In 2045, a huge earthquake, centered in Japan, rocked the Pacific and most of Asia (I learned that it even reached France, but that’s probably an exaggeration). Huge parts of Asia flooded when a Northern Pacific ice shelf broke off and melted, making most of California a lovely underwater community. Unfortunately, even Californians couldn’t sell that, and the economy sank massively. Everyone turned to the scientists, and the scientists turned to the merms.

Merms were created in 2044, but no one gave a flying fart until the quake. They were pretty much mermaids of lore, but they had no gills, and came in a variety of shapes- oh, and they were half-porpoise, not fish (thus, the lack of gills). The merms pretty much took the world by storm at that point, with their ability to stay submerged for long periods of time, and farm underwater (The fact that they were adorably half-flipper could have played a role in that, too).

After the merms became mainstream, it was only a matter of time before the rest came out.