Yehuda 2008

The enormous size of the leviathan is thus illustrated by R. Johanan, from whom proceeded nearly all the haggadot concerning this monster: "Once we went in a ship and saw a fish which put his head out of the water. He had horns upon which was written: 'I am one of the meanest creatures that inhabit the sea. I am three hundred miles in length, and enter this day into the jaws of the leviathan'" (B. B. l.c.). When the leviathan is hungry, reports R. Dimi in the name of R. Johanan, he sends forth from his mouth a heat so great as to make all the waters of the deep boil, and if he would put his head into paradise no living creature could endure the odor of him (ib.). His abode is the Mediterranean Sea; and the waters of the Jordan fall into his mouth (Bek. 55b; B. B. l.c.).
The body of the leviathan, especially his eyes, possesses great illuminating power. This was the opinion of R. Eliezer, who, in the course of a voyage in company with R. Joshua, explained to the latter, when frightened by the sudden appearance of a brilliant light, that it probably proceeded from the eyes of the leviathan. He referred his companion to the words of Job xli. 18: "By his neesings a light doth shine, and his eyes are like the eyelids of the morning" (B. B. l.c.). However, in spite of his supernatural strength, the leviathan is afraid of a small worm called "kilbit", which clings to the gills of large fishes and kills them (Shab. 77b).

Wikipedia entry on Leviathan