Merfolk

Merfolk are the general term for humanoids that inhabit the oceans and swamps while sharing some physical similarities to sea-life and semi-aquatic creatures. Most Merfolk follow the standard humanoid form which means to have the following: opposable anterior forelimb-appendages (i.e. thumbs), visible spectrum-binocular vision (i.e. two eyes) or biomechanic plantigrade-bipedalism (i.e. the ability to walk on heels and metatarsals in an upright position). Merfolk are also known to exhibit specific features of aquatic or semi-aquatic life, such as gills, fins, webbed hands and or feet, and typically have different colored skin compared to land life. However some Merfolk have been observed to have more aquatic qualities, such as not having legs, instead having a tail of a fish, where the legs would usually be.

Etymology
Stories of Merfolk have been spread across the land over hundreds of years, spanning different cultures and empires across the world. "Mer-" in Merfolk comes from a old human dialect, a derivative of the word "Mere" which meant "Sea". Folk, is self explanatory, referring to sapient people in general. The term "Mermen" and "Mermaid" can also be used interchangeably with Merfolk, but refers to a specific sex of Merfolk and is uncommonly used in contemporary society, in favor for the more general term of Merfolk.

Evolution
The evolutionary tree of Merfolk is poorly understood, compounded be the low amount of currently known existing Merfolk, no definitive timeline can be given. Researchers suggest the Merfolk are a subgroup of humans that chose to live in or around bodies of water, and eventually adopt more and more qualities of sea life through a combination of evolution and random genetic mutations. Religious officials meanwhile, suggest that Merfolk come from either a curse put by wrathful Gods as a punishment for disobedient mortals

Anatomy and Physiology
Merfolk are a very diverse species and their physical forms depend on their environment & living conditions, but there are distinct similarities that help identify a Merfolk from non-Merfolk. Merfolk, disregarding the aquatic characteristics seem to be homologous to corresponding aspects of animal physiology. However it is important to keep in mind that not all Merfolk follow these listed characteristics, due to limited study and sample size from current existing Merfolk.

Almost all Merfolk are known to have gills, which they use to breathe underwater. Gills are most commonly observed to be small cut-like holes around a Merfolk's neck or protrude outward as a sort of specialized fin structure. In conjunction with this, merfolk have also been observed to have extremely high endurance and muscle tissue present all over their bodies. It is believed to be caused due to a combination of hydrodynamics and natural selection. Enhanced underwater sight is poorly understood feature of Merfolk, despite being very prevalent in their day to day lives. A very striking feature of Merfolk are their different coloration. Most Merfolk have a different skin tone and skin color, which can range from blues, blacks, greens, and reds. It is however noted, that there could be more that still remains undocumented. It is unclear what pigment the Merfolk create in their bodies in order to produce these vibrant hues of color.

Despite all of these additional parts that make Merfolk more distinct from predominantly land dwelling creatures, according to currently known Merfolk, the most commonly defined body systems in the human race are at least roughly the same as that of Merfolk body systems. These include the nervous, the cardiovascular, the digestive, the endocrine, the immune, the integumentary, the lymphatic, the musculoskeletal, the reproductive, the respiratory, and the urinary system. The only main differences take place in the cardiovascular, respiratory, integumentary, the digestive, and the musculoskeletal systems. While the remaining systems are noted to be very similar to that of humanoid systems, but adapted to marine habitats and life. It should however be kept in mind, that these differences are only noted in currently recorded forms of Merfolk.

One very distinct feature about certain groups of Merfolk are their dual purpose respiratory systems, which allows for survival in both air and water. It remains unclear how this functions, as studying methods remain limited by technology.

Life Cycle
The full life cycle of a Merfolk is inadequately understood by contemporary researchers, but is described to be similar to that of most biological life present. Despite all of this it has not stopped researchers from hypothesizing what a Merfolk life cycle is, using pre-existing data from humans as a baseline. Most Merfolk reproduction is speculated to take place by via sexual intercourse. The average gestation period is unknown, but is speculated to be at least 9 months, or 38 weeks but may vary by up to about 37 days. It is unknown if Merfolk are able to induce early labor, if the offspring needs to be born earlier for medical reasons due to their marine lifestyle. Completely defenseless and helpless at birth, Merfolk are speculated to continue to grow for several years, usually reaching sexual maturity at 15-17 years of age. After this point, it become unclear how many stages of life a Merfolk have, but it has been agreed upon that there is at least an infancy, childhood, adolescence stage among contemporary scientists.

The average lifespan of Merfolk is completely unknown due to a lack of a larger sample size.

Diet
Merfolk are described to be omnivorous, capable of consuming a variety of plant and animal material. Merfolk groups are known to be able to dedicate themselves to eating mostly plant based to primarily meat based nutrition. In some cases, a lack of proper nutrition source in Merfolk can lead to deficiency diseases; however, it has been noted that observed Merfolk groups have nutritionally balanced food sources to mitigate this problem. Despite being humanoid, due to their marine lifestyles, it is currently poorly understood how Merfolk don't need to directly drink water for sustenance, regardless of being completely able to, but instead can submerge themselves in water to rehydrate themselves, as most marine life is currently known to be capable of.

Biological Variation
Due to a lack of a larger sample size, it is unknown how varied Merfolk are able to be. However there is evidence that populations have adapted to various factors. Some Merfolk have evolved highly unique adaptations to very specific environmental conditions, such as those that dwell deeper in the ocean, known as Deep Sea Merfolk, which is a blanket term for any Merfolk that dwells at least 5000 feet below sea level.