Hey there! My eye fell on this mod a while back, and I was intrigued by its central concept, which is very cool. I finally took the time to try it out and poked around a bit with a few short forts, and I enjoyed it a lot, so I have a few notes. I know you're not working on this mod right now (and I'll certainly be giving Long Night a go sometime soon), but I wanted to leave some feedback anyway.
-Cool as the various giant crustaceans and insects are, they retain a certain problem that giant insects in vanilla have: their chitin is like rice paper. I've witnessed a single unarmed dwarf kill a giant fungal crab with a well-placed punch to the cephalothorax. If you want to buff them up a little, you could try replacing the material values for chitin with those of shell, which is a little tougher.
-Love what you've done with the caverns, but I think there's room for more variety of creatures. The mushroom people aren't particularly challenging and neither are the giant fungal crabs, due to aforementioned chitin problem. Incidentally, wormant's name is bugged (displays as "nothing" in game). I was actually pretty surprised when a regular ol' vanilla large rat showed up, is that supposed to happen? It was at least a welcome break from flying mushrooms spooking my dwarves. Anyhow, I have a few suggestions (I haven't seen every aboveground creature by far so forgive me if there's some duplicates): maybe have fungal crabs spawn in slightly larger groups so they can be more of a crundle-like threat, predatory cone snails with nasty syndrome, giant anglerfish, flesh-eating mudskippers, sea stars for the Deep Rot, giant cave lobsters, maybe some 'silk'-spinning tube worm things (as currently, quite a few moods will end in insanity if you don't have relations with a trading partner that has 'mud spiders'), giant and predatory or not. I'm not sure if grazing is possible in the caverns, but maybe some kind of big snail you can farm/graze down there. Maybe scaly-foot snails -these are real life snails that live around volcanic vents and make their shells out of iron, which might be very interesting to dwarves. Generally speaking, creatures based off freaky deep-sea scavengers. Maybe the very largest of the cave crocodiles of the old world survived down there, doing battle with landsharks for the spot as top predator. Evil cave dolphins? Evil cave killer whales? Really big crab okay I'm out of ideas
-Speaking of iron - it's very possible to spawn a dwarf civ with no access to iron at all, and thus no anvils. Might be intentional, idk?
-Tide mushrooms don't seem to work, growing into unusable generic 'plant' that seems to rot away quickly.
-Landrays are cool but very weaksauce due to not having bones to protect their brains. I admit it's hard to come up with suitable war animals for dwarves in this mod and not just going 'some kind of crab' - maybe an adapted, more compact breed of cave crocodile? Some aboveground predator of the old world that survived by adapting to the caverns?
-The new varieties of races are pretty cool, but maybe shrimp men would be better distinguished from their fellow crustaceanfolk (as being crustaceans) and from the dwarves (as being small) by being pistol shrimp men. Kapow!
-Intelligent octopus people. Octopus wizards.
-Lastly, there is a striking lack of butter to serve all these giant crabs with.
Don't know if you'll do anything with any of what I've said here, but here it is anyway. Thanks for making a fun, flavourful mod full of cool stuff!