I'm designing an alien creature that has a similar body structure as a theropod. Now one thing about this creature is that it lives near bodies of water like lakes or shallow seas because that's where its prey lives. However, it does not prey its victims outside of the water but rather going inside tge water to chase them. It doesn't exactly swim though the water though. When it chases its prey it looks more like it is running at the bottom of the sea/lake. Now I got this idea from how hippos are capable of walking underwater, but there are lots of things that are different here. For example, hippos walk underwater not run, plus hippos are big stalky tetrapods while the creature I'm designing is a bipedal theropod. So my question is, what changes should I make to my creature in order to be capable of bipedal running underwater? And also, is such a creature possible?
Expanding on @Renans comment, when trying to walk underwater the biggest problem you have is providing motive force without simply pushing yourself off the ground. To combat this you need a lot of downward force that can be gained in two ways: Density and Power
Density is obvious. Dense things sink, and so they can push off the floor and forward to their prey. The downside, of course, is that you have more weight to move around.
Power is less so, but basically if you swim downwards constantly you can exert force against the floor and use it to push off.
So, to your beasties. They are very muscly (dense), powerful creatures with powerful tails. When chasing prey they use powerful tails to push themselves both forward and down (powerfully), and then use powerful legs for explosive bursts of speed and dangerously fast changes in direction. ‘Walking’ on the bottom gives them an advantage in leverage when it comes to combat manoeuvres, allowing them (with clever use of grasping limbs and positioning) to best theoretically ‘stronger’ prey. They should also be able to burst up to the surface and snatch prey on the surface, or just use that to escape from the seabed should they need to.
Of course, all this requires relatively sturdy ground. No point in using the ground for leverage if it’s made of sucking mud or sand, so places where the waves or tides have exposed bare rock would be best.
I’m envisioning smart ambush predators that prey on creatures trapped in large tidal pools. Basically aquatic raptors...
I may never swim near the coast again.
For speed underwater, if you're going to insist on bipedal motion (or really any motion that isn't swimming) you're going to need to overcome a couple blatant obstacles.
- Water is dense and relatively uncompressible. As a result, it's less able to 'get out of the way' as your creature (which is more dense) moves through the space it so recently occupied.
- Water is under pressure, the deeper you are, the greater the pressure. As you move out of a space, you leave a 'hole' that the pressurized water will quickly move to fill. This water includes the water immediately in front of you (this btw, is how airplane wings work in principle, by creating a lower pressure zone above the wing)
That being pointed out, there are ways to get bipedal underwater sprinting to be a thing that actually happens. Your creature will resemble a tapered wedge at both the front and the back, and the bipedal limbs will resemble (jointed?) oars, able to have a flat, gripping surface that can be turned sideways to minimize drag on the fore-swing. Basically imagine a flatfish with articulated flippers that function like tentacles but are shaped like, well, flippers.
The gradually tapered front gives a aquadynamic design for parting the water, the reversed taper at the back allows the water to be easily and gently "unparted" to prevent the creation of a low pressure zone (or at least minimize the pressure difference). The articulated flippers give the ability to adapt to the underwater terrain without introducing too much drag.
The serious downside to such a design comes with the underwater currents common to essentially every major body of water. One cross-current and your creature is helplessly dragged sideways, due to the almost sail-like form of its body.
As a joke, you could also just chop all but 2 tentacles off an Octopus and hope for the best.
To those who would say a tail is better, remember that evolution
1 Has to work with what it has. A biped can't give birth to a fish.
2 Can only select individuals that make it work in their lifetime and survive to breed. Doesn't matter if an individual represents a step towards the perfect solution if the intermediate steps aren't survivable.
So what would happen here is a bipedal animal that hunts by running on land finds itself low on land-dwelling prey, and faces a sudden pressure to hunt some water-dwelling prey. The individuals that survive the first wave of land-prey-famine do so by running through the water. It's awkward but it works. Individuals that start down the path of becoming fish (webbed feet, legs start to fuse together) are neither good at running anymore, nor good at swimming yet, and die. Running works though, and those individuals live. Although we could see a better solution, the sudden change in selection pressure makes natural selection myopic. It can only see what works right now.
You'd need to be significantly denser than water so that you sink and land on the bottom with weight on your feet. Your feet need weight pushing them into the ground so you can get traction. So your biped is lean, and either has dense bones or something like scales or a shell that doesn't float. As it adapts to running underwater its limbs become very narrow and streamlined. It probably hunches over so that the length of its torso is aligned if the direction of motion. So maybe the first individuals to go in the water were awkwardly hunching themselves over like a bicyclist and then maybe this led to adaptation to a posture more like a T-rex in later individuals.
