Pteropods are small, free-swimming marine snails and slugs found across the world’s oceans. They are often referred to as “sea butterflies” or “sea angels” because of their distinctive wing-like structures. Generally less than 1 centimeter long, their presence in marine ecosystems is globally significant. These tiny organisms are a type of plankton, drifting with ocean currents in the upper layers of the open ocean, typically within 0 to 200 meters of the surface.
Distinctive Features and Locomotion
Pteropods have a modified foot that has evolved into wing-like flaps, known as parapodia, which they use for swimming. This adaptation allows them to move freely through the water column, rather than crawling on the seafloor like many other snails. The motion of their wings can involve a “clap-and-fling” mechanism, where the wings touch and then peel apart, creating propulsion.
There are two main groups of pteropods: thecosomes and gymnosomes. Thecosomes, often called “sea butterflies,” have a thin, transparent, calcareous shell, which can be coiled or triangular. In contrast, gymnosomes, known as “sea angels,” lack a shell and generally have more robust bodies with smaller, lateral wing-like lobes. Both types are small, ranging from a few millimeters to 1 or 2 centimeters in length, and have gelatinous bodies.
Their Role in Ocean Food Webs
Pteropods play an important role within marine food webs, linking primary producers to higher trophic levels. Shelled pteropods primarily filter-feed on microscopic phytoplankton using cilia and a sticky mucous net to trap particles. This method allows them to efficiently collect marine snow, which includes organic debris that constantly sinks through the water column.
These small snails serve as a food source for a diverse array of marine animals. They are consumed by small fish like pink salmon, herring, mackerel, and tuna, as well as larger predators such as squids, krill, seabirds, and whales. Their high fat content makes them a sought-after food source, sometimes referred to as “the potato chips of the sea.” Some sea angel species are specialized predators, feeding exclusively on shelled pteropods. Pteropods also contribute to the biological carbon pump by consuming organic material and excreting it in fecal pellets, which helps sequester carbon in the deep ocean.
Facing Environmental Threats
Pteropods, particularly shelled species, are susceptible to ocean acidification, a direct consequence of increased atmospheric carbon dioxide (CO2) absorbed by the oceans. As the ocean absorbs excess CO2, its pH decreases, reducing available carbonate ions. Carbonate ions are the building blocks that marine organisms, including pteropods, use to form their shells and skeletons.
Pteropod shells are made of aragonite, a form of calcium carbonate, making them vulnerable to corrosive waters. This makes it difficult for them to form and maintain their shells, and in severe cases, their shells can dissolve. Research shows that juvenile pteropods experience extensive shell dissolution when exposed to corrosive seawater. The percentage of pteropods with dissolving shells has doubled in some habitats since the pre-industrial era, projected to triple by 2050. A decline in pteropod populations could have cascading effects throughout the marine food web, disrupting predator-prey dynamics and potentially impacting commercially important fisheries that rely on them as a food source.