Mermaids (Fish)

These are the mermaids most people in this world think of when they think of mermaids. Humans have known about them for as long as humans have lived by temperate shores. And humans have feared them that entire time. If the movie Jaws existed in this world, it would be about a mermaid rather than a shark.

Habitat:
Piscine mermaids live in temperate saltwater (except for a small subspecies that lives in tropical waters) close to the shore. In fact, they're well adapted for living in shallow water for a fish of their size. Unlike most, if not all, other fish they have actually adapted to feeding on terrestrial and avian prey. They actively prefer eating seabirds and terrestrial animals that get into the water to eating other fish - even others of their own species. They employ a system of luring prey similar to that seen in angler fish, but far more sophisticated.

In deep enough water, they will position their bodies straight up and down, with only their lure out of water. In shallow water, they lie on their sides on the sand so that they can bend and raise their heads and display their lure out of the water. In order to move in this position, they have to undulate their bodies against the sand. This requires lifting their tail out of the water, which is why these mermaids are depicted as having a horizontal tail like a mammal's despite it clearly being a fish tail. Mermaids are solitary creatures that each claim a territory for themselves and defend it against other mermaids. Larger mermaids are able to keep larger and more desirable territories, with the most desirable territories being the ones that are shallowest and closest to shore. An ideal mermaid territory is one in which it has just enough room to display its lure without any of its body below the lure visible out of the water.

Physiology:
Despite their most desirable habitat being shallow water, which forces them to lay on their sides, they have not shown any convergent evolution with flatfish. (This is likely due to their reproductive practices making such an anatomy disadvantageous.) Mermaids are bony fish and have narrow bodies averaging about nine feet long when fully mature. They have long caudal fins in a vaguely similar shape to the tail flukes of a whale. Their pectoral fins are long; their dorsal fins are only up to three inches tall but extend along most of their back to fuse with their adipose fins into a single sail-like fin. Their other fins are all very small, ranging from two to four inches long.

Their lateral lines are especially well-developed and adapted to help them detect movement even in the relatively fast-moving currents of the shallows they live in. Their eyes are quite large and they have excellent underwater vision and can see a full range of color. They aren't adapted for seeing out of water, and locating prey is mostly done using the lateral line. Their jaws contain serrated, triangle-shaped teeth which superficially look similar to the teeth of certain sharks such as the great white. However, while great white sharks use a combination of two tooth shapes to effectively feed on large prey, mermaid teeth are uniform due to lack of selective pressure for such specialization. Aside from shape, mermaid teeth have multiple other traits in common with shark teeth; namely that they fall out frequently and are replaced quickly, and mermaids have five series of teeth.

The most distinctive (and arguably the strangest) feature of a mermaid is that just anterior of the gills extend long tentacles. These appendages grow from the mermaids' bodies in an unbroken ring, with the base of each tentacle nearly touching the ones on either side. These tentacles are the only part of the mermaid's body not covered in dark gray scales but are in fact covered in skin. They can change color and texture similar to a cephalopod's skin, though their ability to change texture is somewhat more limited and their ability to change color even more precise. They do not have suckers or the ability to taste like cephalopod tentacles do. These tentacles are about seven ninths as long as the mermaid's entire body and comprise its lure.

Mermaids have adapted to be able to tell what kind of animal is in the water by the specific patterns of movements detected by their lateral line, similar to spiders hunting on their webs. They change the color and texture of their tentacles in specific patterns and using certain sections, then fold the disguised tentacles into the shape of a creature in the same species as whatever prey they detect. Then they "cast their lure" by extending their folded tentacles above the surface. The end result is that right over their heads they place a shockingly convincing false image of another member of their desired prey's species. In the case of seabirds, because they are small enough to not require many tentacles to replicate and they typically fly and swim in flocks, mermaids will typically imitate more than one of them.

Any potential prey that gets too close to these lures will find themselves grabbed by the tentacles that made it up and pulled underwater. Mermaids are incredibly strong and it is rare for even the largest humans to escape once they have been grabbed. They do not eat their prey right away, but instead spend up to half a minute visually examining the prey they have caught in order to maintain the accuracy of their lures. Then they hold the prey to their mouths and eat by tearing off chunks of flesh. This period of examination allows them to keep their human lures "dressed" in the current swimsuit fashions, so it is clearly effective.

Mermaids seem to be capable of a limited form of learning, not just due to their changing their lures based on observations but also because they modify their lures based on what catches prey most consistently. Statistically, when preying on humans a lure that looks traditionally feminine (and visually attractive) is more likely to be successful than a lure that looks traditionally masculine (even if it's visually attractive). As a result mermaid lures almost always look like beautiful women despite them presumably not being able to tell the difference. Because of the length of their tentacles and the shape of human bodies, a lure meant to catch humans will only depict a human from the waist up. Mermaids therefore display their lures as humans whose legs are underwater. The result of all of these factors is that humans see what looks like a pretty human woman wearing a swimsuit, but if the mermaid tries to move their tail comes up behind her. Because they lie on their sides, it looks like the tail is oriented like a mammal's tail despite clearly being a fish's tail. And if someone gets too close to this "woman" they get pulled underwater and eaten. Thus mermaids are depicted as fearsome monsters that are human from the waist up and fish from the waist down.

Evolutionary History:
Despite their similarities to sharks, mermaids are bony fish whose ancestor diverged from other bony fish not long after that trait first evolved. Since then, every phylogenetic line descended from that particular divergence went extinct almost immediately - except the line that evolved into mermaids. It is not known why they were so uniquely successful, nor how they evolved tentacles. As a result, mermaids are considered almost as bizzare as terrifying.

Life Cycle:
When mermaids hatch, they live in deep water and survive via sibling cannibalism. They will feed on other species of fish if they have to, but find them distasteful. Once they are large enough to compete for the least-desirable territories, they begin luring and eating seabirds. However since their territories are still fairly distant from the shore, a large part of their diet still consists of them invading each other's territories to eat each other. The larger they get, the better territories they can win for themselves and the less their diet consists of each other. Cannibalism among adults is rare. Fights for territory don't consist of any actual fighting, but rather visual displays showing off the length and color-changing skill of their tentacles.

Most of the rest of their lives is spent luring and eating. Because of their highly specialized diets, mermaids are thoroughly unappetizing to other fish and have no natural predators. (They do, however, have a few parasites that have evolved to parasitize them. No parasite seems particularly interested in their tentacles, though. This is probably due to the fact that a mermaid with sub-par tentacles will soon be dead.)

During mating season, mermaids will abandon their territories and swim into deep water. Fertilization is external. Females will produce large egg sacs with a tough outer casing covered in a sort of natural glue. They will attach these egg sacks to rocks, mostly. However, while mermaids have been known to capsize small boats to eat the humans inside, mermaids' sole interaction with large ships seems to be to attach their egg sacs to them. It is not known why they do this, since their lateral line should indicate to them that these ships aren't just large rocks. But regardless of the reason, this has allowed the species to spread all over the world.

Males must fertilize the egg sacks before the glue-like material hardens, or it will be impossible. Each egg sack will hatch into a couple dozen mermaid fry, and each female will produce a couple dozen egg sacs. However, only a very small number of her fry will survive. The rest will become food for those who do. After breeding, the adults quickly die.