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Source: Bestiary, pg(s). 204
Merfolk (mermaid f. and merman m.), called sapiaquali-oths in the language of their alghollthu creators, are amphibious humanoids encountered in the temperate waters and coastlines of the Inner Sea region. A merfolk's torso resembles that of a human or elf, while their lower bodies are dominated by a fish-like tail and fins. They are highly xenophobic, and commonly attack members of other races that encroach upon their territory; the only other intelligent creatures merfolk consider allies are the aboleths.
From the waist up, a merfolk's body resembles that of a human, though their features and build are usually more elven in appearance; members of others races sometimes experience feelings of desire due to the attractiveness of a merfolk's upper body. A merfolk's lower body, however, is comprised of a scaled tail and fins similar to that of a giant fish. Female merfolk are generally smaller in size than their male counterparts, but members of the race tend to weigh an average of over 200 pounds and measure between six and eight feet in length.
The hair and scales of merfolk can come in a wide range of colors and hues. Despite the potential variety in the race's appearance, merfolk in shared habitats tend to boast similar pigmentation.
Merfolk are naturally adapted to life underwater, but they are amphibious creatures; merfolk can breathe both in and out of the water. They are even capable of traversing short distances across land, though these journeys are rare, and merfolk tend stay relatively close to their aquatic homes.
The sapiaquali-oths were the result of the alghollthus' first attempt to create humanoids. However, they were quickly forgotten and left alone by their creators, who moved on to other experiments and began interfering in terrestrial societies. The merfolk have forgotten about their origin, but are instinctively xenophobic and prevent intruders from entering certain deep places of the oceans for reasons they do not fully understand—these are actually remnants of the instincts imprinted by the alghollthus in the distant past.
The merfolk race is highly xenophobic; they generally shelter themselves in hidden communities, fiercely protecting the territory they hold from outsiders. Merfolk are especially distrusting of surface races, and go to great lengths to conceal their presence from those that live primarily on land. As a result, most merfolk settlements remain unknown to those on the surface, and encounters between merfolk and surface dwellers are uncommon. It is partially due to the infrequent sightings of merfolk that their presence is sometimes considered an ill omen by sailors.
Merfolk xenophobia extends below the waves as well. Though merfolk will keep aquatic animals for the purposes of companionship, they generally isolate themselves from other aquatic races. The sole exception to this distrust are the fish-like aberrations known as aboleth; many merfolk serve aboleth masters, sometimes willingly.
- Chosovosei is a small, by surface standards, merfolk city located in a trench in the Steaming Sea. They peacefully coexist with the the elves of the Mordant Spire, also located in this region, but are forced to make offerings to placate the krakens of the Endless Eye.
- Located in the Obari Ocean, the city of Jehyseel is protected by a forest of deadly stinging anemones.
- While not an exclusively merfolk-inhabited city, they comprise a plurality of this amphibious River Kingdoms city.
- Stormshoal is a merfolk settlement that floats beneath the waters along the edge of the Eye of Abendego. This isolated fortress is composed of coral and stone, and anchored in place by gigantic seaweed fronds. Native merfolk live in near isolation, the surrounding area made dangerous by the stormy waters of the Eye. 
On other planes
Merfolk have emigrated to the Plane of Water, where they live in seclusion in dimly lit seas. They use illusion magic and other mundane techniques to hide their homes, often to the annoyance of travelers and the plane's other inhabitants.
Paizo published a major "Ecology of the Merfolk" article in City in the Deep 70ff.
- The singular and plural of merfolk are the same.
- Adam Daigle, Patrick Renie, F. Wesley Schneider, and James L. Sutter. (2012). Oceans of Golarion. Raiders of the Fever Sea, p. 58-71. Paizo Publishing, LLC. ISBN 978-1-60125-409-2
- Paizo Staff. (2009). Pathfinder RPG Bestiary, p. 204. Paizo Publishing, LLC. ISBN 978-1-60125-183-1
- James Jacobs et al. (2011). The Inner Sea World Guide, p. 11. Paizo Publishing, LLC. ISBN 978-1-60125-269-2
- Jason Bulmahn et al. (2012). Advanced Race Guide, p. 194-195. Paizo Publishing, LLC. ISBN 978-1-60125-390-3
- Thurston Hillman, Jason Keeley, and Kalervo Oikarinen. (2016). Bestiary. Wrath of Thrune, p. 91. Paizo Inc. ISBN 978-1-60125-824-3
- Dennis Baker et al. (2015). Bestiary 5, p. 172. Paizo Inc. ISBN 978-1-60125-792-5
- Greg A. Vaughan. (2017). Ecology of the Alghollthu. The Lost Outpost, p. 77. Paizo Inc. ISBN 978-1-60125-964-6
- Elaine Cunningham. (2009). When Mermaids Laugh (Dark Tapestry). The Impossible Eye, p. 79. Paizo Publishing, LLC. ISBN 978-1-60125-179-4
- Benjamin Bruck, et al. (2015). Inner Sea Races, p. 6. Paizo Inc. ISBN 978-1-60125-722-2
- China Miéville. (2010). Outsea. Guide to the River Kingdoms, p. 34. Paizo Publishing, LLC. ISBN 978-1-60125-203-6
- John Compton, Paris Crenshaw, Eleanor Ferron, Thurston Hillman, and Jessica Price. (2016). Planes of Power, p. 43. Paizo Inc. ISBN 978-1-60125-883-0