Now the LORD had prepared a great fish to swallow up Jonah. And Jonah was in the belly of the fish three days and three nights. Now the Lord had prepared a great fish - דג גדול dag gadol.
This could not have been a whale, for the throat of that animal can scarcely admit a man's leg; but it might have been a shark, which abounds in the Mediterranean, and whose mouth and stomach are exceedingly capacious. In several cases they have been known to swallow a man when thrown overboard. See the note on Matthew 12:40 (note), where the whole subject of this verse is considered at large. That days and nights do not, among the Hebrews, signify complete days and nights of twenty-four hours, see Esther 4:16, compared with Esther 5:1; Judges 14:17, Judges 14:18. Our Lord lay in the grave one natural day, and part of two others; and it is most likely that this was the precise time that Jonah was in the fish's belly.
Commentary on the Bible, by Adam Clarke [1831].