July is an eventful month for coral researchers like Robert Richmond.

It's the most productive time of the year for the spawning of two of Hawai'i's major corals, resulting in countless coral larvae that will become the valuable reefs of the future.

"This is the big spawning event of the year for them," said Richmond, research professor at the University of Hawai'i's Kewalo Marine Laboratory. "Coral spawning events like the ones we're experiencing this month are the key to the future of Hawai'i corals. This is the replenishment phase. Just like any other living population, they are animals that are dying every year, and they need to be replaced by new individuals."

The coral Montipora capitata began spawning Wednesday night, with scientists expecting countless coral larvae — each a fraction of a millimeter — to drift around looking for a place to settle. The coral Porites compressa is expected to begin spawning later this month. Both are among the major contributors to coral communities in Hawai'i, Richmond said.

It's a critical time, especially considering the stresses and threats that coral reefs in Hawai'i and elsewhere are under.

Land-based runoff and sediment, as well as alien invasive species, are serious threats to corals in Hawai'i. Other stresses include overfishing and ocean debris. Scientists also say global climate change, which can contribute to coral bleaching, is another potential threat.

Coral reefs, sometimes called the rainforests of the sea, are home to thousands of marine plants and animals, and are a valuable part of Hawai'i's culture. They protect shores from erosion and are critical to the state's marine tourism industry.

Coral researchers this month will closely watch the reproduction process for these two corals out in the field as well as in laboratories. For some, it means putting in late hours — M. capitata and P. compressa spawn at night, with the event tied to the lunar cycle, Richmond said.

The coral M. capitata is hermaphroditic, so it releases both eggs and sperm, which float to the water's surface. They interact with other eggs and sperm from different coral colonies, and if conditions are right, fertilization will take place and coral larvae will develop, Richmond said.

P. compressa has separate sexes, so the females will release eggs while the males will release sperm, he said.

The coral larvae will drift around the ocean for a few days to a couple of weeks before settling on the ocean floor. Young corals can usually reproduce after three years and grow large enough to provide habitat for fish after three to four years, Richmond said.

Water quality is a key factor along the reproduction process. A rainstorm, for example, can cause freshwater runoff, which severely cuts the coral fertilization rate, Richmond said. Sediments and invasive algae can make it difficult for coral larvae to find a good place to settle and grow.

Only a fraction of the eggs that are released are fertilized, make it to the larval stage, find a place to settle and survive the first year, he said.

Richmond said he recently advised contractors for the military on Guam about spawning events there so they could plan their dredging operations around it.

"What happens (in Hawai'i) this month is really going to be critical for the next year and the next few years for corals here," Richmond said.