The Ecosystem of Plack Beach
Plack Beach is, in my opinion, one of the most memorable locations in Mario & Luigi: Bowser's Inside Story. Many areas in the game already have a strange theme to their aesthetics, but Plack Beach's giant-teeth-covered sands are so unique, nothing else has ever matched it. Today, I would like to take a closer look at this location and speculate on its creatures, so join me on this dental and marine journey!
For the longest time, I never really thought about the giant teeth that are found all throughout this area. Maybe they come from a long dead giant race? Unlikely. Or they're just teeth-shaped rocks? Boring. But I think I have a more interesting answer.
Real teeth are covered in a hard tissue called enamel, which is primarily composed of calcium, and it protects its softer interior. Corals, specifically hard corals, are marine animals known for protecting their own soft interior with a hard exoskeleton also primarily made of calcium. So what if the giant teeth of Plack Beach are a colony of coral? They're found extensively around this marine environment, and could reasonably also be made of a calcium-based material. Perhaps this hypothetical coral evolved to produce an exoskeleton out of something chemically closer to teeth!
Building off from this, we can then surmise that, like real life coral, this teeth coral acts as the fundamental base for this ecosystem, providing shelter and food for all manner of life.
Toothies are a species of tall Pokey-like creature composed of stacked teeth. Their nature is terribly unclear, but under this coral hypthesis, we have a few options. Maybe they are the coral creatures themselves. I find that unlikely, because most of the coral we see is scattered around, not stacked. Alternatively, they could be a highly-specialized species of Pokey which burrows inside teeth coral and stacks them up to protect its long body as it grows. Or maybe they're actually the spirit of a Pokey possessing a set of teeth, similar to the undead Skellokeys from Mario & Luigi: Partners in Time. Funny how these are also calcium-based, being skeletons themselves.
Another creature which takes refuge inside teeth are the blue viruses occasionally seen alongside Toothies. As stand-ins for the bacteria that cause cavities, it makes sense that they would live in and potentially feed on the "teeth coral", causing the many holes we can observe on them. In nature, this interaction is known as parasitism, since one of the species lives in and causes harm to the other.
As mentioned, Toothies can spit out blue Viruses to help defend them when they are knocked down. Given they help them, the Viruses could be in a mutualistic reloationship with the Toothy. They can hide from predators by living inside them, in exchange for protecting the Toothy when it is in danger. I don't think the Viruses feed on the Toothy, since it is shown to easily spit them out when it wants to, and it has no signs of cavities on its body.
The dentists of the beach
Pendrils are small penguins with drill-like beaks which can be found on Plack Beach. While a penguin in a tropical beach might seem unusual, most real penguins, such as the Humboldt penguin, don't actually live in arctic regions, instead living on temperate beaches and rocky shores.
Pendrils are capable of using their beaks to burrow in and swim through the sand. Given the teeth theme, their beaks seem evidently designed after dentist drills. Perhaps they also them to dig into teeth, looking for food such as the Viruses, who in turn sometimes protect themselves by hiding in Toothies.
The last (new) creatures to be found at Plack Beach are Drillbit Crabs. As their name implies, they appear to be a species of hermit crab wearing drill bits. With these drills, they could also hunt for Viruses inside teeth. It's not known where they get these drill bits from, but due to their similar colors, it is possible they take them from Pendrils.
Maybe Pendrils can shed their beaks? Alternatively, Drillbits take the role of necrophages who feed on dead Pendrils and then take their beaks, but who knows. A little morbid, but it is what it is. As for why/how their drills are so much larger? I don't know. It could be that they go for unseen larger Pendrils, or that their drills come from something else entirely unrelated to them, or maybe their size is a gameplay abstraction so they can work as enemies. No answer seems satisfying to me.
Chuboombas can also be found here, but I can't think of anything for them. They're likely just on their way to and from Cavi Cape, so they don't matter much in the grand scheme of things. Also talking about what is essentially a type of Goomba as if it were a wild animal is... weird. The same goes for Kuzzle and Chakron, two characters who can also be found in this area.
With that said, here's a quick graphic illustrating all the different interactions I mentioned so far. Putting this together gave me a greater appreciation for this area, and I hope it was an interesting journey for you, so thank you for reading!