Worldbuilding Wednesdays: Semi-Aquatic Adaptations

Welcome to Worldbuilding Wednesdays!  Every Wednesday, we spend what is probably far too much time walking through our worldbuilding process.  In this week's post, we cover the perfect organism:  the crab.

What We Have So Far

For the last couple of weeks, we've been working on the wildlife of our world.  We now have sky whales, gliding arboreal predators, giant rabbits who leap a hundred or so feet to safety, and giant centipedes that can skitter across the ground more quickly and safely than a horse.  Things have gotten weird on and above this planet's surface, and weird is good.  It means that our worldbuilding has struck out into some creative directions.

Surely we'll be able to keep up that pace as we head into the depths... right?

Same As It Ever Was

Fun fact about water:  it doesn't really care about gravity.  Which is to say, as long as gravity is present, it's going to act mostly the same once you get beneath the surface.  Buoyancy is a matter of density (mass per volume), which isn't affected by gravity.  Moving through water doesn't become harder or easier as gravity changes.  Displacement is tied to weight, but the weight of the object displacing the water and the weight of the water being displaced are equal, so nothing really changes there, either.

What does all that mean?  Well, mostly what it means is that, when it comes to fully aquatic creatures, we're gonna see a lot of convergent evolution.  There will be plankton, there will be fish, and while there might not be sharks, there will be something you could mistake for a shark.  The biggest fish won't get much bigger than they are right now, and, while the deepest parts of the ocean will be far deeper and blacker than the oceans of Earth (the oceans can be up to 50 km deep in places), the pressure at the bottom is roughly the same.  The average temperature is probably just a little warmer, maybe a tenth of a degree or so, than Earth's oceans.  Kelp forests and other organisms that depend more on structure than swimming (like coral) will get much bigger, but otherwise, you won't see much that will surprise you in the deep oceans.

Truly Flying Fish

The surprises will be happening in the transitionary areas- where water meets air and where water meets land.  For the surface feeders, the biggest difference is that pulling their weight out of the water will be ten times easier.  Since everything else about the water is going to keep them roughly the same size, you can safely assume that what you'll see is sea critters literally leaping out of the water and quite a ways into the air.  And at least a few of those animals are going to adapt to take advantage of that time out of the water.

Consider flying fish.  On Earth, a flying fish can glide for hundreds of feet, given an initial leap at a speed of some 35 mph.  On our world, not only could they reliably glide for ten times as long, they might well be capable of sustained, true flight.  The major limitation would be their ability to survive outside of the water for sustained periods... but if flying fish developed an air bladder that served both to make them slightly lighter and as an oxygen supply, you could see entire schools of fish taking flight, perhaps soaring up to snack on the parasites carried by the sky whale overhead, or simply scattering before a swordfish can leap several dozen feet to try and snatch a few of them mid-flight.

Other aquatic types who could easily develop aerial aids will do so.  Look forward to seeing jellyfish that drift along on the wind instead of the tide, their long tendrils dangling in the water while their main body stays safely above it.

Creepy Crawlies

Intertidal creatures will be even weirder.  Most creatures you see in tidal zones don't move very much, because their hydrodynamically designed bodies just aren't suited to being out of the water.  On this world, however, pulling yourself about on fins or flopping to move are both ten times more effective.  Some creatures might actually end up being faster out of the water than in it, and others might evolve to make that true.  It's happened before; that's how land life got its start on Earth.

As with the terrestrial creatures, the intertidal creatures that will benefit the most from this situation will be those that have a multitude of limbs with friction-defeating features.  Tentacles will be helpful.  Slithering will be doable.  But perhaps no creature will benefit so much as the six-time winner of the Best Body Design award, the crab.  There are already crabs on Earth that can climb trees; the crabs of our world (who can grow to mammoth size, since they don't need to swim) will be armored all-terrain creatures, with coconut crabs the norm instead of outliers.

Prepare for a world where these are possible apex predators.

Some of Column A, All of Column B

There is one other type of aquatic creature that we still need to consider:  the formerly terrestrial creature.  This world's versions of whales, seals, penguins and the like.  As with fully aquatic creatures, those who are swimmers will tend to follow the same lines of evolution as on Earth, with one notable exception.  As we noted last time, terrestrial animals are going to be more likely to wind up with six or more limbs than they would on Earth.  It's entirely possible that those creatures, in turn, will return to the sea, where the extra set of limbs becomes an evolutionary advantage.  There will therefore be at least a few sea creatures with extra fins, or fins mixed with other appendages.  Our world's manatees might prove even more mermaid-like in appearance than Earth's, and other sea mammals might keep their skin flaps as they return to the ocean, so keep an eye out for "flying otters" (gliders rather than true fliers, but they love to slide off cliffs and catch thermals before dropping into the water) as well.

A Note on Recursion

You'll note that some of these potential species arise from considering other animal types first.  It follows that you could come up with other interesting animals by going through and seeing what happens when you adapt what you know of aquatic types to the terrestrial, arboreal, and avian creatures before.  You can also follow that train of logic right back to here and wind up in a loop.  How do you know how much is too much?

When it comes to animals that will wind up being mostly background in whatever setting you're designing, we recommend that you go no further than one more pass through the animal types.  It's actually a callback to the last time we discussed recursion, when we mentioned that you shouldn't worry about any pattern going more than three levels deep.  We're already at "Earth animals with adaptations" and "novel creatures due to low gravity;" once we hit "novel creatures due to other novel creatures," we've hit the third layer and can stop.

The Hilltop

We're finally ready for that hilltop view once more.  The world is dense with life now.  We've shifted the tree so that it's between hills, so that we don't have to worry about falling fruits or diving predators.  We're still close enough that we can see dozens, perhaps more, of various arboreal creatures playing and eating among the massive spread of branches.  Something that looks a little like a griffin (part eagle, part lion) but sized like a leopard, leaps off one branch, trying to dive onto a pack of capybara-sized sugar gliders.  It misses and the would-be prey scatters, even as the psuedo-griffin halts its dive by clawing onto the branch.

In the distance, you can see a fog bank begin to roll in.  At the front of the bank as it skims over the ocean are bright silver flashes that you know to be fish, feeding on the plankton that gets churned into the air by the tides.  Over them, a sky whale casually soars atop the fog bank, its massive wings barely moving.  Near the shoreline, thankfully far from you, you watch as two crabs the size of large dogs chase after a seal.

Next to you, grazing peacefully, is your main mode of transportation.  It is three meters long, about half a meter wide, and looks a little like eight horseshoe crabs decided to permanently form a conga line.  A special saddle that distributes your weight across three sections sits on its back.  Despite its alien appearance, the creature is docile, fairly quick, and lethally agile; were it not for your inability to hold on, you're reasonably certain you could ride "Jitterbug" (as you've affectionately named her) straight up the side of a tree.

Conclusion

With this, we've finished talking about animals, and it only took us three weeks.  Tune in next time, when we finally start talking about people.  That topic will take roughly until the end of time.