Biodiversity on the RocksMacrofauna Inhabiting Authigenic Carbonate at Costa Rica Methane Seeps

Picture the deep continental margin. It’s mostly soft mud, a vast seafloor of silt and ooze. Hard rock is rare. But where methane seeps out, microbes get to work, and the mud turns into geology. Archaea that oxidize methane partner with sulfate-reducing bacteria. Together, they strip energy from methane without oxygen, a process called anaerobic methane oxidation. That chemistry raises alkalinity and pH in the pore waters, tips the balance toward carbonate supersaturation, and precipitates carbonate minerals. Over time, pavements and slabs grow right out of the sediment. These authigenic carbonates become islands of hard substrate in a muddy sea, and they store carbon that might otherwise return to the ocean and atmosphere. In other words, a microbial engine sculpts rock and quietly shapes carbon cycling. On the Costa Rica margin, those rocks form near the sulfate-methane transition, right by the sediment-water interface. They’re cold, like the bottom water, but busy. They host mussels, clams, and tubeworms, sure, the big charismatic seep fauna. But they also host a teeming small world that uses the rock itself — its nooks and its skins of microbes — as home and food. Think rocky tide pools at the beach: hard surfaces that concentrate grazers and build a food web around biofilms. Now push that image a thousand meters down. That’s the link Levin and colleagues chased: how the intensity of seepage, the kind of biogenic neighborhood around a rock, and the location along the margin shape who lives on these carbonates and what they eat. They expected more active seepage to boost density, change who dominates, and leave a lighter carbon fingerprint because methane-derived carbon is isotopically light. They built a field-to-lab pipeline to test it. Aboard the research vessel Atlantis with the submersible Alvin, the team collected thirty-eight carbonate rocks from six sites strung along the margin: Quepos Landslide, Mound 11, Mound 12, Mound Quepos, Jaco Wall, and Jaco Summit. They also took nearby sediment cores for comparison. Underwater, they tagged each rock’s neighborhood as active or inactive based on what they saw. Shimmering microbial mats, bubbles, or big seep animals meant active, while a quiet, bare rock meant inactive. Back on deck and later in the lab, the workflow was simple but meticulous. They washed and sieved the rocks, let hidden critters crawl out, and identified everything down to the lowest practical taxon. They measured stable isotopes in animal tissues — carbon and nitrogen, the workhorse tracers of diet — and in the rocks themselves, teasing apart organic carbon bound in the matrix from the inorganic carbonate. The headline was immediate. Carbonate rocks in active seep neighborhoods were swarming. Densities averaged one hundred eighty point two individuals per two hundred square centimeters, compared with just thirty-three point seven on inactive rocks. Put differently, the same palm-sized patch of rock might hold five or six times more animals if methane was actively bubbling nearby. Habitat mattered within that picture. Rocks sitting inside mussel beds were jam-packed, averaging two hundred forty-six individuals per two hundred square centimeters, and tubeworm patches weren’t far behind. Clam beds and microbial mats trailed. Some single rocks pushed past six hundred individuals. That’s a lot of life pinned to a hand-sized piece of deep-sea limestone. The communities weren’t just denser; they were different. When Levin, Mendoza, Grupe, and colleagues compared composition statistically, active and inactive rocks separated cleanly, with an ANOSIM R of zero point five five six and a strong p-value. The kinds of animals driving that split made ecological sense. On active rocks, gastropods dominated the marquee species, including grazing limpets and snails that mow down microbial films, while polychaete worms were ever-present. Inactive rocks leaned toward crustaceans, cnidarians, and ophiuroids — a fauna more attuned to the overlying water column than to chemosynthetic patches on the surface. If you zoom out to substrate, carbonates versus sediments parted ways too. Sediments were richer in certain polychaete families — ampharetids, dorvilleids, hesionids, cirratulids, lacygonids — while carbonates tilted toward gastropods and polychaete grazers and predators like syllids, chrysopetalids, and polynoids. Same seep, different stage. Diversity tracked that split. Active carbonates supported roughly double the taxon richness, about twenty-six point four species on average, compared with twelve point five on inactive rocks. That’s not a subtle effect. Site mattered as well; some locations, like Jaco Summit, ran high, while places like Quepos Landslide ran low. When the team leveled the playing field with rarefaction, the rocks still won on richness relative to adjacent seep sediments. At Mound 12, for example, a standardized count edged higher on carbonates than in sediments, nine point two versus seven point two. So even when sediments packed in more animals, which they often did, rocks packed in more kinds of animals. Now, about that carbon fingerprint. The rocks themselves carry two flavors of carbon: inorganic carbonate, the mineral that precipitates, and organic carbon trapped in the matrix. The inorganic side averaged about minus twenty-six point ninety-eight per mil in delta thirteen C, that’s the carbon isotope ratio relative to a standard, while the organic side averaged minus thirty-three point