Whale, or great fish
Archimandrite Nikifor’s Biblical Encyclopedia
(Jonah 2:1; Matt 12:40) - the Hebrew tannin and leviathan do not denote a whale in the strict zoological sense but any extraordinarily large or monstrous sea creature. Many naturalists understood Jonah's creature as a shark, a large predatory fish common in the Mediterranean. They argued that sharks follow ships during storms, while timid whales withdraw into the depths; that a shark's throat and stomach can expand enough to swallow people whole, while many whales of northern seas have narrow throats and feed on small animals; and that the Greek ketos used by the Septuagint was an ancient general term for great sea fish or animals.