Sources : Chylon

Aristotle [ca. 350 BCE] (De animalibus, Book 8, chapter 4.2): All fish, except the cestreus [mullet], eat one another, especially the congers. The cephalus and the cestreus alone are not carnivorous. This is a proof of it. They are never captured with anything of the kind in their stomach, nor are they captured with a bait made of flesh, but with bread; the cestreus is always fed upon sea-weed and sand. One kind of cephalus which some persons call chelone lives near the land, another is called peraeas. This last feeds upon nothing but its own mucus, for which reason it is always very poor. - [Cresswell translation, 1887]

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Marine monsters 6.12): Chylon is a marine animal, as Aristotle says. This animal, among all other kinds of animals, does not eat, but feeds only on its own moisture, which issues from it; it is a very viscous and thick moisture, which the animal itself alone takes as its nourishment. Hence it is that this animal is always fasting and yet is strong in body. We see that in the females it is more possible to extend the fast, so that they do not eat for many days, than in males; and this because of moist and viscous superfluities, of which the females abound more than the males. - [Badke translation/paraphrase]