Fantasy worlds in literature often include merfolk, although they are often only mentioned in passing. They exist, for example, in both the Narnia and Harry Potter books, and they have become somewhat more popular of late, often in horror films. Some of these versions are inspired by the relatively benign folk of Hans Christie Andersen's The Little Mermaid, while others reflect humanity's ambivalent relationship with the sea, and our perception of fish as 'ugly'. Earlier myths often do a bit of both, with a particular theme being that mermen are ugly and wont to drown sailors, while mermaids are sexy and seductive.
While in myths, merfolk are sometimes the outcome of some specific magical curse on a previously normal human, they are quite often seen as a race, such that both males and females exist, even if particular stories tend to focus more on one than the other (mermen in the myths, mermaids in more modern fiction, although there are plenty of exceptions).
1EOriginally, merfolk are depicted very much as counterparts of regular humans, only living in the sea. They have no tendency to any particular alignment, have the same intelligence as a regular human, and their basic warriors are roughly as skilled as a 1st level human fighter. In terms of their statistics, the only real difference is a superior armour class, which may just represent that they're agile and difficult to hit, rather than a leathery hide.
The picture shows a fairly classical merman from legend, although without the beard that many classical depictions include. They are human from the waist up, but with gill slits at the base of the throat, and webbed hands. There's no indication that skin and hair colour are any different from those of regular humans. The tail is fish-like, with scales and a caudal fin, and that's about it. Like seals in the real world, they have difficulty hauling themselves about on land due to their lack of legs.
We're told that they live in communities averaging around 300 individuals, which usually consist of no more than a collection sea caves. They apparently don't wear clothes (and, indeed, why would they?), but are adept enough at tool-making to produce crossbows as well as spears, nets, and tridents. Males and females are equally common, and, as is pretty much universal in 1E, the females don't fight. Indeed, the race is referred to as "mermen" in this edition, likely by analogy with Tolkien's use of "Men" to refer to our race, as opposed to elves or dwarves.
They are omnivorous, and evidently live in waters shallow enough to make gathering seaweed a sensible proposition (so 200 metres, at the very most), as well as herding fish, which has no such limitation. As one might expect, they have their own language, and they don't typically learn that of humans, implying little cultural contact between the two.
2EThe 2E version is even closer to the classical look of merfolk, lacking the visible gill slits, as well as making it clear that, in keeping with common artistic convention, the caudal fin is horizontal, rather than vertical, as it is in true fish. It also turns out that, even if the males wear no more than jewellery, the females at least wear bikini tops. The text description seems to imply that merfolk are always caucasian in appearance (they now live only in temperate waters, rather than 'warm temperate and tropical'), while we learn that the tail can be either silver or green.
Merfolk culture is also elaborated on. It turns out that their villages can be three times the size previously stated, and that their leaders are (as one might expect) more skilled than the usual tribesfolk. A few even have priestly magic, although there are no wizards, presumably because writing is going to be difficult underwater unless you want to carve your spell book into a piece of monumental masonry. It's also confirmed that the culture is "heavily patriarchal" with mermaids essentially confined to their homes.
Oddly, while 2E merfolk are obviously capable of breathing air as well as water, they die if kept out of water for long, with a survival time of about three hours being typical. This isn't retained in later editions, and it's not entirely clear why it would be the case, since you'd think they'd be as resistant to dehydration as, say, a seal. Perhaps it's partly a magical effect, tying them to their element.
3EThe image here makes it clearer that merfolk no longer have gill slits, as they do in 1E. The lower body has a long dorsal fin and at least one other fin, in addition to that on the tip of tail. The upper portion remains human-looking, except for blue hair, and what are probably small pectoral fins on the shoulders, but could arguably be jewellery of some sort. They no longer have webbed fingers. Finally, while 2E merfolk are less than six feet in total length, the 3E sort are said to be eight feet long, which fits with the proportional length of the tail shown in the picture.
