Symbolism, Imagery, Allegory
If the transparent shark from the Mariana Trench is a symbol of Tom Stenton, and if the transparent octopus symbolizes Eamon Bailey, we don't need to look far to find the symbolic counterpart of the trench's daddy seahorse.
The seahorse, of course, is a symbol of Ty Gospodinov. Small and reclusive, he shares Ty's temperament and physical stature, and, as the father of a thousand babies, he symbolizes the inspiration that Ty has been to other innovators, programmers, and designers of his generation. This also means that the seahorse's numerous babies are symbols of the brilliant young minds who've been flocking to the Circle to share their ideas ever since the company started to sweep the world.
When the Tom Stenton shark devours the Eamon Bailey octopus, the octopus's death shows us that Bailey's idealism and supposed progressivism have been rendered meaningless by the Circle's true nature as a voracious, insatiable beast. Likewise, when the Ty Gospodinov seahorse and its thousand babies are chewed up and swallowed by the shark, their deaths emphasize how efficiently the Circle will feed off of the idealism of the nation's—and the whole world's—youth.