Sea pens reach sexual maturity between four and six years of age. You can estimate the age of a sea pen by looking at its height and size. Each individual sea pen, or colony, is typically either male or female but there are reports of a few hermaphroditic colonies in which each individual is both male and female. Sea pens typically have a relatively equal sex ratio of one male for every female. They exhibit either seasonal or continuous spawning (the release of eggs and sperm), depending on the species. The vast majority of sea pen species use broadcast spawning to reproduce. During broadcast spawning, male and female sea pens release all of their eggs and sperm at the same time to increase their chances of successful reproduction. 

The timing of spawning has often been correlated with water temperature, the moon cycle and the availability of food. Sea pens produce a lot of offspring, with individual colonies able to release between 2,000 and 200,000 eggs. Once fertilized, the larvae will drift in the ocean for anywhere between a few days to a month, searching for sandy or muddy bottoms to settle in to. They feed on the “yolk” they are provided with at birth until they settle and begin to grow their first polyp and tentacles to catch food. This metamorphosis usually occurs rapidly, and allows the juvenile sea pen to begin actively feeding. If there is sufficient food, the rest of their growth into adulthood is relatively quick at first and then slows down. Like all other corals, sea pens are filter feeders, capturing plankton and detritus (dead plant and animal matter) in their tentacles to eat. The main predators of sea pens are nudibranchs (sea slugs) as juveniles and sea stars as adults.