According to J. Charles Delbeek:
"Observations on Sexual Reproduction in Entacmaea and Stichodactyla
Reproduction in E. quadricolor takes two forms, sexual and asexual. Asexual reproduction has been well-documented by hobbyists and was a common occurrence in this exhibit as well resulting in a large population of these anemones such that the live rock was nearly invisible when all the anemones were expanded. Sexual reproduction also occurs in this tank (see Sprung and Delbeek, 1997) and to the best of my knowledge, is the only such population in captivity that regularly does so each year. Spawning usually occurs between April and May between 0700 and 0900 a few days after a full moon has occurred. In April 2002 we were fortunate to once again witness not only the sperm release but also the release of planulae. Sperm release can occur in the morning but we have also observed it occurring in the late afternoon and this was the case this year. The exhibit was found to be rather cloudy in the late afternoon of April 25th and the incoming water flow was increased to ensure the sperm did not foul the water by being allowed to accumulate for too long and lowering oxygen levels. Four days later, on the morning of April 29th numerous green sphere-like objects were observed floating in the water. Upon closer examination it was found that the tentacle tips of several colonies of E. quadricolor contained these same spheres. Occasionally a few were observed being released from the tips of the tentacles. From previous experiences we knew these to be anemone embryos (see Sprung and Delbeek, 1997 for photos of the embryos and settled juveniles). A few days later there were still some visible in the tentacles and these were observed to be actually rotating and moving within the tentacles, and were more oblong in shape. It appeared as if the embryos had developed into planulae in the anemone and would shortly be released as such into the water. From my observations it would appear that E. quadricolor is dioecious, with males releasing sperm into the water column to be taken in by the females where their eggs are fertilized internally. A few days later embryos/planulae are released into the water column via the tentacle tips. Sprung and Delbeek (1997) also documented the presence of zooxanthellae in juvenile E. quadricolor only a few days after settlement. This time a few embryos were placed under a microscope and were found to contain zooxanthellae as well. Therefore, not only does the female brood the embryos for a short period but she also supplies them with their initial supply of zooxanthellae. As a result it is probable that these planulae can either settle very quickly and metamorphose into fully functional anemones, or they can spend a great deal of time in the plankton till they find a suitable substratum to settle on, without the need for feeding.
As an aside, Stichodactyla gigantea is also dioecious in nature with the females ingesting the sperm and fertilizing their embryos internally. However, unlike E. quadricolor, the embryos are internally brooded, developing into miniature copies of the adult. These juveniles are then egested from the mouth where they then drift in the current till they settle onto the substratum (L. Sharon, pers. comm., 1999). We were lucky to receive four of these juveniles in October 1999 from a coral farm in Belau (Palau); two green ones and two brown ones with an average oral disc diameter of approximately 4 cm (1.6 inches). These were placed in floating baskets in outdoor holding tanks in direct sunlight. Within the following three years one of the brown ones developed blue tentacles and all four had grown such that they had to be separated into individual baskets. At the present time (August 2002) the oral disk diameters of all four average approximately 30 cm (12 inches) when fully expanded. That means that there has been an almost eight fold increase in diameter in three years. I am confident that if the anemones had been directly feed on a regular basis they would have grown even larger (and perhaps faster) in those three years.
This brings up the question of age. Fautin and Allen (1992) proposed that large anemones could be more than a century old. While I do not dispute that some anemones may indeed be very long lived, to infer that something must be old simply because it is large is not entirely valid in my opinion. In the case of our S. gigantea, I think it would be safe to say that they should attain a diameter of close to a meter in less than ten years. We also have a S. mertensii that we have had for 12 years that is almost a meter in diameter when fully expanded. However, I should point that, as with giant clams (e.g. Tridacna gigas ), the growth rates of partner sea anemone species may be fairly rapid in the first decade, only to slow with advancing age. So yes, old anemones may be large but the opposite may not be necessarily true."