Yes, there is a small dolphin under Arethusa's portrait. Arethusa is the deity of a spring that you can still see today in Syracuse, and the fountain is a few metres away from the sea. It is a kind of small miracle!
As for the octopus, it is a very common symbol in Greek thought. The Stoics, for instance, compare the soul (or rather, the hegemonic principle of the soul) to an octopus located at the centre of the body, whose tentacles extend in the limbs.
For more symbolic interpretations See Denyse Berend, "Histoires de poulpes" :
https://it.numista.com/L111892