Story and Photos by Dan Martin

As the sun descends beyond the reef I check my watch, wondering if tonight will be my lucky night. I’ve tended to every detail of which I can think, trying my best to stack the odds in my favor. Oh, but Mother Nature can be fickle. With that in mind, I’ve been up late the last few nights; I am prepared. And so, I check my watch again while the evening unfolds beneath a passing full moon, the sea lapping gently at the night sky and rolling upon itself. With each measured breath, I glance about, soon to see that the heavens have indeed aligned in my favor.

Unknown to many, summer is when some of the Caribbean’s most massive corals undergo a highly synchronized mass-reproduction event. Imagine, as the warm rays wane and you settle into a romantic shorefront evening at your favorite restaurant, that the reef beyond is planning a few activities of its own. Under the guise of darkness, colonies of giant elkhorn and mountainous boulder corals prepare for surprisingly predictable spawning bonanzas that can leave the sea surface streaked with extensive slicks of tiny, floating eggs. Although it can be a momentous event, biologists were wholly unaware of this phenomenon until the early 1980s, for it only occurs a few nights each year, and winds and currents easily disperse the floating masses before the new day dawns. And with each species tending to spawn at a specific time on these particular nights, it is no wonder such events went undetected for so long.

Peering about, I discover I am not the only one who seems to approach the night with a sense of excitement. More than the usual cadre of players seem to be out, looking a little edgy and darting about as if they also wait nervously in expectation. Or is it just me? Am I simply more aware of the goings-on as my senses tune to the action? As I draw slowly on my regulator, I do not imagine too much about what may lie in the dark waters beyond my dive light, but instead marvel at the transformation the reef and its inhabitants have undergone while hidden from the rays of our closest star. While night diving may not be for everyone, I dare say that it is more than worth the effort, and may (as it has for me) become one of your favorite types of dive. A whole new assemblage of fishes and other cryptic beings come out under the protection of darkness to lurk amongst the fusion of corals, and tonight they sense a feast.

As corals come in a great diversity of forms, so do they display a number of what biologists like to call “life history strategies,” or ways of solving the general challenge of survival and reproduction. Stony coralsÑthose that produce most of the hard, white coral skeletons we have all seen on the beach and in shell shops–are responsible for building the bulk of the world’s tropical reefs. Most species of reef builders are made up of tiny individuals of anemone-like polyps living together in massive colonies. Individuals within a colony are genetic clones, and being fixed to the substrate, must release their gametes (eggs and sperm) into open water. There they met gametes from other colonies in order to effect sexual reproduction. A different strategy from the human condition, no doubt, but it seems to work for them.

In what appears to be an attempt at maximizing fertilization success, the polyps of many corals release bundles of lipid-rich eggs that float to the sea surface upon release. Hitching a ride with these bundles of several to several dozen eggs are tiny packets containing millions of sperm. Not long after reaching the surface, the delicate layer surrounding the bundle soon ruptures, releasing its contents. When done in synchrony, gametes from a number of colonies have increased their chance of meeting each other, as all are now at the surface rather than diluted throughout the water column. It is here that fertilization takes place and the tiny new life of a genetically unique coral begins

It is going on 9 PM and I start to see slight changes in the minute polyps of a giant elkhorn. Being careful not to touch the reef, I examine a branch closely and find that within the golden-brown polyps a pale bulge begins to appear. I figure my piscine pals have taken note as the number of Coral Spawning detailsilvery flashes increase about me. Several fire worms, needle-fine setae bristling in my light, also rear their heads in recognition, as does a brittle star that comes out of hiding with whispy arms waving about, tasting the environment via the highly sensitive tube feet with which they are tipped. The bulging I see is like the musicians of an orchestra taking their seats, already tuned and waiting silently for a signal from the conductor: the eggs are readied for delivery.

