Caution; Littorina obtusata and L. fabalis vary with habitat. These are from a sheltered shore. Do not use this image to separate the two Littorina spp. from each other on exposed shores; in some localities dissection is needed to distinguish them.
To gain experience of this pair of species it is suggested that you:
1. Select a rocky shore that is sheltered from strong wave action – The presence of Ascophyllum nodosum (Knotted wrack) indicates shelter.
2. Visit at low water spring tide. (Spring tides occur twice every month – nothing to do with the season spring.)
3. Go to the middle of the zone covered by Ascophyllum and collect a dozen flat spired periwinkles. You may need to move the weed to find them if sunny / dry. Label the box or put a bit of Ascophyllum in. The snails keep best if only enough sea water is added to dampen, not submerge them.
4. Repeat stage 3 at the middle of the zone covered by Fucus serratus (Saw wrack). This zone is nearer low water.
5. Examine and compare the contents of the boxes with each other and the images above. Apart from the odd straggler that has been swept into the wrong zone, box 3 should have the larger L. obtusata; various colours, including shades of olive. Adults will lack the flared lip of juveniles; see Littorina obtusata. Menai Strait. February 2011.
Box 4 should have the smaller L. fabalis. Mainly yellow with the occasional orange one.
PLEASE REMEMBER, the above only applies to sheltered shores.
If you want to take them home for study, they must be kept cool. A refrigerator (about 8 deg C.) is the best place. Summer heat in a car will kill them.
L. mariae = L. fabalis
You may find the flat spired Lacuna pallidula and Lacuna parva , but these are easily distinguished from flat periwinkles by their long groove and umbilicus (hole) along the inner lip of the aperture. Detailed accounts at flic.kr/p/hTnxJa and