Pteropods are small swimming snails that live their entire lives in the open ocean, and they play an outsized role in marine food webs and ocean chemistry relative to their tiny size. Often called “sea butterflies” or “sea angels” depending on whether they have shells, these creatures are found in oceans worldwide, from tropical waters to polar seas. Their importance has sharpened in recent decades because their delicate shells dissolve in acidifying seawater, making them one of the most visible early casualties of rising carbon dioxide levels in the ocean.
Snails That Fly Through Water
Pteropods are molluscs, related to the slugs and snails you find in a garden, but they have adapted to a fully pelagic life, meaning they never touch the seafloor. The name “pteropod” comes from Greek roots meaning “wing-foot,” and it is an apt description. Where a garden snail has a broad, muscular foot for crawling, a pteropod has evolved that foot into a pair of thin, wing-like appendages that it flaps to propel itself through the water column.
That flapping motion is more sophisticated than it looks. Research on the Antarctic species Clione antarctica found that each half-stroke of their wing beat has distinct power and recovery phases of roughly equal duration, generating force through rotational circulation and other hydrodynamic mechanisms similar to those used by flying insects. The comparison is not casual: biomechanics researchers study pteropod locomotion precisely because it sits at the intersection of swimming and flying, operating at sizes and speeds where water behaves very differently from what larger marine animals experience.1PubMed Central. Aquatic wing flapping at low Reynolds numbers: swimming kinematics of the Antarctic pteropod, Clione antarctica
Two Very Different Body Plans
Pteropods split into two dramatically different-looking groups. The shelled pteropods, historically grouped as “Thecosomata,” include the animals commonly called sea butterflies. They carry translucent shells that can be coiled like a tiny snail’s or shaped more like a cone or a vase. The shell-less group, called Gymnosomata or sea angels, have lost their shells entirely and are instead streamlined, often translucent predators that hunt the shelled species.
Recent phylogenetic work has complicated the old classification. The most extensive genetic study of pteropods to date, using mitochondrial genomes and sampling from nearly 100 of the roughly 163 known species, found that the traditional shelled group “Thecosomata” does not hold up as a single natural lineage. The study supported suppressing that name, demonstrating that two subgroups previously lumped together, the Pseudothecosomata and Euthecosomata, are not closely related to each other despite both having shells. The relationships between these major pteropod groups remain an active area of research.2PubMed Central. Between sea angels and butterflies: A global phylogeny of pelagic pteropod molluscs
For the non-specialist, the practical takeaway is that “pteropod” covers a wider evolutionary spread than most people realize. Sea butterflies and sea angels look nothing alike and live very different ecological lives, yet they share a common ancestor that took to the open ocean and never looked back.
Shells Built for Life in the Open Sea
The shells of sea butterflies are remarkable pieces of biological engineering. They are made of aragonite, a form of calcium carbonate that is less thermodynamically stable than the calcite used by many other marine shell-builders. Building a shell out of aragonite in the open ocean is a bit like choosing glass over plastic for a product that gets tossed around: it can be strong and beautiful, but it is inherently more vulnerable to the wrong conditions.
What makes pteropod shells unusual is their internal architecture. Beyond the crossed-lamellar and prismatic microstructures found in many mollusc shells, pteropods also build with curved nanofibers that form a helical three-dimensional structure. This design appears to have evolved before the evolutionary split between coiled and uncoiled shell forms, suggesting it was a key innovation that allowed these animals to thrive as plankton. The curved fibers give the shell maximum strength and flexibility while keeping it thin and lightweight, exactly what a tiny animal needs when it must stay buoyant in the water column.3PubMed. Evolution and biomineralization of pteropod shells
Why Pteropods Matter Ecologically
Pteropods punch well above their weight in ocean ecosystems. Shelled pteropods are a critical food source for a wide range of animals, from commercially important fish like salmon and cod to seabirds and whales. In polar and subpolar waters especially, pteropods can form dense swarms that represent a concentrated packet of calories for predators. Remove them from the food web and you create a gap that is difficult for other organisms to fill, because few other planktonic animals occupy quite the same niche at the same abundance.
