Mycosynth Fiend rulings and poison counters

[Swamp]

if I cast Mycosynth Fiend is its power and toughness locked in based on whatever amount of poison counters the opponent has?

for example, if opponent has three poison counters when I cast the fiend, and then he gains 2 more poison counters, would the fiend be a 5/5 or 7/7?

[Arix]

7/7. The bonus isn't set when the Fiend comes into play, it's constantly updating.