This is another mosaic with Oceanous and Thetys in the lower ranks, above them there is Poseidon, the mosaic now is called (and maybe always was) the Poseidon mosaic.

From Enc. Britt.: In Hesiod's Theogony, Oceanus was the son of Uranus (Heaven) and Gaea (Earth), the husband of the Titan Tethys, and father of 3,000 stream spirits and 3,000 ocean nymphs. In Homer's works he was the origin of the gods. As a common noun the word received almost the modern sense of ocean. Hence in details all sorts of "sea life" appear.