There's more to symbiosis on the reef than clownfish and anemones. (Photo by Will Love)

Editor’s Note: “The Richest Reef” follows members of a scientific dive team as they attempt to pinpoint the center of the most biologically diverse marine ecosystem in the world. Long considered our planet’s most species-rich piece of ocean real estate, the Western Pacific’s “Coral Triangle” is a continent-sized patchwork of habitats, populations, and communities. Expedition scientists are trying to identify exactly which section of this rich mosaic is most diverse, but their effort carries with it far more meaning than a simple dot on a map would suggest. Along the way, they’ll discover dozens of new species, visit dive sites and depths no one has seen before, and gain a better understanding of the factors that promote biodiversity and the role these species-rich areas play in sustaining healthy ecosystems. See below for a list of all posts in the series.

With a rusty hammer in one hand and an equally weathered chisel in the other, Gustav Paulay is busting the hell out of a piece of coral rubble at that edge of the dive resort pool. The two-pound chunk he’s sweating over is pockmarked with six or eight centimeter-wide holes, each one leading into a tiny burrow. Like a kid breaking into a stubborn piggy bank, Paulay, an invertebrate zoologist at the Florida Museum of Natural History, is determined to get to the bottom of every last one. Each time he succeeds, he trades his hammer and chisel for a pair of forceps and carefully extracts a pale little animal, known as a gall crab, that seems both baffled to be discovered and completely ill-equipped to deal with the outside world.

A specimen photo of a gall crab (Cryptochiridae) shortly after its extraction from coral rubble. (Photo by Gustav Paulay)

The connection between gall crabs and corals is just one of countless symbiotic relationships here. You can’t swim ten feet in any direction on the reef without bumping into some animal living alongside, on top of or inside of another animal. And while some of these organisms are competing for space or simply coexisting, others are cooperating–living lives that are tightly intertwined both biologically and evolutionarily. Paulay is at once ecstatic about the prevalence of these “species complexes” and tormented by how little is known about the true nature of many of them. “We just don’t have a clue what’s going on in the majority of associations we see out there,” he says. For most, researchers have only snapshots. They see two organisms living together but can only guess what functions their relationship serves, how critical it is to their survival, how the relationship evolved, and to what extent these types of associations are driving the origin of new species.

An imperial shrimp (Periclimenes imperator) taking a ride aboard a purple-edged ceratosoma nudibranch (Ceratosoma tenue). (Photo by Will Love)

The more you know, the more you realize all that you don’t know, I suppose. From my perspective, Paulay and the other scientists on this expedition have far more than a clue about symbiotic relationships throughout the Indo-Pacific. They can talk for hours about the astounding variety of barnacles specialized to live on an equally incredible range of organisms and very specific body parts; they can describe in intimate detail the life histories of animals that live exclusively on echinoderms, or nudibranchs, or sea fans; and not once do they mention the clownfish and the anemone, the poster species for this biological phenomenon and the animals that typically come to mind when most of us think about symbiosis on coral reefs.

Symbiotic relationships can take many forms, from mildly parasitic, as is the case with the gall crab, to highly cooperative and beneficial to both species involved, as is the case with the clownfish and the anemone. When you boil them all down, though, you see that most of these associations serve one (or both) of two basic functions: giving access to food and other nutrients and providing protection.

Seastar shrimp (Periclimenes soror) are found on the underside of just about any pincushion star you flip. (Photo by Will Love)

Arguably, the most important of all marine symbiotic relationships is the food-generating partnership between reef-building corals and single-celled microorganisms known as zooxanthellae. The polyps of the majority of hard corals–as well many soft corals, anemones, and even giant clams–contain these photosynthetic organisms. In exchange for a steady supply of inorganic compounds, the zooxanthellae harness the sun’s energy to produce carbohydrates, which fuel the host animals’ growth and reproduction. Without these internal fuel cells, the corals simply wouldn’t have the capacity to build the massive colonial apartment buildings that support so much of the life here.

The quest for food has been a powerful evolutionary innovator for countless other species on the reef as well. There’s no shortage of the types of interspecies relationships that enhance an organism’s ability to obtain energy and nutrients. In fact, many animals carry on a number of different relationships at the same time–sometimes playing the host, sometimes the symbiont. And although many of these associations appear messier or less sophisticated than the tidy chemical exchange between coral polyp and photosynthesizing protist, there is no lack of specialization.

Egg cowries hide in plain site amidst beautiful soft corals and feed on their hosts. (Photo by Will Love)

Slow-moving creatures tend to be fantastic symbiotic hosts. Like the hop-on-hop-off buses that traverse major cities, they provide a reliable form of transportation and occasional snacks. Non-stinging varieties are also great for a novice symbiotic observer like me. Now that I know what to look for, I regularly turn over pincushion stars and more often than not, find color-matched shrimp underneath, keeping the starfish free of parasites and remaining vigilant for whatever their ride might stir up. Other species of starfish, urchins, and sea cucumbers tend to have their own symbiotic relationships, with each host paired with a specialized symbiont of its own.

Barnacles tend to be on the other side of the equation, settling on hosts that suit their particular needs. The most common place to find them is on and in corals and sponges, but some species also show up in the gill chambers of crabs, while others inhabit the mouthparts of lobsters. Considering the messy eating habits of crustaceans, there are worse places for a filter-feeder to hang out.

The need for protection is the other powerful driver of symbiotic innovation. Because there’s always something nearby looking for its next meal, most animals hide, and often they hide on or inside of other organisms. Some animals hide in relatively plain sight either by taking shelter amidst the stinging spines of urchins or the tentacles of anemones or by matching the colors and patterns of their hosts so closely that they go unnoticed. Burrowing is another tremendously successful strategy, which is why it’s hard to find a hunk of coral that hasn’t been drilled into by something.

Feather duster worms (Chone sp.) keep all but their filter-feeding parts hidden deep within corals. (Photo by Steven Bedard)

During this morning’s dive, I took a close look at a coffee table-sized shelf of coral. On its surface, I counted more than 200 burrow openings. There were surely gall crabs at the bottom of many of these holes, but it was impossible to tell. The only obvious signs of life were a couple-dozen bristle worms, their crowns of feeding appendages stretched into the current, high above the ocean bottom, waiting patiently for any edible bits to come their way. It was a clear reminder that symbiotic relationships can have multiple functions and that those related to food and protection are not mutually exclusive. In fact, with one need taken care of, there is more opportunity for additional specializations to evolve.

As fun and fascinating as all these evolutionary curiosities are to see in person, I’m learning that they’re more meaningful and relevant to this place and this expedition than I ever could have imagined. Symbiotic relationships, it turns out, are a powerful driver of diversity. Every host-symbiont association provides opportunity for greater specialization, and the more specialized the partners in a species complex become, the more likely it is for that segment of the populations to diverge and become a new species.

The exact impact these types of relationships have had on biodiversity in this incredibly diverse place is anybody’s guess, but they have clearly played a role and will continue to do so. Next, we’ll look at other factors that have helped to make this the richest marine environment on Earth and explore why no two dive sites look the same.

Other posts in this series:

The Richest Reef: Exploring the Most Diverse Marine Ecosystem on Earth

The Richest Reef: No Such Thing as Packing Light

The Richest Reef: Life in Layers

The Richest Reef: A Symbiotic Society

The Richest Reef: A Bagful of New Species

The Richest Reef: Deep Diving into the Twilight Zone

The Richest Reef: Where Have All the Predators Gone?