These pale pink gamete bundles develop and then emerge from the colorless, deeper folds of tissue lining the shallow cavities of the coral’s stony white frame. Hard, reef-building corals are really just a delicately thin layer of living tissue overlying the calcium carbonate lattice that they themselves secrete. A typical coral polyp is simply a circular stalk or trunk with a relatively large mouth at one end. Tentacles tipped with stinging cells, which can be employed to catch tiny food particles from the plankton, ring the mouth. Being of primitive form, corals utilize a simple strategy of releasing eggs through their digestive sacs and out their mouths. Next time you come across a piece of coral on the beach, examine it closely to see the intricate array of geometric patterns created by the septa or divisions within each polyp.

By now I feel as though Mother Nature is teasing me and those who look on; it will be a while longer before the show beings. But not to worry, as my attention has already been diverted. A goldentail eel glides by below, poling its head into holes and crevices in search of a meal or a mate, or something of which I know not. As the eel hardly seems to take notice, I continue to drift down and over the purple splaying barrels of a stove-pipe sponge, and spy one of the reef’s daytime inhabitants bedded down for the night. Deep within one tube is a varicolored wrasse wondering who turned the light on. One of the pleasures of night diving is that you can often give close inspection to fishes that are otherwise shy by daylight–a trick not unnoticed by underwater photographers. And so I soon come face to face with a puffer that is temporarily disoriented by my passing light. It, too, was probably in some sort of quiescent mode, so I pardon myself and move on.

Although the first observations of mass coral spawnings were noted on the great reefs of Australia as recently as 1981, it wasn’t until 1990 that such events were confirmed in the Caribbean region. Temperature and lunar period seem to be the major factors influencing timing among species, but these times vary throughout the world. Pick almost any month and you can probably find some place where some species is expected to spawn, especially as one crosses the equator. But even within species, geography seems to play a role. The magnificent elkhorn coral of the western Atlantic, for example, is known to spawn a month earlier in the Florida Keys than it does in Bonaire, and with slightly different timing. In Bonaire, the fourth or fifth night after a September full moon is the anticipated time, while in Florida it has been two to four nights following the August full moon. Punctuality is also important, given that some species conclude their spawning activities in a matter of minutes, although most last for more than 30.

Returning to my position adjacent the massive branches of an old elkhorn colony, a slender arrow crab tip-toes by with a sideways glance of indifference to me. It’s just after 9 PM and the symphony has begun. Having waited patiently with mouths full, the polyps now evict the cumbersome bundles in a wave that spreads rapidly across the colony. In no time it seems this is a cue to surrounding colonies as a silent crescendo builds, filling the water column with millions of tiny spheres. The sea momentarily resembles a glass of bubbling champagne as I sweep my light about the reef; I am amazed at how quickly a slick begins to form at the surface. The darting fishes are now whizzing by, frenzied and gorging themselves on the annual delicacy presented to them. Off western Australia, large numbers of huge, planktivorous whale sharks reportedly show for that annual event. But the corals’ strategy appears to be a good one, as there are far more eggs floating about than critters to eat them.

The patient conductor of this silent symphony, however, goes unseen and largely unknown. While the long-term cues for synchronicity seem to do with the moon phase and sea temperature, some researchers think the finer minute-by-minute controls among colonies may have to do with a water-borne signal emitted during the final stages of development and release. And within colonies, control may be further mediated by rapid intercellular communications.

As the release of eggs trickle to an end, I check my dive partner (her eyes smiling behind tempered glass), and then my gauges. Although we have been in the water for some time, I still have plenty of air in reserve. Two factors work to your advantage here: some of the best mass-spawning occurs in less than 30 feet of water (even snorkelers can enjoy this activity), and, for most, night diving has a calming effect as many of the sights to be seen will present themselves to you, rather than you swimming about in search of something that piques your interest. From large eels to cryptic minutia, chances are you won’t travel as far at night to see them. So as we surface with air to spare, we pause to inspect the floating masses of pinkish eggs, and it occurs to me that they will not stay this color for long.

