Since 1958 John van Aartsen had a keen interest in molluscs of the Mediterranean, and particularly the differences in malacofauna of the western and eastern parts of that basin. He focussed on Rissoidae, Pyramidellidae and bivalves; published 139 malacological papers and described 105, par­ticularly minute-sized, species. Together with his wife Joz­ina, many trips were devoted to the collection of samples potentially containing small shell species. His collection with an odd 33,000 samples has been included in that of Naturalis Biodiversity Center (Leiden).