When the deep ocean is connected

María de los Ángeles Gallardo – Centro Científico ESMOI, Facultad de Ciencias del Mar, Universidad Católica del Norte

More than 1,000 meters below the surface, shrouded in perpetual darkness and hovering just above freezing, lies a world that until recently remained virtually unexplored. This extreme environment is home to tiny crustaceans known as squat lobsters. Though these distant relatives of king crabs and porcelain crabs rarely exceed a few centimeters in length, they harbor some of the most remarkable evolutionary histories in the Pacific Ocean.

Not long ago, we shared the story of Galathea tukitukimea and Trapezionida diritas, two species discovered at depths above 500 meters. This time, however, we dove much deeper to encounter an entirely different family of deep-sea dwellers: the Munidopsidae.

Thanks to expeditions conducted by the ESMOI Center in 2024 onboard the R/V Falkor (too), and utilizing the ROV SuBastian, we successfully collected eight species of Munidopsidae. One of them, Munidopsis aff. atlantis, essentially “captured itself.” We are not entirely sure how it made its way into the suction sampler container; it most likely remained hidden among the branches of deep-sea corals and was drawn in completely unnoticed. In fact, we only realized it was there when we began sorting through the samples collected from the JF6 seamount along the Juan Fernández Ridge.

Of the eight species discovered, four turned out to be entirely new to science. The remaining four, however, revealed a much grander narrative, a striking contrast between the intense endemism typical of the southeastern Pacific seamounts and the surprising large-scale connectivity that exists across these very same structures at extreme depths, effectively linking the entire Pacific Ocean.

To fully unravel this story, one must understand that depth holds the key.

Within the first few hundred meters of the southeastern Pacific, the ocean is defined by sharp gradients in temperature, oxygen, food availability, and other environmental variables. These rapid changes act as formidable physicochemical and ecological barriers for many species. Furthermore, the Nazca, Salas y Gómez, and Juan Fernández ridges feature highly unique oceanographic conditions, earning these zones a reputation as having some of the highest rates of endemism on the planet.

But what happens below 1,000 meters

At that depth, most of these environmental gradients begin to flatten out, and the surroundings become far more stable. This realm is dominated by a massive water body known as Pacific Deep Water-cold, incredibly ancient, and surprisingly rich in oxygen.

One of the hypotheses emerging from our findings is that this relative environmental homogeneity might foster greater connectivity among deep-sea Pacific populations. In other words, once the environmental barriers found in shallower waters dissolve, deep-sea species can spread across thousands of kilometers, occupying nearly identical habitats.

And that is precisely what we found. Munidopsis comarge, originally described in Australia and New Zealand, turned up living on the seamounts of the southeastern Pacific. Munidopsis aff. atlantis, closely related to a species known from the Indian Ocean, was also present in our samples. Munidopsis tiburon, described in the North Pacific off California, was recorded for the very first time in this part of the world. Even Galacantha rostrata, a species whose known distribution spans the Atlantic, Indian, and Pacific oceans, made an appearance in our expeditions.

Even more astonishing is that the genetic divergence among some of these distant populations is remarkably low, reinforcing the idea that the deep ocean might be far more interconnected than we ever imagined.

Yet, while the species found below 1,000 meters tell a story of remarkable trans-Pacific connectivity, the shallower summits and slopes of these seamounts revealed the exact opposite pattern. There, at depths above 750 meters, we discovered three species entirely new to science: Munidopsis amnos, Munidopsis molinai, and Munidopsis oryza.

The exception to this pattern is Leiogalathea rosula. This species was previously recorded by Paula Rodríguez-Flores in New Caledonia, but has now been found along the Salas y Gómez Ridge, with its formal description utilizing specimens from both regions. Genetically, both specimens belong to the same species. It was collected at depths ranging from 646 to 1,130 meters, demonstrating that certain species can indeed expand their distribution across the hypothetical 1,000-meter threshold.

Seamounts as ecosystems

Munidopsidae do not merely exist at great depths; they also occupy a striking diversity of microhabitats. During our dives, we observed them on fine sediments, volcanic rock, crevices and fractures, small sand-filled cavities, coral rubble (fragments of dead coral), live coral colonies from various taxonomic groups, and even nestled within mollusk shells. Each species appeared to possess its own distinct ecological preferences.

Without a doubt, the most generalist species when it came to finding shelter was Munidopsis comarge. This tiny squat lobster, measuring barely two to three centimeters in total length, was spotted living on various species of both live and dead coral, often forming aggregations of several individuals on a single colony, as well as occupying bare volcanic rock and other available substrates. Everything points to a highly adaptable species capable of exploiting any complex structure that provides refuge.

Yet, perhaps our most striking observation lay elsewhere. The individuals found on white corals displayed a much lighter coloration, whereas those associated with yellow or orange corals exhibited hues that closely matched their host. Is this a mechanism for camouflage? We do not yet know, but this observation opens up a compelling new scientific question that we hope to answer in future investigations.

For a long time, the deep ocean was thought to be a uniform, low-diversity environment. Today, we know the exact opposite is true. Every dive reveals new species, novel ecological associations, and fresh questions about how life managed to colonize one of the most extreme environments on Earth. Ultimately, the greatest discovery of these expeditions was not just the four species new to science, but the realization that, in the depths of the Pacific, seamounts separated by thousands of kilometers remain part of the very same evolutionary story.