PETA get zero breaks at all, they've done bugger all for animal welfare and in some cases have done plenty of damage to the cause with their lies, misinformation and of course, pretending to rescue and rehome animals only to kill them instead.
Pain is a difficult thing to quantify, we can't even be sure that the pain I experience and describe is the same pain you experience and describe.. and we can talk to each other.
However, Professor Elwoods experiments (Queens University, Belfast, 2012) on crabs, and prawns, are consistent with them experiencing pain and learning to avoid it, even when doing so means a trade-off (for example, trading a quality shell for an inferior one, when the quality shell comes with low level intermittant shocks).
Elwoods experiments clearly show that this is more than simply a reflex, so yes, they experience pain.
On a common sense if anecdotal level, it makes no sense for anything to NOT experience pain, pain is how we avoid injury and damage to our bodies.. without the ability to experience pain any living creature is at risk.