Elkhorn coral (left) and staghorn coral (right). (Credit: NOAA Southeast Fisheries Science Center)

As the full moons of late summer and fall rise, so too, does the libido of threatened staghorn (Acropora cervicornis) and elkhorn (Acropora palmata) corals. Awakened from a year of sexual slumber, each species shakes off the shackles of celibacy to engage in a mass-spawning a few days after the brightest nights. Facing declines of up to 97 percent in the past 30 years, these two species have been beaten back by disease, pollution, overfishing and climate change. Their yearly spawning should be a time of celebration. But after millions of years of successful group sex, the very act of reproduction may now be contributing to their ultimate demise.

A report this summer adds to a growing body of evidence that another coral, Acropora prolifera, may be overtaking reef real estate formerly occupied by elkhorns and staghorns. Far from a foreign invader, genetic tests show this coral is in fact the offspring of an elkhorn and staghorn cross. A. prolifera is a hybrid. And its apparent rise is an indication of coral sex gone awry.

Rooted to the seafloor, corals cannot move to mate. Instead, like trees casting pollen to the wind, corals send forth thousands of buoyant bundles of egg and sperm into the sea. Floating silently towards the surface, these pink spheres create an undersea snowstorm – only the “snowflakes” rise up instead of drifting down.

Tiny egg and sperm bundles being released from an elkhorn coral branch. (Credit: Dr. Mark Vermeij, Carmabi Foundation)

At the surface, the bundles burst open, spilling fatty eggs and sperm to mix with gametes from neighboring colonies, which have perfectly synched their own release. When it comes to orchestrating an oceanic orgy, corals have impeccable timing: hundreds of colonies across hundreds of miles of reef all spawn within minutes of one another—they can do all this, and they don’t even have a brain. Such coordination allows corals to “get together” without having to get anywhere.

But for closely related species, such as staghorn and elkhorn, the arrival of all those sperm and eggs at the surface means sperm from one species could bump into and “accidentally” fertilize eggs of another species. Such mismatched fertilizations often kill eggs, or sometimes they result in a hybrid. And while Centaurs and Sphinx may seem like badass offspring, in the real world, hybrids can be bad news for parental species.

First, hybrids are often genetic dead-ends. A mule—the offspring of a mare and donkey—is a good example. They are completely sterile. Second, hybrids that can reproduce often have inferior offspring, a phenomenon known as hybrid breakdown. The first generation hybrid (F1) can be robust and virile, possessing “hybrid vigor,” and even outcompeting its parent species. But later generations (F2 and F3, the product of hybrids mating with hybrids) may prove less well adapted or unable to reproduce—they’re a genetic dead-end, delayed.

Finally, sometimes, hybrids thrive, able to reproduce successfully. In this case, hybrids may or may not contribute to declines in their parental species—it depends how much reproduction is diverted to form hybrids versus the original two species and whether adult hybrids outcompete their parents for resources. A. prolifera is looking a lot like the latter scenario, with the ending still unknown.

Egg and sperm bundles about to be released from the mouths of elkhorn coral polyps. (Credit: Dr. Mark Vermeij, Carmabi Foundation)

Until recently, these mixed offspring were extremely rare. None show up in the fossil record (though that could be a sampling issue), and on modern reefs, they occurred only in marginal habitats—outcasts clinging to the edge of the reef. A few years ago, rumors that hybrid Acropora were on the rise created a great deal of concern over the future fate of already embattled branching elkhorn and staghorn species.

That’s when coral biologist, Dr. Nicole Fogarty at Nova Southeastern University, decided to conduct a few investigations. “We found hybrids were growing like gangbusters. They were next to, intertwined with, or on top of one or both parental species at the majority of the sites we surveyed,” she says.

Fogarty’s genetic studies show that different hybrid colonies are the result of multiple, unique reproductive events: they are not just multiplying clones (a trick corals can do). So why, after maintaining separate, successful identities for millions of years, are elkhorn and staghorn suddenly getting their genes in a tangle?

Fogarty is still working to figure it out. But in a recent conversation, she told me she thinks it may have to do with their dramatic decline—something for which we humans are partially to blame.

Elkhorn and staghorn corals spawn at the same time on the same night but live in different depth zones. “Back in the day,” explains Fogarty, “when there were dense thickets of coral, when each spawned, the eggs were swamped by their own species’ sperm.” But today, a combination of factors, including human-induced pollution, overfishing and climate change, has left colonies few and far between. Eggs float around unfertilized for longer—long enough to bump into sperm from the other species.

A diver securing a collector over an elkhorn coral expected to spawn in a few hours. The released bundles are buoyant and float into a jar at the top of the collector. (Credit: NOAA Southeast Fisheries Science Center)

Fertilization experiments have shown that staghorn eggs are rather promiscuous—accepting sperm from elkhorn corals as readily as their own species. So, if given the chance, hybridization goes off with a bang. For staghorn eggs, ‘tis better to be fertilized than die a virgin.

The good news is that these hybrids may be able to fill the void left by the declining Acropora parents, providing adequate structure for reef residents that rely on corals for shelter and food. The bad news is, in some locations, the hybrids seem to be pushing their parents off the reef.

Fogarty is trying to decipher if more hybrid babies are being born, or if they are just outcompeting young elkhorns or staghorns for space, or both. In some sites, they seem to be doing better than their parental species, showing lower rates of disease, predation, and bleaching – clear signs of hybrid vigor.

And, there is evidence that A. prolifera is indeed fertile, and perhaps in more ways than one. Genetic testing has shown that first generation hybrids can mate back with their wanton parent, the staghorns. This creates a coral catch-22. The better the first-generation hybrids do, the more their mixed up genes clutter spawning events, leading to more back crosses and fewer pure staghorns in the population. More recently, Fogarty and a colleague, Dr. Iliana Baums at Penn State, made preliminary findings that hybrids can also mate with each other, forming second-generation hybrids.

Tiny egg and sperm bundles being released from a staghorn coral branch. (Credit: Raphael Ritson-Williams, Univ. of Hawaii)

This outlook is troubling, but Fogarty warns that there is much we still don’t know about how these species reproduce. She reminds me that the rise of hybrids is not a phenomenon unique to corals, and it’s not always bad news.

As a recent article on hybridization describes, from pizzly bears (the offspring of polar bears crossed with grizzly bears) to dolphins, a changing planet brings with it changes in how (and with whom) species reproduce. Sometimes, the ability to hybridize helps two species produce offspring that can more rapidly adapt to new environmental conditions. Such appears to be the case with wolf-coyote-dog chimeras in New England, and with finches in the Galapagos. It may also be the case with A. prolifera, which thrive in shallower, warmer waters than their parents can tolerate. As humans continue to create massive and rapid shifts in the environment, animals will continue adapt, including mating with species formerly outside their sexual range.

Like with so many other evolutionary processes, the outcome of hybridization remains an unknown. For Caribbean Acropora corals, it could be disastrous, causing the loss of biodiversity; or it could prove to be branching corals’ saving grace—creating a new species better suited to the onslaught of anthropogenic impacts we continue to hurl upon the reefs. Only time, and more studies, will tell.