Leedsichthys was the largest fish known to have existed, at a massive 27 metres 80-90 feet long (nearly as long as the Blue Whale which was 100-110 feet long). It was a filter-feeder, and came under frequent attack from Metriorhynchus, Hybodus sharks, and Liopleurodon. They travelled in shoals much like modern fish, and are indeed closely related to them. They existed 165-155 million years ago.Even though they were the biggest fish ever known they ate the smallest microscopic creatures-Plankton.