Wet Beast Wednesday: common periwinkle
Welcome back to Wet Beast Wednesday. I didn't post one of these last week because I was on vacation in New England, so I decided I'd showcase an animal from that area. I ended up picking the most common one I saw, seriously, if you go anywhere with a rocky shore out there you will see more periwinkles than sand. It's also a species I have some scientific experience with, which I can tell you about in the post because it's kind of funny.
(Image: a common periwinkle climbing on what looks like a piece of sponge. It is a small snail with a round, brown shell. The foot is brown with dark lines. The tentacles are long and skinny, with brown and yellow stripes. End ID)
The common periwinkle (Littorina littorea) is a small sea snail, with a shell that is 16 to 38 mm long (0.6 to 1.5 in) and 10 to 12 mm (0.4 to 0.5 in) wide at the thickest point. The shell is conical and tapers to a point with multiple whorls. Most have dark brown shells, but there is a very wide variation is color, with some being bright colors like orange. The majority of the body, including the digestive organs, is located within the shell. The parts of the body that extend outside of the shell are the foot, mouthparts, and tentacles. The foot is a large, muscular structure that produces mucus and is used to move around. It can also produce suction to keep the snail from being pulled off of rocks. The mouthparts consist of a radula, a tongue covered in teeth that is used to scrape food off of rocks. The tentacles are sensory organs covered in chemoreceptors that allows the snail to detect chemicals in the water around it. At the bases of the tentacles are simple eyes. The body also has an operculum, a hard structure that fits into the opening of the shell. When the snail retracts its body into the shell, the operculum acts like a door and closes the hole.
(Image: the opening of a periwinkle's shell with the boy partially extended. The tentacles are out and the foot is folded up. The operculum is visible as a flat disc on one side of the foot. End ID).
Common periwinkles are found in the northern Atlantic Ocean. They are native to Europe, found from Spain to Russia. They have also been introduced to the east and west coasts of North America. In the east coast, they are found from New Jersey to Nova Scotia and on the west coast they have been documented in California. They are found on rocky shores, mud flats, and estuaries on and near the shore. Periwinkles are omnivores who mainly feed on algae. They target both small algae growing on rocks or sediment and larger seaweeds. While grazing, they also consume biofilms, decaying material, and small organisms like invertebrate larvae and eggs. Periwinkles are also prey to a great variety of other organisms, including birds, fish, crabs, starfish, and mammals. Because they live on the coast, periwinkles must adapt to life in the intertidal zone. The intertidal zone is an extremely harsh habitat with widely changing conditions. Periwinkles have gills and can't breathe air and need to avoid drying out, but may be exposed to air when the tide goes out. To survive, they will suction themselves to the rocks and/or close their operculums and produce mucus, all to hold in water. They can also tolerate a wide range of temperature, oxygen, and salinity levels, letting them survive in tidepools while waiting for the tide to rise again. Periwinkles are ecosystem engineers as their grazing gradually erodes rocks.
(Image: a cluster of common periwinkles sitting in a crevice on a rock. The rock is also covered in barnacles and limpets. End ID).
Common periwinkles are unlike many snail species in that they are dioecious. This means that individuals are either male or female. Many other snail species are simultaneous hermaphrodites, meaning they produce both male and female gametes at the same time. Mating can take place year-round depending on the local climate, but seems to peak between February and June. Males use chemical cues in the female's slime trails to find mates. They will grab onto each other with their feet to copulate. The female will then lay eggs in hard capsules that they hide amongst cracks in rocks or kelp. Each capsule can contain up to 100,000 eggs. The eggs hatch into larvae called veliger that drift in the current, feeding on plankton. After about a week, they will sink to the bottom and begin their adult lifestyle. Periwinkles reach sexual maturity at a year old and can grow up to 14 mm (0.5 in) in that time, with females growing faster. The majority of periwinkles will not survive to their first birthday, but those who do have a better survival rate. Common periwinkles can live five to ten years in the wild and up to 20 years in captivity.
(Image: a microscope image of a common periwinkle veliger. It looks like a tiny translucent snail, but instead of a foot it has two wing-like flaps with cilia on the edges. End ID)
Common periwinkles have not been assessed by the IUCN as far as I can tell, but given that they are highly abundant in their home habitat and introduced habitats, they would likely count as least concern. There is debate over whether or not common periwinkles could be considered an invasive species. The arguments in favor state they outcompete local snails, damage biofilms, and increase erosion on rocky beaches. Arguments against say that the snails are sufficiently naturalized and have enough predators to keep them in check. Periwinkles are edible and eaten mainly in Europe, where they are usually boiled and then scraped out of their shells with a pin. There is not much of an an aquaculture industry for periwinkles, they are mostly harvested right out of the wild. Different species of periwinkles are also eaten in Africa and Asia.
(Image: a grouo of four common periwinkles on the beach. Their bodies are fully retracted and the operculums are closed, sealing the water within. End ID).
Now here's my story with periwinkles. Back when I was doing my master's thesis, the first phase of the project involved collecting and breeding common periwinkles, which led to several stories including me having to explain to a bus driver why I was bringing a bucket of snails on the bus, bringing the snail bucket to lunch after collecting them (there was a lid in both cases) and having to regularly trek down to shore to get fresh seaweed for them to eat. I also needed a way to mark the females and males (btw, sexing a periwinkle is hard) to tell them apart. I ended up buying some pink waterproof nail polish and using it to mark the female's shells. I later released the snails and I like to think I confused some people who were wondering why the snails were pink. And after all that, I pivoted to brine shrimp and didn't use the snail data in the final thesis.
(Image: a common periwinkle climbing on an algae-covered rock. End ID)