Just Elisha wrote with a great question about her Sea Monkeys. I invited Sea Monkey expert George Atamian, President of Educational Insights- the company that markets Sea Monkeys, to help answer.
Question: My monkeys will be 2 weeks old on Tuesday. I noticed some babies! Is it possible that they could have reproduced already?
Sea Monkey Geek: What's more likely is that some of the eggs in the original packet hatched later than others. This is very common. You will probably notice insanely long, um, breeding sessions, before you see a second generation of monkeys.
George Atamian: Normally, it takes 21 days for the Sea-Monkeys to reach reproductive capability. Once they start to mate, several days pass before eggs are seen at the base of the female’s tail. If conditions in the tank are perfect (by hybrid Artemia standards) the female may, in time, give birth to baby Sea-Monkeys sans shells!!
As Keith mentioned, the babies seen may come from the original eggs that may have had very thick shells, which would result in longer hatching time.