color of sin. It has two fissures in its foot, which make three sealed toes, that is, two bad intentions in love to deceive and dishonor, by which a woman is deceived three times, namely in her chastity, in her honor, and in her soul. [Fish 7.20] Some count crabs (cancris) among fish, as he says [i.e. Liber rerum]. They have feet and arms eight in number, and pincers for hands, with which they generally crawl. And this is understood to be only when the crab is in fear and flight. When fearing or fleeing it contracts its tail, it leans back to hide itself in the mud, or in a cave, and is then contained in a very small space. But naturally, when it is without fear, it walks with its face in front and steps forward with outstretched arms. When they are old, two stones of a white color are found on their heads, interspersed with red. Some say that those pebbles are of such power that, given in a drink, they heal punctures of the heart. And of course this is quite credible, because, as Galen says in his Anatomy, the heart is most strengthened by hard, stony substances, such as pearls, sapphires, jacinths, and the like. The crab changes its color when it is in a group, for it becomes red; when, however, it is living it is green or black. What Ambrose has to say about the crab is wonderful; for he says that the crab naturally delights in the flesh of oysters, but as it craves this food, so also in taking it avoids danger to itself. But the hunting of oysters is dangerous for it: for oysters have hard and strong shells, in which they are enclosed; and indeed, if the crab should unexpectedly invade the shells themselves, it could certainly be crushed by the closing of the shells, and killed. In order to prevent this, the crab plots a well-known fraud. Therefore, since all kinds of animals are satisfied with pleasure, the crab sees when the oyster, in places remote from all wind, opens its shell to the sun's rays, and opens the barriers of its shells, so that it may in turn take pleasure in the free air. And then the crab uses a stone to prevent the oyster from closing its shell, and thus, finding the barriers open, the oyster safely enters the shell and devours the internal flesh. [Thomas repeats this story in his description of the pearl-oyster (7.60)] There are many kinds of crab, as Aristotle says. One of these is in Judea, and is called a soldier, because it is the swiftest animal, and when it is cut, no flesh or superfluities are found in it for this reason, because it is not fed with food: for if it used food, it would have superfluities. In the western sea, as Solinus says, there are crabs which drown men when they catch them; but they have hard backs like crocodiles. Crabs lie dormant in winter for five months. In spring they shed their old age like snakes. Crabs follow the warm shores in winter, and retreat in groups in the dark in summer. They are injured in winter, fatten in autumn and spring, but more so during the full moon. Crabs have a long life. In the female the first foot is double, but in the male it is single. The right arm is almost always bigger. The males have two spines between the belly and the tail, which are lacking in the females, which have eggs in the belly. Against the strike of the serpent they heal. When the sun passes through the sign of Cancer, they fight with each other with their horns. When the male wants to mate with his female, he climbs on top of her from the back; but she, feeling the movement of the male behind her wanting to mate, bends over on her side and turns towards him, and thus the mating is completed. A crab that has drunk milk lives without water for many days. - [Badke translation/paraphrase]

Bartholomaeus Anglicus [13th century CE] (Liber de proprietatibus rerum, Book13.26): Also the Crab is enimye to the Oyster. For hee liveth by Fish thereof with a wonderfull witte. For because that hee may not open the harde shell of the Oyster, hée spyeth and awayteth when the Oyster openeth, and then the Crabbe, that lyeth in waite taketh a little stone, and putteth betweene the shelles, that the Oyster maye not close himselfe: And when the closing is so let, the Crabbe eateth and gnaweth the Fish of the Oyster. - [Batman]

Cecco d'Ascoli L'Acerba [early 14th century] Crabs are born in watery caves. They feed minnows until they are big enough to eat, then the crabs grab the fish with their claws, crushing them.