Selkies native to Scotland are one of three known groups of merfolk, although it is possible that there are more less widely known subspecies around the world. They are not as "pretty" and human-like as the sirens of Greece, nor as wild and "monstrous" as the merrows of Ireland. Selkies possess the appearance of gray skinned humans from the waist up, and from the waist down they have silver scaled fish tails. While their torsos are roughly the same size as those of similarly-built humans, their tails are long enough that the average head-to-fin length of an adult selkie is seven feet.
The facial structure of selkies is the same as that of humans, but their teeth tend to be yellowish and more pointed as they are by nature hunters. Their irises are predominantly yellow, ranging from burnished gold to a mellow buttery color. In rare cases, yellow-green or yellow-orange eyes appear. Their lips usually have a slight green tinge to them, and their hair -- which is almost always kept long on men and women -- is dark green.
Merfolk do not wear clothing, as fabric is impractical below the water and the temperature at the bottom of the lake is relatively stable year-round. Jewelry, especially necklaces, is popular with both genders and tends to be made from pebbles, teeth, claws, and other things (such as pearls) that can be found native in the lake. Occasionally something from above the waves makes its way into jewelry, but gemstones and metalwork are both entirely unheard of and never found in native merfolk work.
Pouches and bags are generally made of fish skin or water weeds, and can be carried or worn around the waist to carry items that are needed or being collected by a selkie.
Basic Social Structure
The Black Lake Tribe is a group of Selkies located in the lake between Hogwarts and Hogsmeade. While significantly less violent than their saltwater kin, the survival of the colony is still based on hunting and gathering within the lake. As such, many merfolk are hunters; other occupations include but are not limited to guardians of the village, jewelry makers (who generally use pebbles and teeth native to the lake), and gatherers of food and supplies.
The merchieftainess is a leader rather than a ruler; she is "first among equals" and it is permissible to tell her no. A wise merchieftainess is respected and even venerated, and historically a foolish one may find herself replaced. Unless this sort of replacement occurs, leadership of the tribe is passed from mother to eldest daughter, allowing for wisdom to be passed from one merchieftainess to the next for as long as possible before a new merchieftainess takes over.
The merchieftainess's heir would be given extra respect, but otherwise be the equal of other members of the tribe. The other children of the merchieftainess would be completely the equals of the others in the tribe, although some might view them as advantageous connections to the merchieftainess and may find other members of the tribe seeking to use them to get close to the current seat of power. Attempts to play power-games are not unheard of, but those who do are considered to be especially foolish by those with what is considered a more realistic view.
Physical Contact and Intimacy
Since reproduction is external for merfolk (eggs are laid by the female and fertilized by the male) there is no such thing as accidental pregnancy. Since reproduction is done with a great deal of purpose attached, there is no real reason to restrict other forms of intimacy before a couple is bound together.
Kisses, caresses, and other forms of physical contact are enjoyed and even encouraged in merfolk society. As long as both parties are on the same page about where a relationship is (casual, flirtatious, or a serious courtship) then there is no issue with physical intimacy.
Marriage and Family
Until a couple chooses to be bonded in the merfolk version of a marriage ceremony (which involves literally binding a couple together with seaweed cord), they are free to do whatever they like except for having children. When this stage in a relationship is reached and a couple is married, then they are together for life.
Because it is impossible to become accidentally pregnant due to the external reproduction of merfolk, only bonded couples have children. Parenthood is considered to be a serious decision, and orphans and unwanted children are rare to the point of being nearly unheard of. Children who lose their parents due to accident or death are usually adopted by the tribe.
A child hatched outside the tribe would usually be separated from his or her parents by accident. Eggs are sometimes hidden away by mothers being effected by protective instincts that are unnecessary in the lake, and so if something happens to the mother they may be lost. These children sometimes survive, but usually are found quickly and adopted by the tribe. However, since the lake is large it is perfectly possible for some solitary merfolk to live out in the depths of the Black Lake.
The Written Word
There is no written language among the merfolk, although sometimes stories and important events are recorded in drawings or paintings on large stones. Writing simply isn't practical for a species that lives underwater, and so it was never developed. As such, many merfolk view writing and books as foolish, although some few may be curious about what humans find important enough to write down.
The Spoken Word
Merfolk have vocal cords formed differently than those of humans. They are designed to function and speak under water, and as such even though they are capable of breathing above water they are not capable of forming the sounds necessary for human speech above the waves. As such, while merfolk can communicate with humans under water -- at least, if the human is clever enough to figure out how to speak audibly under water -- they can only speak Mermish in the open air.
Despite being relatively isolated from other races by the fact that they live underwater, merfolk have developed relationships with other magical beings. They have a spotty past with humans, and for the most part simply hold themselves as separate since they find the wizards' insistence of their own superiority to be small-minded and annoying. For example, wizards have made it illegal for merfolk to own wands even though they cannot use them and originally were not considered eligible for Being status because they are incapable of human speech. Currently merfolk, like centaurs and goblins, have rejected Being status in favor of Beast status. While some individual merfolk may hate or be curious about humans, the general stance is one of neutral separatism.
Unlike humans, ghosts are viewed with almost universal curiosity by merfolk. Merfolk do not turn into ghosts -- it is a uniquely wizarding phenomenon -- and so ghosts are seen as interesting and inexplicable.
Centaurs are traditionally allies and friends to merfolk. They refused Being classification when merfolk were excluded by the original definitions. Centaurs and merfolk regularly communicate with each other and occasionally trade supplies as well.
Merfolk do not willingly associate with vampires and hags, and even refuse Being status to avoid being associated with them by wizards. It is possible that if a vampire or hag was caught in merfolk territory, they would be violently ejected.
There is very little interaction between goblins and merfolk, which leads to a neutral opinion of goblins. The two races have little in common, do not value the same things, and so rarely cross paths.
House Elves are viewed with disdain for their willing slavery.
The Perpetual Hunt
For as long as it has inhabited the lake, the Giant Squid has been hunted by the merfolk who live there. This animal is not the same creature that muggles know as a giant squid, but rather a magical relative. Seeing as it is not dangerous, that is not why the selkies hunt it. Rather, it is for sport: the Squid is semi-sentient ant so poses a unique challenge for the selkie warriors. The squid has never been caught or significantly wounded in a hunt.