Now keep in mind I haven’t taken a biology class since high school, however, I have a few intresting ideas for each case
Case 1- The traditional mermaid
In the event that the mermaid is completely humanoid from the waist up with the exception of possibly gills or some other method by which they breathe water I believe it would make the most sense that they have live birth due to them having breasts and presumably the nursing habits and instinct to care for their young up to a certain age that is shared by all mammals. These mermaids would pretty much have the same mating habits at humans, just underwater. For examples look to sharks or other fish that give live birth
Small aside: you could take this a step further and have mermaids modeled after existing sea mammals such as whales that have to breathe air, would be cool and give them an excuse to interact with humans more as they have to regularly go to the surface anyways.
Case 2- the standard siren
The most common siren I have seen examples of have the fallowing traits. Predatory, scaled, uses illusions to lure sailors to their doom. From this we can construct the second hypothetical mermaid, focusing more on the mer side of the equation. These would be completely scaled from head to toe but use some form of illusion to make humans think they are beautiful maidens (or beautiful men, they are adaptive). These are completely fish like but can hold their breath for awhile to hunt their out of the water prey and have NO BOOBS. I repeat ALL SIGHTINGS OF BOOBS ARE ILLUSIONARY. Fallowing with their casting as villains and completely fishy nature they would likely lay eggs en mass. And because of this would be unlikely to try and care of their young
What society they do have would reject the swarms of young tiny sirens (perhaps these younger forms do not even yet have limbs or resemble humans, as that would be reserved for the adult stage of their lifecycle.) alternatively they COULD care for their young but would be canablistic, the young ones eating eachother and the adult eating these younglings in times of hardship and the adults feeding the excess food they cannot eat themselves (not really a way to preserve things in the ocean) to the younglings. Either way this would result in a small crop of adults reaching maturity each year from the hundreds laid.
I could come up with more but my brain is tired and I need to clean the bathroom so let me simply address the other points you have for both of the ideas
Siren-yes, fish eggs and therefore in theory caviar, although actual caviar is actually only from one very specific type of fish and the size of the fish eggs could be pretty big as the size of the fish that spawned them is person sized at minimum
Where do they come out of?
Mermaid- same place as a whale
Siren- you know every animal on earth has some kind of birthing hole right? Even fish?
Mermaids- A bra if they even bother, it would probably be more practical to construct some kind of binding out of seaweed instead
Siren, no titties, could wear seashell bra with some kind of “it gives something for the magic to build off of” but probably nothing.
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