Color is another aspect of corals that hints toward their fascinating life histories and how they are exquisitely adapted to their delicate environment. Ever wonder why some of these animals look so much like plants with their branching forms that seem always to be reaching beyond areas of shade, as do leaves of a tree? Many of the colors you see, especially when observing the large reef-builders, come from tiny single-celled algae living symbiotically within their tissues. Here, living in the outer layers, algae are exposed to the sunlight they need, and return favor by supplying corals with compounds essential to their health.

Researchers are still unraveling the complexities of algal-host associations, but it has become clear that for many corals, this symbiosis is necessary for proper skeletal growth, and thus reef growth. (A coral which loses its algal symbionts during times of stress may not survive long without them. Such individuals are described as “bleached” since their tissues now lack color pigments, and soon resemble the bone-white pieces you find on the beach. Various factors may cause coral bleaching, but increased sea temperature is believed chief among them.) The pinkish, floating babies of the elkhorn will soon take on the color of a colony as they settle and transform into their adult form, culturing a new crop of algae as they do.

As we return to shore, the dive-talk is lively and everyone is feeling invigorated rather than tired. It is a good thing, too, because the evening is just beginning for the developing coral larvae and those who study them. The larvae will soon expel much of the fatty substance that floated them to the surface, allowing them to sink in the open waters where they will develop weak swimming behaviors and ride the currents. In as little as a few days, these tiny, spindle-shaped larvae will begin searching for a place to attach to the sea floor and begin the next stage of their life cycle.

But the Atlantic is a deep ocean, the Caribbean an expansive sea. How does a minute larva know where to settle on the sea floor? Once it begins cementing a hard skeleton to the bottom, it is committed. Too deep and there will be no light for their crop of algae; too shallow and they may not be near other colonies of their kind, leaving them unable to mass-spawn. As has been discovered, coral larvae possess tiny chemical receptors that work much the way our smell receptors work when we distinguish between the various foods we eat.

After many experiments, researchers have found that the sequence of events initiating settlement behavior and growth into the adult life form is triggered when larvae “smell” a certain type of algae common to coral reefs. The alga of choice is usually a thin, encrusting form (often purple or reddish in color) readily found on the undersides of corals and covering coral rubble. Curiously, these “crusty” algae do such a good job of holding coral rubble and other bits together that they themselves are credited as important members of the reef-building community. By cementing smaller pieces together, they stabilize the substrate long enough for the miniscule coral larva to cement its own sturdy base and begin a new colony. A fine strategy by the coral larva, especially when considering that their attachment responses are best elicited by particular species of encrusting algae; the strongest responses often come from algae whose depth of distribution is common to that of the adult coral’s habitat.

Not that you need to keep all these facts in mind to enjoy a night dive. However, as is often a consequence of new knowledge or insight, I find myself having a deeper appreciation for the amazing reef communities that embrace these beautiful islands. As demonstrated in the life history strategy of the corals, life on the reef is woven together inextricably; a community, indeed, as overall health and success can be quite dependent on individual members. So next time you think your night life is getting complicated, think of the corals, or go for a night dive. And don’t forget to make plans for next summer’s events. (Rumor has it that dive charters to see coral spawn at the Flower Gardens Bank of Texas have a two year waiting list.)

Editor’s Note: The exact dates and timing of this event have yet to be documented for the Turks & Caicos, although it is estimated to take place the week after the eighth full moon of the year between 9 and 11 PM.

It’s not really that rare an event once you get the schedule: just a few nights later I was treated to another, more dramatic show by the massive boulder corals–the timing even more impeccable as their little mouths yawned in synchrony, releasing their pearly beads of life.

Author Dan Martin spends most of his time as a research associate in marine science. He is currently working on a long-term krill project in Antarctica, jelly fish in the Gulf of Mexico, and teaching freshman biology. He spent several seasons assisting the Morse lab at the University of California, Santa Barbara, with their investigations of larval coral settlement cues–work that may someday allow us to “re-seed” damaged reefs.