what's interesting about the nile river, is that volcanic activity blocked some of the rift valley fresh water lakes from draining into the nile, otherwise it would be even longer, extremely longer. some of these lakes no longer drain anymore, and are accumulating salt. this is where intersting semi-saltwater fish come from and are called cichlids. they are, for fish at least, extremely intelligent, very colorful and have extremely eccentric behaviors and very specialized mouth parts that border on galapogos island type diversity and speciation.
one of my favorite is Ophthalmotilapia ventralis
http://www.aquatic-photography.com/gallery/showimage.php?i=4577 it's kinda spectacular. the little spots on the tip of the two thin fins are egg spots.
from the following link http://kingdappercombo.com:8082/GAAS/ttopics/1999/optimism.htm
O. ventralis are mouth brooders. They spawn in shallow pits in the sand. Sometimes the female will lay her eggs without the male being present. She will pick them and carry them in her buccal cavity. When the male finally comes home from wherever, the female will mouth the egg dummies as if trying to pick them up. The male being stimulated by this will release his milt, which will be sucked in by the female, and so the eggs get fertilized orally. I guess technically this is not sex, but as long as I get fry I don't really care what you call it. The female carries the eggs for 3-4 weeks until the fry are developed well enough to be free swimming.
you can purchase these for your home enjoyment.