Zostera bed. Llŷn Peninsula, Wales. September 2009. Before 1934, R. membranacea was widespread and common in north-west Europe, primarily on the strap-like leaves of the marine vascular plant, Zostera marina. From 1934 and through the 1940s, disease caused the destruction of most European Zostera beds, and a consequent drastic reduction, or local extinction, of R. membranacea populations at most sites. Many Zostera beds have now re-established themselves, such as the one illustrated above. But, despite intertidal searches by experienced members of the Conchological Society of G. B. & Ireland, no Rissoa membranacea have been found here in recent years, apart from a few dead shells that probably originate from a previous population. It seems that the species has been unable to re-establish itself here, despite the recovery of the Zostera marina bed from disease damage. This situation may be general in North Wales as many locations are shown there on NBN maps for Zostera marina, but all NBN R. membranacea records in North Wales are of dead shells, except for one site.
30 Rissoa membranacea
