Coral Reefs (1992) 11:33-35 Coral Reefs 9 Springer-Verlag1992 o~ Detection of estradiol-17] during a mass coral spawn S. Atkinson and M. J. Atkinson Hawaii Institute of Marine Biology, University of Hawaii, P.O. Box 1346, Kaneohe, Hawaii 96744, USA Accepted 17 September 1991 Abstract. The steroid estradiol-171~ (E2) is associated with female gametogenesis in all vertebrates and many inver- tebrates. This is the first report of estrogens in scleractin- ian corals. Seawater and egg slicks were collected during a mass coral spawn at Ningaloo reef, Western Australia for the measurement of total phosphate (TP) and E2. To- tal P in the water column increased 600 times, from 0.5 gM to 300 gM. Concentrations of E2 increased nearly 8 fold during the spawn, from 55 to 420 pg/100 ml seawater. Coral eggs collected from egg slicks contained 368 + 40 pg E2/g dry wt of eggs. Estrogen may be a key hormone in a simple endocrine system of scleractinian corals that synchronizes growth and development of coral oocytes. Its potential role in triggering spawning via chemical messengers in the water column warrants fur- ther research. Introduction Corals reproduce sexually in two ways: (1) brooders, in which fertilization and embryonic development occur in the coelenteric cavity of the coral, or (2) broadcasters, in which gametes are synchronously released into the water column where fertilization and embryo development take place (Campbell 1974). The latter may have reproductive schedules that are seasonal, monthly or continuous de- pending on the length of the gametogenic cycle for a given species (Fadlallah 1983; Richmond and Hunter 1990), but they are always synchronized to some degree. Gametogenesis in higher invertebrates is controlled through neurosecretory or endocrine systems. The neurosecretory cells of marine invertebrates appear to be under photoperiodic control, providing the necessary sensory mechanism to link reproduction and environ- mental cues (Giese and Pearse 1974). Stimulation of the gonads with gonadotrophic hormones results in the pro- duction of steroid hormones that are associated with, and necessary for, proper gamete development. Sex-steroid hormones (C18, C19, C21 steroidal compounds) of verte- brates have also been detected in many invertebrates (Sandor 1980). Thus it seems likely that corals would syn- thesize steroids. Estrogens that are synthesized in female gonads are potent hormones that promote tissue growth. They cause a rapid replacement of epithelial cells by stimulating cell division. A decrease in the concentration of estrogen just prior to ovulation in mammals leads to a thinning of epi- thelial cells and a reduction in mitotic divisions (Turner and Bagnara 1976). It is possible that an endocrine sys- tem may control the synchrony of spawning in scleractin- ian corals. Thus, we tested the hypothesis that coral eggs contain a ubiquitous form of estrogen, specifically E2, which is released into the water column before and during a mass spawning. Phosphate (P) is involved in a variety of intracellular processes, particularly any process requiring an exchange of intracellular energy. Phosphate concentration is low and usually constant over shallow water reef communi- ties (Atkinson 1987). To get an estimate of the concentra- tion of biological compounds and estimates of suspended particulate phosphate released into the water column during the spawn, we measured the concentration of total phosphate (TP) in whole water samples. Materials and methods Mass coral spawning of Ningaloo Reef, a fringing coral reef system off Western Australia, was predicted for 23-25 March 1987. The sampling site was about 300 m offshore of the seaward point at Coral Bay, Western Australia. The water depth was 3 m with I-2 m of bottom relief. Seawater samples were collected between 1400 and 2215 hours. Visual observations indicated that a prespawning pe- riod began around 1930 hours, but the actual release of eggs peaked around 2130 hours. At the height of the spawn there were egg slicks for more than a kilometer down current. Eggs were collected from these slicks. A background or control sample of open ocean water was collected 0.5 km seaward of the reef at 1500 hours of the day of spawning. All samples were frozen at - 2 0 ~ C.