Sources : Berus

Pliny the Elder [1st century CE] (Natural History, Book 32, 5): Licinius Macer relates that the murena [moray] is female only, and conceives out of serpents... - [Rackham translation]

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Serpents 8.6): Berus is a most wicked kind of serpent, as the Experimentator says, and more cunning than all other serpents. This serpent summons the sea moray by hissing at the shore of the sea, and flatters it, that it may excite it to copulation. The sea moray is soft-hearted and tolerates intercourse, but this leads sometimes to its own ruin. For, having been intercepted by the snares of its adversary, before it can return to the waters, it is slain and pays the penalty by its destruction. - [Badke translation/paraphrase]