Goodeids are livebearers that have a different way of being sexed than typical mollies, platies, etc. One of the common names for the group called goodeids is splitfins. The reference is to the way they are sexed. A female goodeid has much the same looking anal fin as a typical livebearer that we all know. It is a basic triangular shape like in this picture of a Xenotoca Eiseni female.
The male is distinguished by having a split in the anal fin. Usually the outer part of the fin is shorter than the other part and the split off part is used by the male as a sex organ much like a
gonopodium is used. This is a male Xenotoca eiseni.
In these particular goodeids, the orange tail helps you spot the dominant male in a tank but the other males will not have the orange colored tail. The common name for this particular fish species is orange tailed goodeid. All of the goodeids are sexed in the same way although the color patterns and even body shapes vary a lot within the goodeid group.
Goodeids vary from other livebearers in another very important way. Unlike the guppies, swords, platies and mollies, The goodeid females actually provide nourishment to the fry through a structure that is much like an umbilical cord. Goodeid fry are quite large when born because they can survive long enough inside the female to actually develop some size to them. A typical goodeid fry is close to 2 cm long instead of the 0.5 cm more typical of common livebearers.
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