So maybe like a T-rex with legs that are wide back-to-front with muscles for running, but narrow side-to-side to slip through the water, and a streamlined head like the front of a fish to slip through the water. The smaller muscles on the side of the legs would be OK underwater where it isn't as easy to topple over, but eventually your creatures might find that their legs are no longer suitable to balancing their top-heavy bodies on land.
I like Joe Bloggs' answer, but since I can't give that one anymore, I'll offer a slightly more exotic one (especially since you said "alien" in your description).
Your creature has developed a beneficial, almost slightly symbiotic, trait which allows it to not require massive amounts of power or weight to run underwater. Imagine it has developed something similar to the hook part of Velcro on its feet. The lake bottoms around it are covered with a strongly rooted plant which resembles the loop portion (it may also gets= most of its nutrition from the scattered remains of your theropod's prey which meets its end underwater, symbiosis is always interesting).
This allows your creature to still move forward as the hooks all point backwards on its feet, something similar the special skis people use to actually walk up mountains and then ski down (Ski skinning). But instead of retarding rearwards motion, the hooks and loops combine to prevent upwards motion. Your creature is still free to 'run' forward using leg power alone.
If your creature can control how these miniature hooks on its feet come out, it can change directions quite easily or release itself in case of entanglement.
Just a more off-the-wall idea (which can likely be built on) if your world allows for such things.
Hippos are fast underwater because of their low centre of gravity, their sheer weight and their special skin. A therapod could angle down on all fours and push forwards. Smooth skin, and as others have suggested a powerful tail would help. Penguins are modern therapods... Perhaps your therapod has wing like powerful forearms the work well under water?
Dreamworks got around a similar problem by making one of their dragons - in How to Train Your Dragon's television series, the speed stinger grow webbed feet so they could actually walk on water. I mention this because the speed stinger was also a wingless therapod. I direct you here: http://dreamworks-dragons.wikia.com/wiki/Speed_Stinger hope that helps :-)
You can't. Water is dense: it provides bouyancy and considerable resistance to motion. To even walk underwater, your creature first has to be a good bit denser than water, otherwise the force of a leg against the bottom will simply push it upwards. That means that at best the motion will more or less resemble astronauts moonwalking rather than any sort of running.
Now once we get that moonwalk going, realise that in water the legs have to swing back and forth against water resistance. This uses energy, and probably more energy than could be obtained from prey, so your creature starves and becomes extinct.
That's not the biggest problem, though. Even if we overcome the first two, the creature is trying to move its upright, unstreamlined body against water resistance. This means that it will be very slow (try it yourself if you have a pond or stream handy), and will not catch prey. Thus it will starve, and again, the species becomes extinct.
Really, there's a reason why things as diverse as dolphins, tuna, ichthyosayrs, and submarines all have the same basic shape: physics dictates that that's the shape you must have if you want to move at speed in the water.
A fish hook with legs...
No, but seriously; there are many fishooks that are shaped so that they will dive into the water when you reel. See https://www.google.com/search?client=ms-android-samsung&ei=aox-W8_eE9nF0PEP64WWwAk&q=diving+lure&oq=diving+lure&gs_l=mobile-gws-wiz-serp.3..0i71l5.0.0..30138...0.0..0.0.0.......0.kyE5YgwE8PA
In order to go faster in the water your creature needs to stay low to the bottom so it can push against the ground and gain momentum. It will probably have strong legs, small, fined arms that either pull against the body or streamline behind it, and a tail with webbed spines to give it even more momentum.
Another part of going fast underwater is being very streamlined. There's a reason almost all fish look like rounded arrowheads, and that's because they evolved to be streamlined. Your creature will need a narrow, tube like body if its to keep up with anything underwater. It's already at a disadvantage with legs, since they have drag, plus it will need a head shaped like a wedge to keep its feet properly pressed against the bottom, which also generates drag.
All in all I would make this creature closer to a lizard than a theropod, a frilled neck to act as a wedge and keep it low, webbed feet that work similarly to a frogs in that they stay under and beneath its body, minimizing drag, and a strong thick tail that has a fin on the end made of webbed spines that can be flattened on land, which it uses to gain an additional burst of speed when catching prey, and also as a counterbalance on land to keep it on two feet. It catches prey in its mouth and then takes it back to land to eat so it can spit the excess water out.