I think they're fish, which will be explained a little when the boss dies.
I found Blade level 3 a good weapon against this boss. The fish home in on you, so you can't just sit still at the boss and fire.
One tactic that works is to fire the blade at the centre of him when he opens, then run away to the side shooting the blades towards the fish chasing you (which should be mostly sideways and easier to hit).
Good luck. I also found this boss very hard!