Fucking seadwellers, how do they work?
The very basics! Eridan is 5'11", although as a highblooded troll he's probably not done with his growth yet and might get a little bit taller. Things he inherited from Dualscar: broad shoulders! Things he didn't inherit: the gunshow. Think swimmer body- broad shoulders, narrow waist, long muscled legs, but overall not exceptionally bulky, just long and fairly lean.
He's got a pretty masculine face- strong jaw, straight nose, high cheekbones and a perpetually sneering look, although that's just him, his face doesn't always look that way. Basically he looks like this, only I always imagine that he has the goofy lightning bolt eyebrows this artist draws him with (I think they are cute, okay) His teeth are like shark teeth, which is to say razor sharp and triangular, although less like the ones you'd see in a great white (which are pretty snaggletoothed) and as even as human teeth. I also think his hair is actually curly/wavy and a little on the longish side when it's down, depending on how it dries (or gets messed up), so he has to spend a lot of time in the morning straightening it as he combs it back and up and it takes him a good hour+ to get it done exactly right. The purple part also takes up a rather significant part of the front of his hair- not overly dominant, but large enough that it's not just a small streak at the temples but an actual large bunch of hair (see above example!) It's exceptionally soft because he uses about three different shampoos a day and also never really goes swimming enough to truly damage it, although it has a tendency to get greasy if it's touched too much from all the product and hair spray used to keep it back. I don't know that I really need to cover horns but I will anyway because there's some discrepancy over the shape in fanart- they go back, and to either side as they get taller, and the bend is to the back and not the front, so they're angled at something of a diagonal line in relation to his head. They definitely look like lightning bolts. His eyes are lighter than what Hussie claims to be orange, definitely more of a shade of gold, and posses a second, nictitating membrane for underwater. And although it would be very hard to tell unless you were staring right up underneath his chin or looking really closely at his face, just behind his jaw on the underside and on his cheeks close to his nose are little clusters of ampullae to do the same electrorecepting sharks can (they basically look like little enlarged pores or freckles).
He does also have freckles, though. He freckles easily. I also think seadwellers can smell blood like sharks, but that has nothing to do with how he looks.
AND MY FAVORITE PART: THE FINS. The fins actually inhabit the same space that his ears would and function somewhat similar- the ear canal actually lies just in front of the fins at their base, but the fins themselves aren't connected to them or protect them the way regular ears would. They work pretty much the same way betta fish puff themselves up when they're feeling threatened- if Eridan is angry, or feeling aggressive, they'll get bigger and open up and out in an attempt to make himself look larger and more intimidating. Like so. When he's upset, they fall back and get a little droopy; they twitch when he gets nervous and open when he gets excited, too. They're little mood barometers that he has absolutely no control over, and they're very sensitive and delicate despite the rest of him being pretty tough.
Working down from there- I've based seadweller anatomy primarily on sharks, which means that instead of one large gill like most fish (which a humanoid body would have difficulty accommodating due to the size it would need to be to adequately supply his body with oxygen) he has six. Unfortunately this is really difficult to explain just where this would sit in the body and how without a chart, and the best chart I have is in a book that I can't scan for fear of destroying it. This chart is the best that I can find to explain the difference; the gill rakers would simply attach to the spinal column and close proximity to that would allow blood directly into the artery anterior spinal artery. Three of them are located in his neck, and three of them are located at the bottom of his ribs, between the very last two and one just below. When he's above water this system closes off, although the gills can be pulled slightly open when he moves and stretches, and he posses lungs for breathing in air. Both respiratory systems share the same windpipe, but when swimming this above-air system is pressurized, collapsed to avoid the problems associated with deep sea diving, and closed off so that no water can get into his lungs and water can be passed through the mouth and out the gills. If he really and truly concentrates above water, he can manage to open his gills, which are bright purple like his blood and also usually very soft and dry instead of being slimy, but unless something goes horribly wrong, he can't force his body to open the land-breathing system under the water (it's mostly internal muscles, too difficult to exercise and control.)
