Coral Reefs in July: Underwater Wonders at Their Peak

Field report

From a mass coral spawning to sea turtles nesting on moonlit beaches, this month is perfect for slipping underwater and taking in the reef.

While fireworks will dominate the evening skies on the Fourth of July across the U.S., the rest of the month will feature spectacles of a different kind, from the annual mass coral spawning—one of nature's most synchronized events—to the arrival of whale sharks at summer feeding grounds.

With July being the warmest month for tropical seas in the Northern Hemisphere, it only makes sense that coral's most dramatic event would follow suit. For those lucky enough to have calm waters, the mass coral spawning (frankly, the most extraordinary underwater event of the year) will occur just after the full moon on July 2-3. Peak release will come on the evening of July 3, when entire reef sections erupt in a blizzard of egg and sperm bundles.

This month's spawning is the largest synchronized reproductive event in the animal kingdom. Called broadcast spawning, coral colonies across hundreds of miles release their bundles simultaneously—triggered by water temperature, the lunar cycle, and the time of sunset. The rising egg bundles look like pale pink snow drifting upward through dark water, an event so thick it can be seen from boats on the surface.

In addition to the coral spawn, this full moon period has also been linked to sea turtle nesting (green and loggerhead females come ashore on these bright nights), the hatching of shorebird chicks on barrier islands, and the peak of manatee mating herds in Florida's warm springs. Locals also call this the Conch Moon, coinciding with the summer harvest of queen conch from the shallow grass flats.

July's Calm Weather Reveals the Clearest Underwater Visibility (July 15-20)

"It's not common for bees to use parts of plants for nests,"saidDr. Jerome Rozen of the American Museum of Natural History (AMNH), a member of the team in Turkey. "There's a demand for biologists to know bees nowadays," he added. "They are the foremost animal pollinators of plants, and tremendously important for maintaining ecosystems — not only crops but also for conservation."

To perform this beautiful task, mother bees bite off the petal from the flower and fly it back, one by one, to the site. She starts the nest in a small burrow, layering petals in a surprisingly orderly fashion. Asdescribed in the research, which was published in the AMNH publication, American Museum Novitates:

Marine Photography / Coral Reef Archives / CC 2.0

Swim Among the Season's Whale Shark Aggregation (July 17)

Just in time for July's calmest seas, whale sharks—the largest fish in the ocean—will reach peak numbers at seasonal feeding sites. These gentle giants, which can grow over 40 feet long, arrive each summer when plankton blooms concentrate near the surface, their enormous mouths filtering thousands of gallons of water per hour.

A precursor to the more popular daytime reef experience, the night dive reveals an entirely different ecosystem that comes alive after dark. Octopuses emerge from their dens to hunt, parrotfish secrete their mucus sleeping bags, and basket stars unfurl their feathery arms to catch plankton in the current. To catch the night reef at its best, descend after the last light fades and shine your dive light along the reef crest.

While the spotted eagle ray isn't known for appearing in large groups, what it lacks in quantity it more than makes up for in quality. These rays, with wingspans reaching nine feet and striking white-spotted patterns against dark backs, glide over the reef like birds in slow flight. On calm late-July mornings, they patrol the reef crest searching for buried mollusks and crustaceans, their duck-billed snouts probing the sand.

According to a long-term study of eagle ray movements, it's estimated that these rays return to the same reef sections year after year, a site fidelity that makes them vulnerable to habitat degradation but also means patient divers can reliably find them at their favorite cleaning stations.

The Return of the 'Ghosts of the Reef' — Bioluminescent Ostracods (July 30)

Bioluminescent ostracods—tiny seed shrimp no bigger than a grain of sand—are among the reef's most magical nighttime residents. On warm July nights after the moon sets, male ostracods rise from the reef and release pulses of blue-green light, creating trails of glowing dots that hang in the water like underwater stars. Each species flashes a distinct pattern, a luminous Morse code visible only in the dark. The display is brief—lasting perhaps ten minutes—but so vivid that the reef itself seems to breathe with light.

Which sounds perfectly reasonable and undeniably beautiful ... and still provides all kinds of wonder more than a decade later.

This story was originally published in 2018.