Firstly, I sketched out something very roughly. I've always liked the enemy explosions in Shadow Dancer on the Mega Drive, so I took that 'pillar of flame' look as the central idea, but surrounded it with chunks of standard exploding bits.
The circle in the centre was just for positioning, but when it came to the second pass, where I drew everything neater and clearer in one colour, I kinda liked it as it could represent some concentration of energy, or a small black hole or somesuch. The wispyness also reminds me of the Onibi Yokai which appeals to me.
The colouring was pretty simple. The red surround with blue interior should contrast against most background art. It's only a few frames, but they are over so quickly that the effect works well enough.
I've also updated the 'hit enemy' flash with some animation, changing the colour to pink to match the player sprite and not clash with the explosions.
The Space Sharks now have a walking animation, rather than sliding around. This was constructed with my usual method - I draw out everything in monochrome first. I find this is pretty important as it allows you to refine the look and positions because once you've shaded the whole set it's a royal pain to fix (saying that I did adjust a few minor areas after the fact).
All in all, they look nice and smooth. I maybe could have put more wobble in some areas, but I'm trying not to go too over the top with animation frames as I want to keep the feel of a certain era of gaming.