Their role in ocean chemistry is equally significant. When shelled pteropods die, their aragonite shells sink, carrying carbon from the surface ocean to the deep. This “biological pump” helps regulate how much carbon dioxide the ocean absorbs from the atmosphere. Pteropods also produce fecal pellets that sink relatively quickly, further shuttling organic carbon downward. In some regions, pteropod shells make up a substantial fraction of the carbonate sediment on the ocean floor.
Ocean Acidification and Dissolving Shells
The reason pteropods have become poster animals for ocean acidification is straightforward: as the ocean absorbs more COâ‚‚ from the atmosphere, seawater becomes less saturated with the carbonate ions that these animals need to build and maintain their shells. When the water becomes undersaturated with respect to aragonite, pteropod shells begin to dissolve, sometimes while the animals are still alive.
This is not a theoretical concern. A survey along the Washington-Oregon-California coast in 2011 found that large portions of continental shelf waters were already corrosive to pteropods. On average, about 53% of pteropods collected near shore and 24% of those collected farther offshore showed severe shell dissolution damage. The researchers found a strong positive correlation between the proportion of individuals with severe damage and the percentage of undersaturated water in the top 100 meters of the ocean.4PubMed Central. Limacina helicina shell dissolution as an indicator of declining habitat suitability owing to ocean acidification in the California Current Ecosystem
Separate research in the same California Current system found that pteropods in upwelling regions, where deeper, more acidic water rises to the surface, build thinner shells compared to those living offshore. Dissolution marks were concentrated on the oldest parts of the shell, the inner whorls that had been exposed to seawater the longest.5PubMed Central. Pteropods make thinner shells in the upwelling region of the California Current Ecosystem The picture these studies paint is that acidification is already reshaping pteropod biology in real time, not just in laboratory projections.
Warming Makes Everything Worse
Ocean acidification alone is bad enough, but pteropods rarely face just one stressor at a time. Ocean warming compounds the problem. Experiments on Southern Ocean pteropod larvae exposed to combined warming and acidification found larval mortality as high as 39%, along with extensive shell malformation, dissolution, and reduced shell growth. The researchers concluded that even short-term exposure to hotspots of warming and acidification poses a serious threat to pteropod recruitment and long-term population survival.6PubMed Central. Southern Ocean pteropods at risk from ocean warming and acidification
Work on the Antarctic species Limacina helicina antarctica helped disentangle which stressor does what. Temperature turned out to be the primary driver of mortality: pteropods held at ambient temperatures survived well even under low pH conditions, but adding heat on top of acidification caused survival to drop sharply.7PubMed Central. Combined stress of ocean acidification and warming influence survival and drives differential gene expression patterns in the Antarctic pteropod, Limacina helicina antarctica In other words, acidification damages their shells, but warming may be what actually kills them.
Physiological studies have also shown that pteropods under high COâ‚‚ conditions mount increased antioxidant defenses, a sign of cellular stress. Compared to copepods, small crustacean zooplankton that share their habitat, pteropods appear more vulnerable to oxidative damage under acidified conditions and seem to have a lower adaptive ceiling for coping with combined stressors.8PubMed Central. Eco-physiological responses of copepods and pteropods to ocean warming and acidification
They Can Fight Back, but at a Cost
Pteropods are not entirely passive victims. Research has shown that they can repair their shells after dissolution damage, patching eroded areas with new layers of aragonite. This capacity for shell repair demonstrates a resilience to acidification that initial studies may have underestimated. But the repair comes at a metabolic cost: energy spent rebuilding shell material is energy not spent on growth, reproduction, or escaping predators.9PubMed Central. Pteropods counter mechanical damage and dissolution through extensive shell repair
Gene expression studies add nuance to the resilience picture. Antarctic pteropods showed significant transcriptomic plasticity when exposed to changing temperatures, meaning they could ramp up or down a suite of genes in response to thermal stress.7PubMed Central. Combined stress of ocean acidification and warming influence survival and drives differential gene expression patterns in the Antarctic pteropod, Limacina helicina antarctica Whether this flexibility is enough to keep pace with the rate of environmental change projected for the coming decades is an open and pressing question. Having a compensatory mechanism and having a sufficient one are different things.
