Okay, soooooo I'm gonna go off of the fish I had chosen for the Mermay stuff since... there's a lot. I'm mainly just gonna be listing stuff about the fish alongside whoever is associated with said fish and how it ties into them (I am a Mario blog, after all, lol!) Hope you're seated for this one.
Parrotfish (Mario, Luigi, and Viviana)
Aight, so these dudes are gorgeous. They live near reefs and coral. They primarily eat all sorts of good stuff like coral, algae, plankton... and rocks (Yes. Rocks. Fun fact, that's actually how beaches are made by these dudes!). They're able to eat all this thanks to the little beak they have (hence the name). In the bros' case, they just keep the beaches all neat and tidy (they do eat algae and coral though-)
When it's nighttime, they secrete a mucus cocoon to protect themselves from any predators from smelling them, or from any parasites attacking. Mario doesn't do this as often since... well, it is Mario. Luigi, however, does this mainly to protect his family (despite Daisy having spikes, but I'll get into that later).
Also, a win for the us trans folks, these guys are certified trans icons! No joke, a majority of parrotfish are born female, but transition to males later on (for mating purposes). This may or may not have been the primary reason I made Mario and Luigi based on parrotfish-
Jellyfish (Peach)
I mainly based Peach's mermaid design off of a jellyfish because... I mean... they're gorgeous. Terrifying because of them tentacles (I mean... toxins??), but they're basically floating parasols in the water that can electrocute you. Super Princess Peach anyone?
Their big bulbous heads (also called the "bell") acts as a shield for them and sense their surroundings since they don't have eyes. They also don't float as they lack bones. Instead, they use currents to drift/float around wherever they want to go (the epitome of "go with the flow" as you will"). In Peach's case, she does have a bell, but it's more of a glorified headdress. HOWEVER, she does have tentacles, which she will use as both defense and transportation (she does use her tail, but her tentacles are pretty strong).
Probably the most well-known things about them is that they are bioluminescent. Now... in terms of Peach, she usually just uses this as comfort (particularly when it comes to Mario or her kids... but mainly Mario). Sadly, jellyfish are not this nice 😅
Lionfish (Daisy)
Daisy is pretty, but she can be vicious if she wants to due to her lionfish (also called "feathered soldier" or "devil firefish" appearance) genetics. Just like them, she has spines that carry venom should any unsuspecting enemy come near; needless to say, don't come near her Luigi or Peasley, or she'll send you straight to the hospital with anaphylaxis and paralysis (... okay, maybe just avoid her altogether-).
Whenever a lionfish wants to hunt... they essentially act exactly like lions. They wait quietly among the reef or coral and slowly stalk their food (usually stuff like crabs) before cornering them and pouncing (like you see up here ☝️). Also yes... mermaid Daisy does actually eat crabs. Shell and all 😐. And you best believe she is as vicious as possible once she's done stalking-
Ironically, lionfish do actually eat parrotfish (primarily due to them being an invasive species). Thankfully, Daisy isn't about to eat Mario or Luigi, but the two of them know she could easily overpower them (not that she would, but still). Needless to say, if you find the merbros just casually swimming around, Daisy is probably drifting nearby.
Stonefish + Great White Shark (Bowser)
Yeahhhhh Bowser is a combo of both these guys purely on just how terrifying they are. Great whites are fast, stealthy and just overall menacing-looking. But stonefish? Let's just say there's an entire dance ritual explaining why you and they should never EVER meet.
Bowser, much like a stonefish, will simply camouflage himself among coral or rocks to silently observe the bros and everyone else doing... whatever it is that mermaids do, ig. He could easily outright ambush someone if he wanted (and... he does), but other times, he simply likes to let his food come to him... usually with his spikes out to induce some delicious venom. Yeah... Mario and Luigi are usually on guard for this.
As for what he looks like... uh... imagine a shark chasing after you and it looks like molten lava. With spikes. Sweet nightmares. 🙃