Who are they?

The periwinkle is a marine gastropod that lives in the intertidal zone of temperate marine ecosystems. This zone corresponds to the part of the beach between high tide and low tide. It therefore usually lives close to the water’s surface, although it has, on very rare occasions, been observed at depths of up to 60 metres! Native to the north-eastern Atlantic coast, it is found along all the Atlantic coasts of the northern hemisphere. It is thought to have been introduced to the North American coast in the mid-19th century, although this hypothesis is still debated today 1,2. It thrives quite comfortably in areas where predation pressure remains low 2.

It plays an important role in the ecosystem. It is a herbivorous animal that feeds on green and brown algae as well as diatoms 3, which it grazes on using its radula (its mouth and teeth) as it moves along the seabed. In this way, it regulates the proliferation of these algae 4 and maintains a balance in their growth 5. It also helps to feed smaller organisms by breaking these algae down into smaller pieces (whilst contributing to the carbon cycle) 5.

The periwinkle is often dark in colour. Like many other gastropods, it has a muscular foot and most of its internal organs are housed within its shell. As an adult, it measures around 2.5 cm. Its main predators are crabs and starfish. It should not be confused with another gastropod: the turbinate monodont , Phorcus turbinatus. 

A clear vulnerability

Periwinkles are vulnerable to climate change and pollution, but also to parasites.

Firstly, their shells are made of calcium carbonate (CaCO₃), which is highly sensitive to acidification; this makes periwinkles, like many other marine organisms, vulnerable to Ocean acidification 8. Indeed, an acidic environment causes the calcium carbonate to dissolve.

It has been shown that periwinkles produce shells of varying thickness depending on the predation pressure to which they are exposed 9,10. For example, periwinkles living in the presence of crabs tend to produce significantly thicker shells (and larger ones 11) than those of periwinkles that do not encounter crabs 9. However, periwinkles exposed to lower pH levels (= more acidic water) produce a thinner shell, making them more vulnerable to predators 9. Ocean acidification therefore has a direct effect on periwinkles (and other marine organisms) but also an indirect effect via changes in metabolism and an increase in predation pressure 9.

Changes in temperature also have a considerable impact on periwinkle populations 12. Indeed, their tolerance to high temperatures varies depending on the rate at which the temperature rises and the season in which they were born, but they appear to suffer in all cases when temperatures exceed 28 °C 13. Embryonic development is possible between 5 °C and 23 °C, but below and above these temperatures, periwinkle populations do not develop properly 7. Temperature affects not only embryonic development but also its duration 7. Adults, too, suffer from changes to their environment caused by climate change. Rising CO₂ concentrations and temperatures have direct implications for their physiological processes. An increase in CO₂ concentration alone causes a 31 % reduction in the respiratory rate of periwinkles and, more generally, in their metabolism. When combined with temperature, effects are observed on growth, population dynamics and, ultimately, all ecosystem processes 14. Recent studies show that periwinkles have low resistance to extreme temperatures. They cease to cling to the substrate and enter a ‘heat coma’ from 31°C onwards (with the temperature increased by 1 °C every 5 minutes) 12. 50 % of individuals die after 330 minutes (5.5 hours) of exposure to 39 °C, 90 to 120 minutes at 40 °C, or 60 minutes at 41 °C 13. The indirect consequences of temperature changes, arising from alterations to the ecosystem (plants, predators, etc.) in which young periwinkles develop, must also be taken into account and constitute a significant impact.

As periwinkles live in the intertidal zone, they are significantly exposed to chemical and plastic pollution in their habitat, as well as to heavy metals; indeed, they are organisms that are sensitive to pollution in their ecosystem 15–17. High concentrations of silver have been found in their gills, kidneys and stomachs following exposure to water containing dissolved silver 16. Even at low concentrations, a large proportion (80%) of individuals are affected even more than 24 hours after exposure. Furthermore, the greater the exposure and the higher the concentrations, the less the periwinkles feed 15. Studies on plastics and microplastics show that 100 per cent of the periwinkles studied had ingested microplastics and had them present in their bodies 17. These plastics originate from human activities. They have been released into the environment (as waste or when synthetic fabrics and clothing are washed) before being carried to the ocean by rainfall and drainage systems. 

Finally, periwinkles are regularly infected by parasites, particularly trematodes (worms). These parasites significantly reduce the periwinkles’ appetite, causing them to graze less on seaweed on the seabed, thereby altering the natural balance between seaweed and periwinkle populations 18.

They are now recognised as a species that is important to understand within the context of evolutionary ecology 19.