In a study of seven "massive" Caribbean corals, Soong (1993) identified major differences in reproductive behavior between species with large maximum colony size (>100 cm2 in surface area), including Diploria strigosa, and species with small maximum colony size. The four large species studied broadcast gametes during a short spawning season and had a relatively large "puberty" size. The two smaller-sized and one medium-sized species brooded larvae during an extended season (year round in Panama) and had a puberty size of just 2 to 4 cm2.
Prior to the early 1980s, for over 200 years, all corals were believed to be viviparous (brooding). It is now known that most reef-building corals release, or "broadcast", eggs and sperm into the water column during periodic and often synchronous spawning events. For decades researchers have speculated about and worked to identify environmental entrainment factors that might influence sexual reproduction and the eventual release of gametes. This synchronization is generally believed to operate on at least three interrelated temporal levels: (1) the time of the year; (2) the lunar cycle; and (3) the time of night. It is clear that nighttime is required for gamete release, but a consistent global relationship between lunar phase and the timing of spawning is less clear, given that most corals on the Great Barrier Reef in Australia spawn at neap tides, while the same species in southern Japan spawn at spring tides. It has seemed reasonable to assume that the time of the year for gamete release is linked to optimal sea surface temperature (SST). van Woesik et al. (2006), however, have argued that solar insolation (energy from the sun), is a better predictor of gamete production for many corals.They tested this hypothesis using data for 12 species of corals distributed throughout the Caribbean (tropical west Atlantic), including Diploria strigosa. Regarding temperature, they found that the cumulative dose of SST measured through time and the rate of change in temperature correlated poorly with the timing of coral spawning, although the average temperature during the month of spawning was significantly correlated with spawning. For solar insolation, they found that the rate of change and the cumulative response of solar insolation cycles was a better predictor of gamete release, although solar insolation intensity at the time of spawning was not. All of the coral species they examined showed highly significant positive relationships between spawning date and the cumulative dose of solar insolation, and 11 of 12 species, including D. strigosa, showed a significant response to the rate of change in solar insolation. Solar insolation and temperature are obviously related phenomena since solar irradiance ultimately drives SST, but because of the high specific heat capacity of water, maximum SST generally lags 1 to 2 months (or more) behind maximum solar insolation. Time delays in SST fluctuations are latitudinally predictable but vary with cloud-cover and windstrength. van Woesik et al. concluded that solar insolation influences the reproductive schedules of Caribbean corals, but water temperatures must be optimal (28–30 C) to allow maturation and gamete release. (van Woesik et al. 2006 and referencess therein)
Broadcast spawning by corals is a tightly synchronized process characterized by coordinated gamete release within 30 to 60 minute time windows once per year. Vize (2006) asserts that for shallow water corals, annual water temperature cycles set the month, lunar periodicity the day, and sunset time the hour of spawning. This tight temporal regulation is critical for achieving high fertilization rates in a pelagic environment. Given the differences in light and temperature that occur with depth and the importance of these parameters in regulating spawn timing, Vize notes that it has been unclear whether corals in deeper water can respond to the same environmental cues that regulate spawning behaviour in shallower coral. Vize used a remotely operated vehicle to monitor coral spawning activity (including that of Diploria strigosa) at the Flower Garden Banks (northwest Gulf of Mexico) at depths from 33 to 45 m, All recorded spawning events were within the same temporal windows as shallower conspecifics. These data indicate that deep corals at this location either sense the same environmental parameters, despite local attenuation, or communicate with shallower colonies that can sense such spawning cues.
In a study in Puerto Rico, D. strigosa spawned in August and/or September. Development of oocytes (egg-producing cells) began in October–November, between 5 and 7 months before spermatogenesis (sperm production). Development of spermaries (sperm-producing structures) started 6–7 months after oogenesis, in May to June. Spermatogenesis lasted 3 to 5 months, with sperm cells maturing rapidly and reaching full maturity at the same time as the eggs, in late July and August. A high proportion of colonies had mature spermatocytes and oocytes 4 days after the full moons of both July 28 and August 30, 1999, indicating that in the year of this study, a split spawning occurred. No mature gametes were found in these colonies in tissue samples collected in September of 1999. Spawning occurred after 11 p.m. on nights 9 and 10 after the full moon for D. strigosa. (47.6 and 50.03 eggs/polyp in 1999 and 2000, respectively). There was no significant correlation between colony size and mean polyp fecundity for D. strigosa. Even the smallest colonies sampled (140 cm2) were sexually mature, and there was high variability in fecundity; minimum reproductive size must therefore be below this size.
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