As mentioned already on the main page, nearly every boss in the game is modeled after a sea creature.
There was going to be a Darius III, but Taito thought that the original plot (Which involved the main characters returning to Darius, only to find it bombed to hell and back) was too depressing, and retooled it into a separate game called Metal Black.
Divorced Installment: The obscure Taito shmup Metal Black was originally devised as Darius 3, but it was deemed too depressing for the series' setting and so was spun off into its own game. Some artifacts of its lineage remain, most prominently in the inexplicable cameo of Yamato in the first stage and a few fish-like enemies.
Keep Circulating the Tapes: Darius, Darius II, and Dariusburst Another Chronicle do not have Arcade Perfect Ports, due to their multi-monitor gimmicks. You can find home ports that use more standard aspect ratios in the case of the former two games, however.
Word of Dante: The gag boss in Akkanvader known as My Home Daddy's Girl got her name on a fan site.