Ok, the way hydras are worded:
They spawn a head when they take damage equal to 1/(1+mark) of their max health.
They also regen over time suggesting that the % damage can come out of what they healed.
However I constantly see them producing 15-20 heads suddenly (single hydra here, MkI or MkII) when they reach the % of their health that they would be expected to spawn one, when hit with many small attacks. That's at 50% for MkI, or 66%/33% for MkII. These heads are spawned suddenly.
Is this the proper behavior? It seems to me that they are hit by a small attack, spawn a head, heal a little, get hit again, spawn a head, etc. This caused me to nearly lose my regen golem on an attack to just 4 MkII Hydras when they spawned about 350 heads over a very short time. I was not amused.
Currently it seems that they are overwhelming against any ship that cannot do much damage to it in a single hit, but weak to any ship that has a multiplier vs. it's hull.
Finally, are all marks of hydras supposed to spawn MkI hydra heads?