I am going to briefly sidetrack here to talk about body temperature because this is a thing that bothers me. My headcanon is fully that higherblooded trolls are warmer than lowerblooded trolls, and that seadwelling trolls are NOT cold-blooded (at least not in the sense that we would think of it, in that I think they possess warm-blooded traits at minimum.) I have a few reasons for this. The first is that cold-blooded traits for marine animals don't really match what canon seems to tell us about troll species. Metabolism slows with colder temperatures, which means less eating is necessary but also less energy, which means that cold-blooded marine organisms are only capable of moving in very short quick bursts of energy, which seems contrary to the very active lifestyle trolls appear to live as an aggressive species. This would also be a pretty poor adaptation to have if you were going to be at the top of the hierarchy. Most of the largest and most potent marine predators are warm-blooded (they're also mammals, but there are exceptions IN SHARKS, you knew that was coming). Furthermore, warmer body temperature = easier to fight off infections, viruses, and bacteria, which would mean they would be healthier and probably live longer than other trolls. Thirdly, I am basing a majority of all of this off of shark biology, and there are in fact sharks which can be considered either warm-blooded or posses warm-blooded traits. (The lab I work in calls these "endothermic" capabilities, which is not the same thing as being warm-blooded, but for the purposes of Eridan's body temperature it is.) The lamnidae family of sharks, including Great Whites which is basically what I use as a template for everything, have thermoregulating capabilities that allow them to boost their temperatures up to 57 degrees F (14 degrees C) higher than their surrounding temperatures. One can even boost it up to 70/21 higher than subarctic temperatures, and this is mainly why I say that Eridan is perpetually warmer than pretty much everyone around him all the time always.
Okay so junk. Eridan carries his inside his body, in what is biologically called a vent. What this means is that at his groin there's essentially a bulge (not his penis-analog bulge, but an actual swelling) where it sits inside, and a slit in the covering of the skin for it to emerge. Like so (I promise that picture is not at all dirty, you are safe to click on it.) When he gets aroused it simple pushes forth out of his body. His shame globes/testicles also lie in this...for lack of a better word, pouch of flesh, although they're not obvious until his actual bulge emerges and the skin draws back. Although it's a bone bulge I've almost imagined it's more cartilaginous than bony because that would allow it more freedom of movement, although the cartilage is ridged like a seashell and also slightly kinked in the middle like his horns (though not nearly so severe or pointy, that would just be painful). His genetic material is inky! By which I mean it has the same consistency and smell as what an octopus or a squid would excrete when startled, so essentially his shame globes double as ink sacs, only his ink is purple. This is really not that exciting, I just wanted to be able to talk about ink sacs in smut threads and make jokes about him inking himself.
Small, final details! He smells like sea salt, even when he hasn't been to the sea all that recently. He does have webbed feet and hands, although the webbing between his fingers is retractable so it can't catch and rip on things and thus he can wear all his gaudy rings. The webbing between his toes isn't; his toes are also a little on the long side (not grotesquely so, but just. Long toes, okay) to provide more of a flipper-like coverage. He has large, long feet, hands and fingers for this reason as well. Also no body hair! No nipples and no bellybutton because he is not a mammal, his is a fish/insect boy. Also he has swimmer's ass, which means it is round and firm. I fully subscribe to the idea that Eridan does know how to swim and be a proper sea troll, he just chooses not to, because anything else seems silly no matter how adorable it is.
He's got a pretty masculine face- strong jaw, straight nose, high cheekbones and a perpetually sneering look, although that's just him, his face doesn't always look that way. Basically he looks like this, only I always imagine that he has the goofy lightning bolt eyebrows this artist draws him with (I think they are cute, okay) His teeth are like shark teeth, which is to say razor sharp and triangular, although less like the ones you'd see in a great white (which are pretty snaggletoothed) and as even as human teeth. I also think his hair is actually curly/wavy and a little on the longish side when it's down, depending on how it dries (or gets messed up), so he has to spend a lot of time in the morning straightening it as he combs it back and up and it takes him a good hour+ to get it done exactly right. The purple part also takes up a rather significant part of the front of his hair- not overly dominant, but large enough that it's not just a small streak at the temples but an actual large bunch of hair (see above example!) It's exceptionally soft because he uses about three different shampoos a day and also never really goes swimming enough to truly damage it, although it has a tendency to get greasy if it's touched too much from all the product and hair spray used to keep it back. I don't know that I really need to cover horns but I will anyway because there's some discrepancy over the shape in fanart- they go back, and to either side as they get taller, and the bend is to the back and not the front, so they're angled at something of a diagonal line in relation to his head. They definitely look like lightning bolts. His eyes are lighter than what Hussie claims to be orange, definitely more of a shade of gold, and posses a second, nictitating membrane for underwater. And although it would be very hard to tell unless you were staring right up underneath his chin or looking really closely at his face, just behind his jaw on the underside and on his cheeks close to his nose are little clusters of ampullae to do the same electrorecepting sharks can (they basically look like little enlarged pores or freckles).
He does also have freckles, though. He freckles easily. I also think seadwellers can smell blood like sharks, but that has nothing to do with how he looks.
AND MY FAVORITE PART: THE FINS. The fins actually inhabit the same space that his ears would and function somewhat similar- the ear canal actually lies just in front of the fins at their base, but the fins themselves aren't connected to them or protect them the way regular ears would. They work pretty much the same way betta fish puff themselves up when they're feeling threatened- if Eridan is angry, or feeling aggressive, they'll get bigger and open up and out in an attempt to make himself look larger and more intimidating. Like so. When he's upset, they fall back and get a little droopy; they twitch when he gets nervous and open when he gets excited, too. They're little mood barometers that he has absolutely no control over, and they're very sensitive and delicate despite the rest of him being pretty tough.