It's in this edition that the name switches from 'merman' to 'merfolk', and there is no longer any hint of a patriarchal culture in the description. They are described as playful, and living in semi-permanent settlements rather than in (presumably permanent) caves as previously. In terms of their abilities, they are slightly fitter and more physically agile than humans, but it turns out that their warriors do, after all, wear 'leather' armour (shark-skin, perhaps), which explains the apparently tough skin of earlier editions. While priestly spell-casters still exist, bardic magic is now more common, and its practitioners tend to be the community leaders.
Where the previous versions tamed barracudas to use as guards, this one associates primarily with porpoises - albeit not presumably for the same purpose. In line with the reduction of the number of languages generally seen in 3E, merfolk no longer have their own tongue, having adopted that of water elementals.
5EWhile the stats of merfolk remain similar in this edition, allowing for relevant changes in the rules, both physical appearance and culture have changed significantly. The mermaid in the picture is blue-green, although we're told that they have a wide range of different skin tones, just as humans do. More significantly, she has a vertically arranged caudal fin, double pelvic fins, and highly webbed fingers, in addition to the dorsal fin on the tail. The pectoral fins are now on the lower arm, rather than the shoulder, and there are particularly odd fins below the greatly elongated ears and on the top of the hairless head.
Merfolk also appear to have barbels projecting from their foreheads, which may be sensory in nature, although, at least within the granularity of the rules, merfolk senses are similar to those of humans, so that they need to carry light sources when venturing deep. For some reason, the one in the picture is wearing a skirt, which seems impractical.
Culturally, merfolk are far less technically advanced than previously, and they certainly don't have crossbows. There's no indication in the core rules that they can still use magic, but that hardly proves they can't. We're told that they don't even have the know-how to 'shape stone' to make buildings, although the next paragraph states that they can 'carve rock' for the same purpose, so that's all right (?) In any event, the great majority are now nomadic tribes, with the settlements of the first two editions now being rarities, though not unknown.
They apparently deal more with humans than in previous editions, since they typically speak the Common Tongue alongside their native Aquan.
They apparently deal more with humans than in previous editions, since they typically speak the Common Tongue alongside their native Aquan.
While most fantasy RPG settings do seem to include mermen, or something similar, they necessarily have limited contact with land-dwelling races, and so are usually little mentioned in core books. In the Forgotten Realms they are settled enough to have long-lasting kingdoms and often associate with other marine races, such as aquatic elves. In Mystara, at least one group are closely allied with one of the human nations, trading with them and sharing naval/military defence. In Golarion, in contrast, they are generally deeply xenophobic, hostile towards other aquatic races as much as to surface-dwellers. In Eberron, they aren't mentioned at all, but do apparently exist, and are as varied as humans.
Since they are such a common mythic concept, similar races exist in many other fantasy RPGs, too. In the default Savage Worlds fantasy bestiary, for example, they are hideous shape-shifting monsters, partly inspired by the Sirens of Greek myth. In Yrth, the default setting of GURPS Fantasy, they are a widespread tribal stone-age culture, capricious and aloof. They are specifically said to reproduce in the mammalian manner, and can survive at depths of no more than about 100 metres without difficulty. In Glorantha, there are multiple different races that fit within the broad 'merfolk' category, with one of the most significant having the lower half of a dolphin, rather than a fish; like dolphins, but unlike most classical merfolk, they can stay submerged for extended periods, but have to return to the surface to breathe.
Merfolk BiologyLet's turn to the question of how merfolk might work biologically. To start with, there is the physical shape. Most depictions, in D&D and elsewhere, show the tail fin as spread out horizontally, not arranged vertically as it is in typical fish. This implies an up-down swimming motion with the tail, as used by dolphins and porpoises, rather than the side-to-side motion of fish. Some pictures show a dorsal fin, which would help with stability, but, while webbed hands might help a little with propulsion, the arms would mainly be used for steering. It's harder to see why the arms would have fins on them, as they often do.
In terms of the skeleton, merfolk are typically shown as having hips, which means that they must have a pelvis, but all that's behind this is a long tail formed from the backbone. (Some artists give a shape to the tail that implies human leg-bones, with knees and so forth, inside them, like an actual woman in a mermaid costume. But most go for a more piscine form).
