To milt or not to milt?
For most fishes, reproduction involves eggs and milting, which is like crop-dusting with sex cells (aka gametes). The vast majority of fishes are oviparous, which means they lay eggs that are fertilized and develop outside the mother's body. In these situations, males typically milt, which is the release and spreading of their gametes, onto the eggs that have been deposited in the environment. In ovoviviparous fishes, the eggs develop inside the body of the mother, and male gametes have to be passed into the females’ body through specialized structures, such as claspers (modified pelvic fins) in sharks or gonopodiums (modified anal fins) in guppies. Live birth (viviparity) has also evolved in a number of lineages of fishes, including sharks, guppies, and rockfishes. In viviparous fishes, the young develop within the mothers’ body.
Male, female, or both?
While most fishes have separate sexes, a number of lineages are simultaneous or sex-switching hermaphrodites. Some fishes, such as clownfishes, can change their sex once during their lifetimes either from female to male, or male to female depending on environmental and/or behavior scenarios. A small number of fish species are simultaneous hermaphrodites capable of producing both male and female gametes at the same time (e.g., lancetfish, some species of moray eels as seen above). Scientific studies have identified that at least some of these species (e.g., the mangrove killifish Kryptolebias marmoratus) are capable of self-fertilization!