These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Black Comedy: Although the film is fairly unrelentingly grim throughout, the inclusion of the young Miss Crustacean appearing again as one of the dead bodies in Main Street, having been ravaged by crustaceans is certainly a deliberate irony.
Hell Is That Noise: The sourceless screaming of the isopod victims, as the parasites near maturity.
Nausea Fuel: Mass fish kills, larvae-infested fish, mountains of chicken droppings, vomiting, hideous skin lesions, blood, corpses, isopods eating their way out of victims' skin ... it's probably easier to list things from the film that aren't nausea fuel.