My mom had a pregnant Mickey Mouse platy fish, but she just died today. We performed a little fishy c-section and got about 15 eggs out of her. Is there anything I can do to ensure they still hatch without their momma?

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My research suggests that it could be possible, but the prognosis depends on how soon have the eggs been removed from the platy fish after it died.

Platies are fish from the genus Xiphophorus of the family Poeciliidae. Xiphophorus genus contains 28 recognized species; all of them are called either platies or swordtails, and this distinction depends on whether the male specimens have elongated lobe(s), or "sword(s)" on their caudal fin; those species in which males possess this feature are swordtails, while those in which males lack this feature are platies.

Almost all fish within the Poeciliidae family, including platies and swordtails, are ovoviviparous livebearers; the eggs are retained in mother's body mostly for protection and are otherwise thought to be independent from her. However, there are differences among the ovoviviparous species of fish in regard to the support provided to growing eggs by the mother.

In the Poeciliidae family, many species are considered to be lecithotropic, which means that mother fish is just a "live egg bag", and does not provide support for developing larvae besides facilitating the O2/CO2 gas exchange; however, other species are matrotrophic, which means that mother has more or less developed placenta and nourishes the larvae developing in eggs.

Maternal provisioning is quantified via "matrotrophy index" (MI for short), and it is an estimation of dehydrated body mass of the newborn fish divided by an estimation of dehydrated egg's mass at the point of fertilization. If fish larvae inside eggs develop independently from the mother, all the biomass has to be already contained within the eggs themselves in the form of yolk which nourishes the developing larvae; in such cases MI values are less than 1, and this is the case in most species of livebearer fish.

However, in some other species MI could be as high as 120, which means that newborn fish contains 120 times as much dehydrated biomass as the egg at the point of fertilization - all the difference is provided via the mother's nourishment, and these eggs would not be able to develop outside of the mother.

Good news in the context of your question is that, according to this source, platies have MI of less than 1; in other words, they are lecithotrophic, lack a placenta and this means that developing eggs are independent from the mother in terms of nourishment:

[...] published reference genomes of three related lecithotrophic species: the Trinidadian guppy (Poecilia reticulata), the Amazon molly (Poecilia formosa), and the Platyfish (Xiphophorus maculatus). These latter three species are lecithotrophic (MI < 1), and lack a placenta. [...]

One important thing is, the mother facilitates gas exchange for larvae developing inside the eggs. I would assume that the mechanism of facilitating gas exchange for eggs relies on the mother performing her own gas exchange - in other words, it relies on her being alive and breathing. Once the mother dies, she stops breathing and at this point I would assume that the eggs inside her start suffocating, as they are somewhat isolated from the outside world by her body; what is more, decomposition processes that quickly start after mother's death could themselves begin to consume any leftover oxygen from within the body and further deprive eggs of it. Because of this, the chance of successfully hatching the retrieved eggs is probably the higher, the sooner they have been removed from the body of dead mother.

After retrieving the eggs, it is important to place them in water that is oxygenated enough and has some flow to facilitate more efficient gas exchange for the developing eggs. I would also suggest not placing the eggs in a brightly lit place, but rather in shade to mimic the conditions inside the body of their mother.

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