THE ANIMAL FILES

Common periwinkle

Littorina littorea

A snail on a rock may be grazing a film you can barely see.

Common periwinkles are intertidal snails found on rocky shores and in some estuarine habitats. They scrape food with a radula, a toothed feeding ribbon. Their young begin in the water column before settling, linking a crawling adult life to a drifting start.[1]

Follow the evidence ↓
Common periwinkle
Image: Photographed by Guttorm Flatabø (user:dittaeva). · Commons · CC BY-SA 3.0
Food tool
A toothed radula[1]
Main food
Algae and surface films[1]
Early life
Planktonic larvae before settlement[1]

Scraping the surface

The radula scrapes algae and detritus from rock and other surfaces. In salt marshes, food can be taken from cord grass or biofilms on mud. Small invertebrates may also be eaten. “Algae grazer” describes the main emphasis without turning the species into an animal that can consume only one kind of plant-like food.[1]

Living between tides

Rocky upper and middle intertidal zones are common habitat, with small pools and some muddy estuaries also used. Conditions change as tides expose or cover the shore. Individuals can withdraw into the shell in response to harsh conditions. The shell’s pointed shape and variable grey-brown pattern are useful features, but shell appearance alone does not resolve every identification.[1]

A drifting beginning

Egg capsules are released into the sea and produce plankton-feeding larvae. After a period in the water column, larvae settle to the bottom. Local climate affects reproduction timing, and published observations differ in settlement dates. The dispersing stage can therefore move differently from the adult crawling along a rock.[1]

A COMMON MIX-UP

Does every stage crawl along the shore?

No. Larvae spend an early period in the plankton before settling.[1]

TRY A LITTLE REASONING

How could this snail feed on a rock with no visible seaweed frond?

Reveal the explanation

Its radula can scrape thin algae and biofilm from the surface; food need not form a large visible frond.[1]

Sources & editorial notes

Adapted and translated from the cited Wikipedia revisions, with source-linked reasoning. Image credit and licence are listed separately above.

Adapted and translated from Wikipedia contributors; exact revisions are linked below. CC BY-SA 4.0

  1. Wikipedia contributors — Littorina littorea (adapted and translated; exact revision)
  2. NCBI Taxonomy — Littorina littorea (identity and game lineage)

Last fact check:

One discovery leads to another

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