It is a sea creature found in medieval bestiaries and the Physiologus. It has been mentioned in traveler’s myth and lore in Greece, Egypt, throughout Europe, and in the Latin world.
It has been variously described as either resembling a large whale or vast sea turtle, and a giant sea monster with huge spines on the ridge of its back. It was often mistaken for an island and appears to be rocky, with crevices and valleys with trees and greenery and having sand dunes all over it.
It was said to rise to the surface from the depths of the sea, to trick sailors into believing it was an island - for them to land on - and then pull them to the bottom of the ocean and drown them.
The Aspidochelone also emits a sweet smell that lures fish into its trap where it then devours them.
In the Latin text of the Physiologus, it reads:
“There is a monster in the sea which in Greek is called aspidochelone, in Latin "asp-turtle”; it is a great whale, that has what appear to be beaches on its hide, like those from the sea-shore. This creature raises its back above the waves of the sea, so that sailors believe that it is just an island, so that when they see it, it appears to them to be a sandy beach such as is common along the sea-shore. Believing it to be an island, they beach their ship alongside it, and disembarking, they plant stakes and tie up the ships. Then, in order to cook a meal after this work, they make fires on the sand as if on land. But when the monster feels the heat of these fires, it immediately submerges into the water, and pulls the ship into the depths of the sea. Such is the fate of all who pay no heed to the Devil and his wiles, and place their hopes in him: tied to him by their works, they are submerged into the burning fire of Gehenna: for such is his guile.“
Pliny The Elder’s Natural History tells the story of a giant fish, which he names “Pristis”, of immense size; he also relates the tale of sailors landing on its back, only to discover that it was not in fact land when it submerged.
In Irish folklore, there was a giant fish sea monster that breached the boat of Saint Brendan. It had a different name of “Jasconius”. It was also mistaken for an island. The sailors celebrated Easter on the back of the sleeping monster, but awakened it when they lit a campfire. They race to their ship, and Brendan explains that the moving island is really Jasconius, who labors unsuccessfully to put his tail in its mouth.
The sea monster is also known as “Zaratan” in the Middle East, found in Arab and Islamic legends. It is found in the works of:
“The Wonders of Creation”, by the Al Qaswini in Persia.
“Book of Animals” by a Spanish naturalist named Miguel Palacios.
It is also mentioned in the first voyage of Sinbad the Sailor in the “Tales of the Thousand and One Nights”.