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.

Of all the divers’ possessions, the collecting bags and the specimens they contain are the most precious, the least replaceable, and so they’re always handed up first. The clear plastic Whirl-Pak bag that’s just come onboard is treated no differently. It’s passed toward the center of the boat and placed carefully into one of several buckets containing many other collecting bags, but, from where I sit, it looks like there’s been some mistake. The specimen itself seems to be missing, as if the organism that was meant to be collected slipped out along the way, leaving behind nothing more than several cups of what may as well be dirty dishwater. A moment later, Terry Gosliner comes up the ladder and climbs into the boat. “Slug city!” he says, beaming.

Of course Gosliner’s collecting bag contains more than dirty water. Inside are nearly 20 tiny nudibranchs, all different, all interesting enough for the lifelong sea slug expert to even bother collecting. About a quarter of the species in the bag will turn out to be completely new to science. Gosliner, a curator of invertebrate zoology at the California Academy of Sciences and one of the scientific leads on this expedition is not the type who’s easily impressed, but he’s pretty happy with this haul. He’s spent the better part of an hour on his hands and knees 60 feet below the surface—alternately tossing up bits of algae and rubble to see what falls out and staring through his makeshift magnifier constructed of PVC and thick glass lenses in search of these elusive grains of life—and he has as many as half a dozen new species to show for it. This mucky spot in the middle of Batangas Channel at the edge of Mindoro Island has turned out to be one of the revelations of the expedition for Gosliner. But it hasn’t been so fruitful for everyone.

Almost without exception, a dive site that pays big dividends for one or two scientists on the team leaves others slightly bored and working hard for a modest number of specimens. Because the underwater terrain here is so varied, however, those fortunes can be reversed in a very short boat ride. Venture around the corner of this tiny peninsula—to a slightly different depth, to a site with a steeper slope that’s subject to a different set of currents—and it might be Gosliner who strikes out, while the expedition’s coral and barnacle specialists strike it rich and come to the surface bursting with specimens and superlatives.

One of the most striking things about this place is its variety. I’m nearly 30 dives in and I can honestly say that no two sites have looked the same. I’ve swum along vertical walls that drop toward unknown depths; I’ve parted groups of fish that brought to mind the term “swarm” more than “school”; I’ve seen silty eelgrass beds, mangrove thickets, sandy bottoms, rubbly bottoms, gardens of coral, and nearly every imaginable hybrid in between. Some spots are nearly overrun by the coral-munching crown of thorns starfish, while others look like triggerfish conventions, and still others appear nearly devoid of life altogether, until you look more closely. My personal experience with this place has certainly been influenced by the scientific sampling protocol, which has the team exploring the widest possible range of habitat types. But what we’ve all seen is not an inaccurate portrayal. This is an astoundingly diverse place, not just in terms of the organisms that live here but also the habitats available to them. That habitat variability, in fact, is integrally linked to and is one of the most powerful drivers of this area’s unprecedented biodiversity.

In general, the region is perfectly positioned to receive a diverse collection of out-of-town visitors. It is sandwiched between the Pacific and Indian Oceans and surrounded by nearly a dozen seas. Many of those bodies of water have spent long periods isolated by ice ages and low sea levels and, as a result, possess unique organisms and communities. A lot of those organisms have ended up here, and because of the Verde Island Passage’s diverse habitats, they've been able to find suitable conditions and have established new populations. “What makes this place special is not that you have unique fauna,” says Rich Mooi, an echinoderm specialist from the California Academy of Sciences and another of the expedition’s principle investigators. “It’s that the organisms are uniquely combined. There’s an incredible overlap of ranges of species from everywhere.”

A steady source of energy and nutrients is a necessary factor in that diversity and abundance. Cold, nutrient-rich water wells up at either end of the channel and is carried by currents to every corner and bay. There it consistently feeds productivity and keeps communities thriving. This link between the deepest deep and the shallowest flat is a powerful reminder of the connection between any one habitat and another.

No matter how different they appear, no matter what lines we draw between them, even the most disparate habitats are connected. Mangroves and eelgrass beds serve as nurseries and the primary-production houses of the life that keeps reefs stocked with fish and filter-feeders filtering. Which is why, Mooi says, “If you want to save coral reefs, you can’t just protect the corals. You need to keep in mind—and preserve—all the many habitats that support them.” And to do that effectively, you have to know what lives where. Which is exactly why the team here scrambles each day to learn as much as they can about as many of the organisms and habitats as possible, while there’s still time left.

In the next post, we’ll dive even deeper to explore one of the least understood of all the habitats here, a place that fewer humans have visited than have walked on the moon. Next stop: the twilight zone.

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?