The strong current in the Mersey narrows keeps small areas of hard substrate free of the mud that is deposited from the turbid water in most of the estuary. But, except near high water mark, no fucoid algae grow as there is insufficient light penetration for photosynthesis. Consequently, Littorina obtusata and Littorina fabilis do not live in the estuary.
Mersey Estuary, Liverpool seen from the Wirral, Cheshire. October 2010.
Full account at: 02 Differentiation L. obtusata/L. fabalis

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