Pteropods, often called ‘sea butterflies,’ are small, free-swimming marine mollusks that inhabit the open ocean. These gastropods have adapted their foot into wing-like flaps, known as parapodia, which they use to propel themselves through the water column. Found across all major oceans, pteropods are particularly abundant in polar and subpolar regions, where they can form dense swarms near the surface. Most species are less than one centimeter long, making them a significant component of the zooplankton community and the marine food web.
Defining the Two Major Pteropod Groups
The Pteropoda order is divided into two groups, and their physical differences directly influence their feeding habits. The Thecosomata, commonly known as “sea butterflies,” are the shelled pteropods. They possess a delicate, transparent shell, which requires them to adopt a largely passive feeding strategy.
The other group is the Gymnosomata, or “sea angels,” which are entirely shell-less in their adult form. Gymnosomes are more robust and streamlined than their shelled relatives, and their parapodia are relatively smaller. This lack of a shell allows them to be active predators with specialized feeding structures. The two groups share their small size and pelagic lifestyle, but their internal structures and behavior diverge.
Primary Food Sources of Pteropods
The food sources for pteropods vary between the two groups, reflecting their different lifestyles. The shelled Thecosomata are largely planktonic grazers and filter feeders, consuming microscopic organisms suspended in the water. Their diet consists mainly of phytoplankton, which are single-celled photosynthetic organisms. These plants form the base of the marine food web and provide the bulk of the Thecosomes’ nutrition.
Thecosomes also feed on small particulate matter known as marine snow. They are considered omnivorous grazers because they also consume very small zooplankton, including bacteria and protists. In contrast, the Gymnosomata have a specialized diet, feeding almost exclusively on their shelled relatives, the Thecosomata.
Feeding Mechanisms and Food Acquisition
Thecosomata capture their microscopic food through a passive filter-feeding mechanism involving a large, sticky mucus web. The pteropod secretes this net from its foot and deploys it into the water, where it can be many times the size of the animal itself. The pteropod remains motionless, allowing passing phytoplankton and detritus to stick to the web. When the net is full, the pteropod gathers the entire mucus structure into a ball and ingests it through its mouth.
The naked Gymnosomata use an active method of food acquisition, functioning as raptorial predators. They actively hunt their prey, the shelled Thecosomata, often swimming faster than their target during a chase. To capture and consume the prey, Gymnosomes use specialized head structures, such as a protrusible proboscis and buccal cones, which can sometimes be equipped with suckers. They use these appendages to grasp the shell and then employ hooks and a toothed radula to pull the sea butterfly from its shell.
Ecological Importance in Marine Food Webs
Pteropods serve as a link between microscopic primary producers and higher-level consumers in the ocean ecosystem. Their high abundance, particularly in polar waters, makes them a dense and nutritious food source.
A wide range of marine animals rely on pteropods as a component of their diet, transferring energy up the water column. Predators include:
- Various fish, such as herring, salmon, and mackerel.
- Larger zooplankton and cephalopods.
- Baleen whales.
- Several species of seabirds.
Pteropods can make up a substantial portion of the diet for juvenile salmon, underscoring their influence on commercially important fisheries.