Molluscs are a very important element in marine ecosys­tems. They are a useful indicator for the quality of the natu­ral ecosystems in marine areas. We thoroughly analysed the status of all extant Dutch marine mollusc species known to live in the Netherlands marine waters from 1900–2013. For the statistical analyses, data from all available sources (pro­fessionals, volunteers, and museums) from these years were brought together in a database which was the basis for an ecological mollusc atlas of the Netherlands published in 2013.

Of the 255 species found alive in the Netherlands from 1900–2013, statistical trends could be determined for 202 species, comparing the years 1900–1985 with 1985–2010. Of the 202 species, 21 were not recorded since 1985, 51 have decreased, and 46 show a stable or unclear trend. Thirty-five species expanded their range and/or their pop­ulation size and 49 species were newly recorded from the Netherlands since 1985. The most important factors that have contributed to these enormous changes of the Dutch marine mollusc fauna are fisheries, habitat changes caused by water management constructions such as the Delta Works, the introduction of exotic species, seawater pollu­tion, and climate change.

In a future publication, we hope to describe the changes in the Dutch mollusc fauna during the last 10 years.