Update: I think the circumstances of the fight make a big difference. I just had another session where the same PCs were out hunting sharktocrabs--it turns out they're delicious, so restaurants will pay well for them. The first time, I started in media res with the PCs in rowboats being attacked by a pod of sharktocrabs surfaced unexpectedly. The PCs got pretty torn up very quickly, thanks to some lucky rolling on my part, so I held back on the reaction attacks and allowed the sharktocrabs to be driven off at half hitpoints to prevent the whole thing from becoming a bloodbath. The second time, the PCs were able to spot the sharktocrabs from a distance and prepare to attack. I had the same number of beasties, but the combat ended with a grand total of one hit on one PC--and that was only because the paladin decided to get within melee range. Otherwise, thanks to hold monster and the mystic's mind control powers, the sharktocrabs were completely unable to attack.