Sources : Testeum

Aristotle [ca. 350 BCE] (De animalibus, Book 8, chapter 3.2): The testacea which are immovable live by a fluid which percolates through the dense parts of the sea, and being digested because it is lighter than the sea water, thus returns to its original nature. That this fluid exists in the sea, and is capable of infiltration is manifest, and may be proved by experiment; for if anyone will make a thin waxen vessel, and sink it empty in the sea, in a night and a day, it may be taken up full of water, which is drinkable. - [Cresswell translation, 1887]

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Marine monsters 6.52): Testeum is a marine animal, which can have its name from the fact: for testeum have a shell, says Aristotle. It is born in the Arabian Sea. This animal is weakened by the fact that the internal heat in its body cannot have escape by respiration through the middle pores, because the thickness and hardness of its skin prevents this; When it is sick, I say, it seeks fresh water and drinks from it for a while, and by the change of fresh water its skin becomes fine, and it sweats from the narrow pores, and it recovers. When the animal is healed, it returns to the salt water and returns to its original disposition. Now it is evident that there is fresh water in the sea; and it can be proved in this way: take a wax vessel and close it on all sides and throw it empty into the sea for the space of a day and night, and afterwards you will find the vessel itself full of fresh water. This is the largest animal in the Red Sea. - [Badke translation/paraphrase]