Working down from there- I've based seadweller anatomy primarily on sharks, which means that instead of one large gill like most fish (which a humanoid body would have difficulty accommodating due to the size it would need to be to adequately supply his body with oxygen) he has six. Unfortunately this is really difficult to explain just where this would sit in the body and how without a chart, and the best chart I have is in a book that I can't scan for fear of destroying it. This chart is the best that I can find to explain the difference; the gill rakers would simply attach to the spinal column and close proximity to that would allow blood directly into the artery anterior spinal artery. Three of them are located in his neck, and three of them are located at the bottom of his ribs, between the very last two and one just below. When he's above water this system closes off, although the gills can be pulled slightly open when he moves and stretches, and he posses lungs for breathing in air. Both respiratory systems share the same windpipe, but when swimming this above-air system is pressurized, collapsed to avoid the problems associated with deep sea diving, and closed off so that no water can get into his lungs and water can be passed through the mouth and out the gills. If he really and truly concentrates above water, he can manage to open his gills, which are bright purple like his blood and also usually very soft and dry instead of being slimy, but unless something goes horribly wrong, he can't force his body to open the land-breathing system under the water (it's mostly internal muscles, too difficult to exercise and control.)
I am going to briefly sidetrack here to talk about body temperature because this is a thing that bothers me. My headcanon is fully that higherblooded trolls are warmer than lowerblooded trolls, and that seadwelling trolls are NOT cold-blooded (at least not in the sense that we would think of it, in that I think they possess warm-blooded traits at minimum.) I have a few reasons for this. The first is that cold-blooded traits for marine animals don't really match what canon seems to tell us about troll species. Metabolism slows with colder temperatures, which means less eating is necessary but also less energy, which means that cold-blooded marine organisms are only capable of moving in very short quick bursts of energy, which seems contrary to the very active lifestyle trolls appear to live as an aggressive species. This would also be a pretty poor adaptation to have if you were going to be at the top of the hierarchy. Most of the largest and most potent marine predators are warm-blooded (they're also mammals, but there are exceptions IN SHARKS, you knew that was coming). Furthermore, warmer body temperature = easier to fight off infections, viruses, and bacteria, which would mean they would be healthier and probably live longer than other trolls. Thirdly, I am basing a majority of all of this off of shark biology, and there are in fact sharks which can be considered either warm-blooded or posses warm-blooded traits. (The lab I work in calls these "endothermic" capabilities, which is not the same thing as being warm-blooded, but for the purposes of Eridan's body temperature it is.) The lamnidae family of sharks, including Great Whites which is basically what I use as a template for everything, have thermoregulating capabilities that allow them to boost their temperatures up to 57 degrees F (14 degrees C) higher than their surrounding temperatures. One can even boost it up to 70/21 higher than subarctic temperatures, and this is mainly why I say that Eridan is perpetually warmer than pretty much everyone around him all the time always.
Okay so junk. Eridan carries his inside his body, in what is biologically called a vent. What this means is that at his groin there's essentially a bulge (not his penis-analog bulge, but an actual swelling) where it sits inside, and a slit in the covering of the skin for it to emerge. Like so (I promise that picture is not at all dirty, you are safe to click on it.) When he gets aroused it simple pushes forth out of his body. His shame globes/testicles also lie in this...for lack of a better word, pouch of flesh, although they're not obvious until his actual bulge emerges and the skin draws back. Although it's a bone bulge I've almost imagined it's more cartilaginous than bony because that would allow it more freedom of movement, although the cartilage is ridged like a seashell and also slightly kinked in the middle like his horns (though not nearly so severe or pointy, that would just be painful). His genetic material is inky! By which I mean it has the same consistency and smell as what an octopus or a squid would excrete when startled, so essentially his shame globes double as ink sacs, only his ink is purple. This is really not that exciting, I just wanted to be able to talk about ink sacs in smut threads and make jokes about him inking himself.
Small, final details! He smells like sea salt, even when he hasn't been to the sea all that recently. He does have webbed feet and hands, although the webbing between his fingers is retractable so it can't catch and rip on things and thus he can wear all his gaudy rings. The webbing between his toes isn't; his toes are also a little on the long side (not grotesquely so, but just. Long toes, okay) to provide more of a flipper-like coverage. He has large, long feet, hands and fingers for this reason as well. Also no body hair! No nipples and no bellybutton because he is not a mammal, his is a fish/insect boy. Also he has swimmer's ass, which means it is round and firm. I fully subscribe to the idea that Eridan does know how to swim and be a proper sea troll, he just chooses not to, because anything else seems silly no matter how adorable it is.