Survivors of Past Catastrophes
One reason to be cautious about writing off pteropods is their deep evolutionary track record. Fossil and molecular evidence shows that all major pteropod lineages, including both shelled and shell-less forms, were already present during the Cretaceous period. That means they survived the asteroid impact that ended the age of dinosaurs about 66 million years ago, and they made it through the Paleocene-Eocene Thermal Maximum roughly 56 million years ago, an event that involved massive carbon release and ocean acidification broadly analogous to what is happening today.10PubMed Central. From the Cover: The origin and diversification of pteropods precede past perturbations in the Earth’s carbon cycle
This survival record is both encouraging and tricky to interpret. Past acidification events unfolded over thousands of years, giving populations time to adapt, shift ranges, or evolve new shell-building strategies. The current rate of ocean pH change is much faster. Still, the fact that pteropods have weathered extreme carbon-cycle disruptions before suggests the group has evolutionary tools that are not fully understood yet.
Canaries in the Ocean’s Coal Mine
Scientists increasingly use pteropods as biological indicators of ocean acidification, and for good reason. Their thin aragonite shells respond visibly and measurably to changing water chemistry: decreased shell size, reduced thickness, and increased surface dissolution are all quantifiable signs that conditions are deteriorating.11Frontiers in Marine Science. Assessing pteropod shell dissolution to advance ocean monitoring techniques: a methods comparison of SEM, CT, and light microscopy Because pteropods are found in essentially all of the world’s oceans, they offer a globally applicable metric rather than a regional one.
An indicator screening assessment found that pteropod shell dissolution scored highly compared to other candidate indicators of marine ecological integrity. The analysis argued that pteropods are sensitive enough to detect early changes, specific enough that the signal is clearly tied to acidification rather than some other stressor, and quantifiable with relatively low-cost methods. As the socioeconomic consequences of changing ocean chemistry grow, having straightforward biological metrics becomes increasingly valuable for water quality assessment and ecosystem-based management.12Ocean Acidification International Coordination Center. New ocean, new needs: Application of pteropod shell dissolution as a biological indicator for marine resource management
Multiple techniques are now used to assess shell condition, from scanning electron microscopy to micro-CT imaging, with ongoing work comparing these methods for accuracy and practicality in routine monitoring programs.11Frontiers in Marine Science. Assessing pteropod shell dissolution to advance ocean monitoring techniques: a methods comparison of SEM, CT, and light microscopy The goal is to make pteropod monitoring reliable enough to inform policy decisions, not just academic publications.
Where They Live and How They Behave
One assumption about pteropods that has recently been challenged is the idea that they are strong vertical migrators, rising toward the surface at night to feed and descending during the day to avoid predators. This behavior, called diel vertical migration, is common among many zooplankton groups. But detailed depth-resolved sampling of Limacina helicina in the Northeast Pacific found no statistical difference in their vertical distribution between day and night, with typical mean depths sitting around 50 to 70 meters regardless of the time of day.13ICES Journal of Marine Science. Depth-resolved vertical distribution of the pteropod Limacina helicina in the Northeast Pacific and its implications for exposure to ocean acidification
This matters for acidification research because many exposure risk models assume pteropods migrate through different water layers, encountering varying pH levels throughout the day. If they actually stay at a relatively consistent depth, their cumulative exposure to corrosive water could be quite different from what models predict. Getting their real-world behavior right is essential for accurately forecasting how populations will respond as ocean chemistry continues to shift.
What the Loss of Pteropods Would Mean
If pteropod populations decline significantly, the consequences ripple outward. Predators that depend on them would need to find alternative prey, which is not always possible, especially in polar ecosystems where food web options are limited. The carbon export they provide, moving carbon from surface waters to the deep sea through sinking shells and fecal material, would diminish, potentially altering the ocean’s capacity to buffer atmospheric COâ‚‚. And the loss of a sensitive, globally distributed bioindicator would leave scientists with fewer tools for tracking the biological impacts of acidification in real time.
The science on pteropods is still evolving, with new findings regularly updating what we know about their genetics, physiology, and behavior. What is already clear is that these tiny, translucent animals occupy a position in the ocean that is far more consequential than their size would suggest. They are simultaneously participants in and witnesses to one of the largest chemical experiments ever conducted on a planetary ecosystem.