This is a fun idea, and I do love manta rays, just such strange creatures.
Looking at this image the perspective is really bothering me, given how many roof tops are visible and how many lines should be converging it really sticks out. If you aren't to that phase yet disregard of course.
From the composition stand point it feels very crowded towards the right side of the image, the manta, light source and more detailed building all push the balance that way very heavily, but there is nothing on the left for the eye to drift towards to make for a more pleasant layout. Given the cityscape aspect of it, I am sure you could easily add something in, perhaps a mid-ground detailed building on the top left third intersection, or a completely unique building, something domed would stick out.
GL with the image, I also love the glowing dots along the mantas back and the attention to detail with the rear fins.