Estuarine Livebearers of Abeli

The mosquitofish, being the only large organism on the planet that spends its whole life cycle in the water, was naturally the first of Apterra's "big seven" to begin colonizing its vast oceans. With time, it will expand into thousands of species across the globe, but for now it is limited by a simple fact: the mosquitofish is a freshwater minnow. Gambusia, however, are hardy animals, and the array of open niches they found as they approached the sea proved too tempting to pass up.

A few founding individuals possessed genes that granted a slight salt tolerance (this has evolved convergently many times among their relatives on Earth). While such a trait would’ve been useless in the mosquitofish's natural habitat, and equally so in the waterways of the Panapterran interior, it proved advantageous in the coastal waterways of northern Abeli, nor far from the initial site of introduction. Within less than a hundred generations, brackish waters around Abeli were teeming with the small poeciliids. 

This complex system of estuaries is in many ways an ideal home for the minnows. Various small and large rivers that weave their way across the nearby coastal plain all converge and meet the ocean here. Masses of dead grass wash up here often, providing ample detritus for the fish to feast on. This debris is also consumed by bacteria, protists, and microcrustaceans, all of which Gambusia find appetizing. While the fish were busy expanding into this habitat, a parallel trend of increased salt tolerance also created a line of estuarine Aedes, which now serve as an ample source of protein. The high population of Gambusia in this habitat leads to a low density of adult mosquitoes compared to regions with smaller fish populations. As a result, the land around the north Abelian delta is one of the most pleasant places for land animals to live at this time. Here, terrestrial vertebrates can wander freely, worrying little about parasites or the diseases they carry.

Of course, even where mosquitoes are common, some of Apterra's animals have bigger problems to deal with right now.