The latter is actually the perfect setup, for both frog and spider. As the spider feasts on prey, discarded remains begin to pile up. This (along with the spider's appetizing eggs) attracts ants and other hungry pests, which the tiny amphibian eats up before any damage can be done to the nest. In exchange, the spider does not kill and eat the frog. That's ... pretty much it. These two radically different species have come to an unspoken understanding.

Francesco Tomasinelli
Or maybe a spoken one. We don't speak spider-frog.

Of course, this is a giant, angry, hungry spider, and sometimes they forget themselves. Should Xenesthis ever lose its shit like Oscar Madison on bath salts and start picturing its pet Felix roasted on a plate with an apple in its mouth, the suddenly-not-so-harmless froggy will unveil its secret weapon: toxic skin. Biting into it isn't enough to kill the spider, but will certainly persuade it to cancel its dinner plans and go back to chewing on something safer. Like a cobra.

Emperor shrimps are natural hitchhikers, and they'll ride just about anything in a pinch (they might even choose you, which is certainly a better fate than if a shark decided to do so) -- but their ideal whip is some venomous creature, like a sea cucumber, medusa worm, or nudibranch. Those animals are usually brightly colored, a striking visual that can easily convince potential predators to stay the fuck away or risk a very painful death indeed.

Unlike most other creatures that travel from place to place via living, breathing taxi, these shrimps are not parasites. They don't harm their trusty steeds in any way, and might even help them by nibbling on actual parasites that stupidly try to take root. More often, though, they simply hitch a ride to wherever it is shrimps go when they're not slathered in butter on somebody's plate, feasting on any waste or debris that the steed kicks up during the ride.

Steve Childs
"I'm going to call you Falcor."

Emperor shrimps are so dependent on their free rides, in fact, that if they can't get one, they'll just sit around and mope until something sittable comes along. But when one does, the scene's more magical than a hundred "soldiers on leave surprising their children" videos put together. Such as in the video below: If worm whispering isn't your thing, just skip to 5:20 and enjoy the new couple's beautiful first dance together.

Keyhole limpets are typical cone-shelled mollusks, in that they don't do a whole lot with their day aside from laying there, sticking to shit, and suffering recurring nightmares about someday being fried and dunked in mayonnaise.

Chilean Seafood Eirl
Mmmmm ... limpy.

Their shells offer decent protection from predators, so long as those predators are too stupid to realize that meaty mollusk goodness is but a single flip away. The problem is, they're usually not, and this results in the limpet spending the rest of eternity as starfish poop.

But limpets aren't stupid (well, as not-stupid as a near-brainless organism can be), and many have set up a home defense system to scare off those who might want to eat it. Since their species isn't covered under the Second Amendment, guns are sadly not an option (they have no cold, dead hands from which to pry a rifle anyhow). Their alternate weapon of choice? A big, ugly, pissed-off attack worm.

wallawalla.edu
"I'm standing my ground ... well, ocean floor. You know what I mean."

Known as Arctonoe vittata (which rhymes with "Hakuna matata," in case Weird Al is reading this and needs ideas), this cuddly bit of bait lives inside the limpet's shell, happily eating and napping all day while enjoying the good life. But unlike your lazy, apathetic cat, who clearly takes being a pet for granted, the scale worm is extremely protective of its cozy home. Should some shithead predator attempt to dine on its master, the worm will quickly (and viciously) dissuade it from doing so via a series of ferocious ninja bites.

The biting part could easily be avoided if the limpet would simply stick up a "Beware of Worm" sign somewhere in its territory, but clearly it would rather hand would-be intruders a brutal, toothy surprise. They're assholes like that.

Dave Cowles
It's basically giving you the finger in this pic.

But when it comes to finding something to drink, they're all about milking the living. Their favorite cow is the leafhopper, an insect famous for ... well, hopping. But also for regularly getting wrangled up by the big tough meat ants and harvested for butt residue. The hoppers produce a sweet resin that the ants just adore, both for consumption and for food preservation. In return for the never-ending butt-ffet, the ants offer protection from predators who value the leafhoppers less for their resin and more for their crunchy, yummy bodies.

If need be, the ants will actually carry the leafhopper (or any other sweet-pooping insect it might rustle up with its metaphorical lasso) away to safety so the two can coexist more peacefully.

brisbaneinsects.com
"I can show you the world. Shining, shimmering, splendid."

The hoppers are so dependent on their masters for survival, they might not even mind the ants turning them into steak once the drink finally runneth dry.