Illustration of Sea Serpents from Gessner's 'Historia Animalium', volume 4, 1558. Later hand colouring. Gessner's accounts included creatures which exist, and creatures which we now know do not. There were small sea snakes in the tropics, and eels in more temperate waters. There must have seemed every chance that such leviathan Sea Serpents roamed the deep. They hardly seemed less likely than the great whales. His accounts were based on first hand reports as well as the 'authorities' of earlier authors such as Aristotle and Pliny.