Natural History Mystery

What is that light-colored circle in the pond shallows?

A male bluegill is guarding his nest!

Territorial male bluegills become sexually mature around 7 years of age. They pluck vegetation with their mouths and fan their fins to create silt-free circular nests. While males defend their nest, they may establish nests in large colonies to attract schools of females. In courtship, females and males swim in tight circles with the female dipping and rolling to her side to release eggs while the male releases sperm. In a single day of spawning a territorial male may have 30,000 eggs from multiple females in his nest. The territorial male guards the eggs and fans his fins to keep them oxygenated for the week or two until the young have dispersed. 

There are also male sunfish who mature at an earlier age, are a smaller size, and live shorter lives – satellites and sneakers. Satellite males resemble females and can fool a territorial male into thinking he has two mates. The satellite male positions himself between the territorial male and the female and during the female’s dips, both males will attempt to fertilize her eggs. Sneaker males quickly swim past a mating pair while releasing sperm and fertilize some eggs. A sneaker can transform into a satellite when they get older. These alternate strategies are effective – on average 25% of bluegill eggs are fertilized by satellite and sneaker males (not the territorial male of the nest). Bluegills may spawn several times during the summer so keep your eyes out for their underwater drama!