Coral Reefs not spawning any mobs

Put the summary of the bug you're having here

What I expected to happen was...: 
I entered a coral reef biome expecting to see tropical fish.

What actually happened was...:
No mobs were spawning within the confines of the coral reef biome. Not any squid either. nor drowned. the biome was completely devoid of mobs.

Steps to Reproduce:
1. enter coral reef biome
2. search for any sign of mob activity
3. fail to find a single mob.