Adult killifish are small and mega-bright and hard to kill, which makes them popular pets (there's even an International Killifish Association that caters to "killi" hobbyists worldwide). The species pictured here—known colloquially (and hilariously) as the mummichog—is a favorite of scientists because of its unusual breeding patterns. Up to eight times a year, female mummichogs will spurt out a passel of fertilized eggs onto empty mussel shells and shallow-water plants; the embryos like to develop in the open air. Once they get a little larger than the ones shown here, it's possible to look right through the egg and see the fishlets' beating hearts.