Source: Wrath of Thrune, pg(s). 90
Merlucents are a species of half-human, half-jellyfish aberrations with the natural ability to sniff out and reproduce arcane magic. They reproduce by implanting polyps into humanoid hosts who will be devoured by the growing young.
A merlucent resembles a translucent, faintly glowing humanoid crowned by long tentacles and supported by a crystalline skeleton. Merlucents range from four to five feet in height and weigh around 180 pounds, but smaller or bigger specimens exist.
Merlucents are likely artificial creatures, spawned by some accident in the distant past, but have never been linked to any of Golarion's races normally associated with fleshwarping. Sensitive to arcane magic, merlucents see it in the same way a human sees soft music or pleasant lights, and claim small coastlines suffused with it, where they hunt fish, shellfish, and occasionally birds. They do not need sleep and often drift among the currents, snatching prey that approaches them. They can wait for weeks and drift for hundreds of miles between meals, but can deplete entire coves within days at their most ravenous.
Every few years, merlucents reproduce by spawning hundreds of ant-sized polyps which must be implanted inside humanoid hosts, and enter an aggressive frenzy to capture surface dwellers to serve as unwilling hosts. During days or weeks depending on the individual host, the polyp takes over the host's mind and converts its flesh into jelly, and its skeleton into crystal. Merlucents prefer to use arcane spellcasters as hosts for their young, despite having no need to do so, and young merlucents rarely inherit a host's spellcasting ability.
Elder merlucents that reared enough young are taken over by an instinct that drives them into the deepest oceanic trenches, where they form large colonies around hydrothermal vents and cultivate jellyfish and tube worms. They rarely eat their livestock, but aggressively defend them from all threats.
Merlucents are simple-minded and mostly concern themselves with eating and creating art, outside of their mating frenzy. They magically grow coral reefs to form elaborate settlements, complete with air pockets where captives are kept until they can serve as hosts.
Surface dwellers often see merlucents as pests for depleting fishing grounds, but they also keep larger aquatic predators at bay. In turn, merlucents view humanoids as intelligent but alien and needlessly complex, and sometimes sell information, fish, and scavenged goods in exchange for magic items, which they integrate into their coral colonies instead of actually using them. Some merlucents view their long-time neighbours as herds or pets, and aid them by growing barnacles to keep waters clean, or dragging the corpses of sea beasts to shore for them to eat.
Fascinated with magic, merlucents treasure potions and wands like delicate trinkets and view arcane spellcasters as performers. They sometimes follow a particularly powerful wizard or sorcerer, but such groups inevitably disperse after the leader's death.
Elder merlucents constantly compete against deep merfolk over the richest hydrothermal vents. Captive deep merfolk are never used as hosts for merlucent young, but are cooked alive in the hottest black smokers.