Pteropods are tiny marine mollusks, typically less than a centimeter long, that inhabit the upper layers of all major oceans worldwide. These free-swimming gastropods are a form of zooplankton, meaning they drift through the water column, often carried along by ocean currents. Their name, which comes from the Greek for “wing-foot,” refers to a modified body part that allows them to swim. The shelled types are called “sea butterflies,” and the unshelled types are known as “sea angels.” They are a widespread and diverse group, representing substantial biomass in many ocean ecosystems.
Classification of Pteropods
The division of pteropods into two main groups, Thecosomata and Gymnosomata, is based on their physical form. Thecosomata, known as “sea butterflies,” are distinguished by thin, coiled or straight shells made of calcium carbonate. They are filter-feeders, using a specialized mucous web to capture phytoplankton and other suspended particles. Thecosomata are considered herbivores and omnivores, occupying a lower rung of the food web.
Gymnosomata, the “sea angels,” are entirely shell-less in their adult stage and exhibit a streamlined, torpedo-shaped body. These unshelled pteropods are specialized predators, with many species feeding exclusively on their shelled relatives. They employ sets of curved hooks or tentacles to grasp prey and pull it out of its shell.
Locomotion and Habitat
Pteropods are unique among mollusks because their muscular foot has evolved into a pair of wing-like appendages called parapodia. These parapodia beat rhythmically, allowing the animals to “fly” through the water in a graceful, flapping motion. This adaptation permits them to be holoplanktonic, spending their entire lives swimming or drifting in the open ocean.
They primarily inhabit the pelagic zone, often concentrating in the top 10 to 200 meters of the ocean surface where food is abundant. Many species exhibit diel vertical migration, moving closer to the surface at night to feed and then retreating to deeper waters during the day to avoid visual predators.
Foundational Role in Marine Food Webs
Pteropods act as a key intermediary link between the microscopic base and larger marine life. The shelled species consume vast quantities of phytoplankton, efficiently converting this primary production into a food source for higher trophic levels. This transfer of energy is significant because pteropods often reach high population densities, forming a substantial part of the zooplankton community’s total biomass.
They are a primary food source for a diverse array of commercially important fish, including juvenile salmon, cod, mackerel, herring, and tuna. In Arctic and sub-Arctic waters, pteropods can constitute up to 60% of a pink salmon’s diet. They are also consumed by various seabirds, squid, and baleen whales, which filter-feed on swarms of the mollusks.
Shell Vulnerability to Ocean Acidification
The delicate shells of Thecosomata are particularly susceptible to the effects of ocean acidification, a direct consequence of the ocean absorbing excess atmospheric carbon dioxide ($\text{CO}_2$). As human activities release $\text{CO}_2$ into the atmosphere, a significant portion dissolves into the ocean, where it reacts with seawater to form carbonic acid. This reaction increases the concentration of hydrogen ions, which lowers the water’s pH and makes the ocean more acidic.
This change in chemistry reduces the availability of carbonate ions, the building blocks that shelled marine organisms, like pteropods, need to construct and maintain their shells. Pteropod shells are made of aragonite, a form of calcium carbonate that is more soluble and therefore more vulnerable to dissolution in corrosive water than other shell types. When the water becomes undersaturated with aragonite, it becomes chemically unfavorable for pteropods to grow their shells, and existing shells can begin to dissolve.
Researchers have documented severe shell damage in pteropods, such as Limacina helicina, in the Southern Ocean and off the US West Coast, where naturally low $\text{pH}$ conditions are intensified by increasing acidity. Laboratory experiments have shown that pteropod shells placed in seawater conditions projected for the year 2100 slowly dissolve over a matter of weeks. This dissolution impairs their ability to swim and survive, affecting the many species that depend on them for food.