Adam Clarke on Jonah 1:17

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 Mat 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 Est 4:16, compared with Est 5:1; Jdg 14:17, Jdg 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.