To be frank, gastropods are one of those taxa that I find to be a little unsettling. You see, I’m not formally trained as a biologist, so when cataloguing animals, I tend to generalize. Chelicerates, for example, are easy—you’ve got sea scorpions and horseshoe crabs at the base, nobody knows what’s up with the sea spiders, and then in arachnids proper you have the spiders, scorpions, mites and ticks, harvestmen, pseudoscorpions, whipspiders, whipscorpions and the like. When I see one of these, I can tell right away what it is. Insects are similar—anything with two wings is a fly, fly-looking things with four wings are actually wasps, it’s a beetle if the forewings are solid, true bugs have half-opaque wings and sucking mouthparts; that sort of thing. Gastropods, though, have countless ranks of identical-looking white shells and identical-looking gooey innards (beetles suffer from a similar fate, where you have a dozen drab, barely visible whatsits for every bright-colored one). This, unfortunately, makes them rather forgettable.
Unfortunately, I say, because they are far from boring—while terrestrial slugs and snails have rather conservative morphologies, their true home is the oceans, where they show a staggering array of surprisingly attractive forms. The poster children of slug-kind, of course, are the nudibranchs, dashingly handsome animals that come in every color imaginable—often in dazzling combinations. The quintessential sea slugs, nudibranchs are without exception carnivores, and can often be found “grazing” on sessile animals—those feeding on sea anemones may even steal the undischarged stinging cells of their prey for their own use, concentrating them on horn-like outgrowths that, in some species, can be detached when the animal is threatened (the pelagic nudibranch Glaucus atlanticus uses the same trick, although on much bigger prey—this tiny animal, barely three centimeters in length, hunts floating behemoths like the Portuguese man o’ war, the tentacles of which can extend for some 50 meters and are packed with enough nematocysts to be fatal to humans). Others, not content to rely on the defenses of another animal, secrete acid on their own skins (and I don’t mean wimpy organic acids—at least one nudibranch coats itself with sulfuric and hydrochloric acids), instantly transforming from a tasty morsel into an unpalatable, vomit-inducing glob when confronted by fish.
Equally elegant are the free-swimming gastropods, which come in three varieties—heteropods, sea angels and sea butterflies. Heteropods are a relatively obscure group, though I very much appreciate the gradual change in form exhibited by their three extant families: atlantids are little more than tiny snails that have swapped the muscular gastropod foot for an equally muscular swimming fin, while the flimsy shells of carinariids cover only their viscera, and the animal is larger and more streamlined—a trend that is further apparent in pterotracheids, the adults of which do away with the shell and become agile, roughly barrel-shaped predators. Much more popular are the sea angels, famous on the internet both for the lazy grace of the unprovoked animal and the sheer brutality it shows in the presence of prey—the cute, cat-like head of the sea angel in fact conceals six spiky, vicious tentacles, designed to rip flesh from within the shells of pelagic snails. These pelagic snails are the third group of open-sea gastropods, the sea butterflies, which like the atlantids are thin-shelled, swimming snails (convergent evolution at work here, though atlantids have a single fin, while the feet of sea angels and sea butterflies are split into two paddles). Unlike the visually hunting heteropods, sea butterflies behave like marine spiders, spitting a web of mucus to ensnare small animals caught in ocean currents.
The very same hunting mechanism is employed by a vastly different kind of snail, the worm shell—where sea butterflies have sacrificed their hard shells to pursue a pelagic existence, vermetids fortify their defenses by fusing into rocks. A young worm shell is outwardly little different from your run-of-the-mill sea snail, except that it is sessile, having cemented itself to the rock it inhabits—but as the animal ages, the coils of its shell begin to loosen, eventually becoming a roughly spiral tube that is firmly embedded into the surrounding rock (like barnacles, vermetids are often found in aggregates, where their shells fuse with each other and create a network of interconnected spirals). The animals cannot graze for algae or sessile marine life, being sessile themselves—so they also produce a mucus web and drag small animals and organic debris back into their waiting radulae. And while worm shells glue themselves to their surroundings, carrier shells do the exact opposite: instead of secreting themselves a larger or more spinose shell, these snails pick up debris (usually the shells of other snails) from the environment and glue it to themselves, enhancing their natural defenses with a makeshift armor (hermit crabs face a similar problem with their shells, as they frequently outgrow them—some handle this problem by cultivating symbiotic anemones that secrete extensions for their shells).
I’d like to say “this does it for slugs and snails,” but no, it really doesn’t. This is a huge group, and there is a lot more to say about it (I didn’t mention starfish-hunting parasitic snails, for example, or the clam snails, with their split shells). But I’ll end it here for now, on the note that the title refers to “Uzumaki,” the magnum opus of Junji Ito—and a recommended read for everyone. It’s just the right mixture of horrific and goofy, and it has people turning into snails. What’s not to like?