The Cosmic Ocean

"The Cosmic Ocean" is the 9th episode of season 4 and the 87th of the overall. It aired on March 13th 2016.

Official Description
The Turtles journey through an ocean-like area of space.

Characters

 * Leonardo
 * Donatello
 * Raphael
 * Michelangelo
 * April O' Neil
 * Casey Jones
 * Fugitoid
 * Splinter (Hologram)
 * Lord Dregg
 * Armaggon
 * Hiidrala
 * Daagon Soldiers
 * The Vreen
 * Cthugga

Trivia

 * First and only appearance of Hiidrala, the Daagon Soldiers and Cthugga
 * Splinter was a hologram in this episode.
 * The electrical seaweed, that Raph warned about, was referenced from an underwater level in the 1989 video gamefor the NES. Fugitoid's reference to how fast he could make it through (two minutes, twenty seconds) is the time limit given in that stage.
 * The image Mikey thinks up in which he imagines what Cthugga would look like, is character art of the Archie version of Ray Fillet.
 * The names Daagon and Cthugga are based on Dagon and Cthulhu of H.P. Lovecraft's writings.
 * The Daagon are based and nicknamed after Merdude, a character from the 1987 TV series.
 * The Captain of the Daagon guard has an eye-patch on his right eye, as a reference to the Merdude action figure released in 1992.
 * When being chased by Cthugga, Leo tells Fugitoid "we need more speed". Fugitoid replies in a Scottish accent "I'm giving her all she's got, Captain!" This is a reference to the popular catchphrase by Chief Engineer Scotty on Star Trek: The Original Series. In this exchange, Leo acts as Captain Kirk.
 * It's revealed that the Sea Monster used by the Kraang to protect their underwater base in the first season comes from the cosmic ocean of Varuna, not Dimension X, as they're used as mounting by the Daagon.The character Hiidrala seems to be inspired by Hydras, not the many-headed serpent in Greek mythology, but the genus of small, fresh-water animals.
 * Much like Hiidrala, the Hydras have multiple tentacles and are generally sedentary or sessile, but do occasionally move quite readily.
 * Biologists are especially interested in Hydra because of their regenerative ability – they appear not to age or die of old age.