eighty-three. Organic was consistently lighter than inorganic, which fits a microbial origin. Activity mattered here too. On active rocks, both pools shifted lighter: organic by about ten per mil, inorganic by roughly fourteen. That’s a signature of methane carbon showing up in both the mineral and the organic films that coat it. The animals echoed that complexity. On average, tissues on carbonates sat around minus thirty-one per mil for delta thirteen C and roughly five point seven per mil for delta fifteen N. But don’t get stuck on the average — the range was huge, a telltale of mixed microbial diets. One dorvilleid worm came in at an eye-popping minus one hundred point five per mil in delta thirteen C, an extreme pull toward archaeal methane-derived carbon. Others were far heavier, closer to minus sixteen, suggesting more conventional organic matter or sulfide-oxidizing bacterial pathways. When Levin’s team lined up animal delta thirteen C next to the rock’s organic delta thirteen C from the same sample, there was no significant difference on average, a hint that the rock’s carbon pool and the animals’ diets are linked. While animals on active and inactive rocks didn’t differ in delta thirteen C overall, location did — Mound 12 animals skewed lighter than those at Jaco Summit — and nitrogen isotopes were heavier at some sites than others. All of that says local context steers the menu. What did that mean for trophic structure? By conventional community-wide isotope metrics, active and inactive rocks didn’t separate cleanly. Using a relaxed threshold, there were hints that active rock assemblages had a narrower range in delta fifteen N — a tighter spread of trophic positions — but occupied a broader isotopic niche area, something ecologists call S E A c. The real signal came within rocks. Gastropods and polychaetes sharing the same carbonate often carried distinct carbon and nitrogen signatures. That’s resource partitioning in action: some species scraping bacterial films, others tapping archaeal biomass tied to methane, and still others exploiting filamentous mats. Same stone, different diets. The environmental context mattered most when the seep was quiet. In multivariate models, inactive rock communities showed a clear, moderate link to background conditions. About twenty-eight percent of the variability in who was there could be explained by depth, temperature, oxygen, and the carbonate’s isotope signatures, with a RELATE statistic around zero point four two. On active rocks, those same variables lost their grip, and nothing stood out as predictive. That’s a beautiful result in its simplicity. When the seep’s chemical engine is on, it resets the rules. When it’s off, hydrography takes the wheel. Zooming back to substrates, there’s a counterintuitive twist. Adjacent seep sediments actually held higher macrofaunal densities on average, around three hundred nine individuals per two hundred square centimeters, than the carbonates. So if you’re just counting bodies, the mud wins. But when you count kinds of bodies and ask who they are and what they’re doing, the rocks stand out. Carbonates supported substantially higher richness in many cases and a different, chemosynthesis-tuned community. They added heterogeneity to the seep landscape, not just as outcrops, but as engines that help diversify the food web. A quick note on how these patterns played out across habitats within seeps. Mussel beds and tubeworm aggregations on rocks concentrated the most animals. That’s probably not just because they’re busier neighborhoods; their physical structures trap particles, slow flow, and create microhabitats on the carbonate surface. Microbial mats and clam beds, by contrast, supported leaner rock-attached communities. Inactive rocks, divorced from live seepage, drifted toward crustaceans and cnidarians — animals less tied to scrubbing chemosynthetic films and more tied to what the overlying water column delivers. There’s also a neat feedback hidden in the carbonates themselves. The inorganic minerals and the organic films on the same rock shifted together toward lighter delta thirteen C on active sites. That suggests the microbial precipitation of carbonate and the microbial production of biomass are in sync when methane is flowing. You can feel the system coupling: methane oxidation drives carbonate growth and feeds the biofilm that feeds the grazers that feed the predators perched on the same stone. Put all of this together, and the picture that Levin and colleagues painted is crisp. Authigenic carbonates at methane seeps are biodiversity engines. They concentrate life when seepage is active, they tilt the balance toward grazers and chemosynthesis-linked worms and snails, and they host a nutritionally diverse food web where even neighbors don’t eat the same thing. When seepage wanes, those same rocks still matter, but their communities relax into patterns set by depth, temperature, and oxygen. Compared with the surrounding mud, the rocks don’t necessarily pack more bodies, but they do pack more kinds — a crucial distinction if you care about ecosystem function. That has a practical edge. Monitoring and conservation in the deep sea often think in terms of mud, corals, sponges, and vents. This work by Levin, Mendoza, Grupe, Gonzalez, Jellison, Rouse, Thurber, Waren, and their coauthors argues for adding seep carbonates to that list. They’re common along continental margins. They lock away carbon. They build their own version of a rocky shore, far below the waves, where methane-fueled microbes set the table for an entire community.