Poeciliidae
The live-bearing species fish can be different from each other depending on how much the mother fish does for the developing larvae. Many fish in the family Poeciliidae are lecithotrophic (the mother provisions the oocyte with all the resources it needs prior to fertilization, so the egg is independent of the mother), but others are matrotrophic ("mother feeding": the mother provides the majority of resources to the developing offspring after fertilization). Lecithotrophy and matrotrophy are not always completely separate. Most scientific studies use a matrotrophy index (MI) to say how matrotrophic an animal is. This uses the dry mass of fully developed offspring (how much they weigh when they are not wet) divided by the dry mass of a fertilized egg.