The most significant question about merfolk biology however, is how they can breathe both air and water. Even those real-world fish that can extract oxygen from the air aren't as adept at it as mermen are said to be, but there is one species of living lungfish that does have both fully functional lungs and gills (the other species lack gills as adults, only breathing air). So it's clearly not impossible.
That arrangement - having both lungs and gills - is clear in the 1E merman, and also described in early depictions of 'aquatic elves', but not so much in later versions. Such a merman would presumably breathe as a fish does, regularly swallowing oxygenated water through the mouth, and pushing it out through the gill slits in the throat. The narrow and short human neck means that these gills must be small, and so probably far more efficient than those in real-world fish. The merman would have to close off his trachea and redirect his internal bloodflow to the relevant respiratory organs at the same time, but, again, Australian lungfish can, in fact, do this.
Merfolk, however, are frequently shown without visible gills. This leaves us with two possible mechanisms for their breathing. Firstly, their rib cage might actually contain gills, not lungs, remaining full of water while on land, but still breathing through their nose. That would help them resist the bends after diving, and maintain themselves under high pressure, but implies a magically effective means of transferring oxygen from air to water at the larynx while above the surface. They'd also have to snort water out of the nose whenever they surface, to keep the upper respiratory passages free (they couldn't speak in air otherwise), which seems unlikely.
The alternative is that, while underwater, they breathe through their skin. Which would at least fit with them wearing such minimal clothing, although that would weigh them down underwater anyway, which is reason enough to go near-naked. This would also, it has to be said, require a magically effective means of transferring oxygen into the body, since the skin doesn't have a large enough surface area for this to work on the scale of a physically active merman, and, anyway, it's mostly human-like or scaled, which wouldn't be much use as a respiratory membrane. They'd also need some means of avoiding the bends, as gases in the lungs create bubbles in the blood when the external pressure drops, but this is something that dolphins, whales, and seals can all do perfectly well in reality.
Speaking of which, how deep can merfolk dive? The GURPS figure, as we've seen, is about 100 metres, which means that merfolk can only reach the bottom in coastal waters, and a few, particularly shallow seas, such as the North Sea in the real world. Elsewhere, they'll have to float about in open water, if they wish to visit at all. Most D&D merfolk, however, appear to be able to go much deeper than this, something that might be possible for short periods of time, as whales attest, but would likely require some sort of magical adjustment if they can prolong it for more than a few hours. (Of course, it wouldn't be an issue if they naturally evolved at greater depths, but then they'd have the opposite problem on surfacing - it's the wide range of survivable pressures that's the issue here).
3E gives merfolk Low Light Vision, which makes sense given that the light is dim under even shallow depths of water, but, like humans, they do need at least some light to see. Other editions don't have this level of granularity - it's Darkvision or nothing - which is likely the only reason it's absent in 5E.
This brings us to the question of merfolk reproduction. Here, the evidence seems stacked in favour of a mammalian system. Certainly, we can see from their upper bodies that mermaids are mammalian. Most images also show merfolk having navels, which means they must have an umbilical cord, and hence, aren't born from eggs or fish spawn - merfolk pregnancy is therefore surely similar to our own. This would also explain the wide, human-like, hips that mermaids usually seem to have.
That implies a similar female reproductive system which, in turn, at least makes it likely that the male system is also similar. The necessary parts must be hidden in some kind of fold in the hind portion of the body, as they are in dolphins. Indeed, aside from the scales and rayed fins, it's plausible that the internal structure of a merfolk's tail resembles that of dolphins more than it does that of fish.
Modelling D&D style merfolk in other systems that don't already have something suitable requires accounting for their slow movement on land, and a slightly raised agility, stamina, and probably charisma (or equivalent), but few other changes beyond their ability to breathe both water and air. In general, they are supposed to be the aquatic equivalent of humans, with the same variety of personalities, skill sets, and cultural foibles as we have.
While some other aquatic races from the setting are capable of surviving for extended periods on land, however, the lack of terrestrial mobility of merfolk makes them unsuitable as PCs outside of highly specific scenarios.