Angels and Butterflies
Pteropods are planktonic sea snails. Some are tiny, at 0.5 mm, while others are just small, 10 mm or so. The largest is Clione limacina, subspecies Antarctica, which reaches 10 cm. It’s a predator that feeds on other, smaller pteropods. It uses microscopic hooks to grab its prey.
The fully-shelled pteropod species belong to the Euthecosomes group, while the Pseudothecosmomata and Gymnosomata groups are partially shelled or have no shells. The shelled and partially shelled species are Sea Butterflies in the group Thecosomata, while the shell-less Gymnosomata are called Sea Angels. And yes, I capitalized those on porpoise (I did that on purpose, too).
Systematic and taxonomic revisions are ongoing in these snails, and the number of species is unresolved. Among the Sea Butterflies, there are, perhaps, 100 species. They appear closely related. Despite this close relation, Sea Butterflies’ shells are diverse, ranging from coiled to nearly flat to long and slender to little pyramids.
Shelled species have been the focus of acidification research due to the potential effect of changing acidity on shell formation and dissolution. Their shells are made of aragonite, a calcium carbonate crystal. Their aragonitic shells are highly sensitive to changes in ocean chemistry. Carbonate fixation becomes more difficult and energy-absorbing as pH drops. The shells of these open ocean snails are thin, very thin, and fragile. Add low-pH environments, and thin becomes thinner.
Because of these vulnerabilities, pteropods are often considered indicator species for monitoring the effects of ocean acidification. Specific results of studies have shown that lower pH levels weaken pteropod shells, making them thinner and more susceptible to dissolution. Laboratory experiments demonstrate that even a slight decrease in pH can cause visible damage to their shells over a short time.
Acidic conditions also affect pteropods' growth rates, reproductive success, and overall survival. Slower growth and fewer offspring mean smaller populations, which can have cascading effects up the food chain. The shell degradation reduces pteropods' survival rates, making them more vulnerable to predation and other environmental stressors, such as temperature changes.
Smaller Pteropods feed on plankton; some produce a mucus web that is used to trap organic molecules and tiny plankton. Larger ones feed on other pteropods. These are then fed on by a number of fish species including herring and salmon.
The future for sea snails looks bleak. The combination of stressors—warming and lower pH—is most damaging. Arctic species are valuable members of food webs, and their demise will disrupt marine food webs.
Figure 1. A Sea Butterfly, Limacina helicina. This shelled pteropod has a delicate, coiled shell for buoyancy. Its large, wing-like parapodia are used for swimming, giving it an almost fairy-like appearance as it drifts in the ocean. These snails use a mucus net to capture phytoplankton. After it fills its net, it consumes the net along with the plankton. Photo, Alexander-Semenov.
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Figure 2. Clione limacina is shell-less. It has a soft, gelatinous body that uses wing-like appendages (parapodia) to “fly” through the water. This species is a specialized predator of shelled pteropods like Limacina helicina. It uses tiny hooks to capture prey, a top predator in this small world.
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Figure 3. Hyalocylis striata, the Glass Sea Butterfly, is a detritivore, feeding on organic particles suspended in the water column. |
For the future Snailologist:
Pteropods are placed in the Euopisthobranchia, closely related to Aplysia, the sea hares, and Bubble snails, and slightly more distantly related to the nudibranchs.
Sources and Further Readings:
Lalli, C. M., & Gilmer, R. W. 1989. Pelagic snails: The biology of holoplanktonic gastropod mollusks. Gastropod Mollusks. Stanford University Press.
Van der Spoel, S. 1976. Pseudothecosomata, Gymnosomata and Heteropoda (Gastropoda). Utrecht: Bohn, Scheltema & Holkema.
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