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Ocean ecosystems have been subjected to anthropogenic influences for centuries, but the scale of past ecosystem changes is often unknown. For centuries, the European flat oyster (Ostrea edulis), an ecosystem engineer providing biogenic reef habitats, was a culturally and economically significant source of food and trade. These reef habitats are now functionally extinct, and almost no memory of where this ecosystem once existed, at what scales, or its past form and functioning, remains. The described datasets present qualitative and quantitative extracts from written records published between 1524 and 2022, which show: (1) locations of past oyster fisheries and/or oyster reef habitat across its biogeographical range, with associated levels of confidence; (2) extent of past oyster reef habitats, and; (3) species associated with these habitats. These datasets will be of use to inform accelerating restoration activities, to establish reference models for anchoring adaptive management of restoration action, and in contributing to global efforts to recover records on the hidden history of anthropogenic-driven ocean ecosystem degradation.

Anthropogenic activities have impacted marine ecosystems at extraordinary scales. Biogenic reef ecosystems built by the European flat oyster (Ostrea edulis) typically declined prior to scientific monitoring. Collating >1,600 records published over 350 years, we created a highly resolved (10km2) map of historical oyster reef presence across its biogeographic range, including documenting abundant reef habitats along the coasts of France, Denmark, Ireland and the United Kingdom. Areal extent data were available from just 26% of locations, yet totalled >1.7 million hectares (median reef size = 30ha, range 0.01 - 1,536,000ha), with 190 associated macrofauna species from 13 phyla described. Our analysis demonstrates that oyster reefs were once a dominant three-dimensional feature of European coastlines, with their loss pointing to a fundamental restructuring and ‘flattening’ of coastal and shallow-shelf seafloors. This unique empirical record demonstrates the highly degraded nature of European seas and provides key baseline context for international restoration commitments.

Only a few records exist of the sponge crab Dromia personata in the North Sea. Most records are in local languages, and have limited availability. On 11 August 2016, D. personata was found by divers in the Eastern Scheldt, a former estuary in the south-western part of the Netherlands. In the months to follow, at least four other individuals were observed by divers at the same and nearby locations. This paper lists these and former records in the North Sea. The distribution pattern, and the possible migration of the sponge crab in relation to environmental conditions such as seawater temperature are discussed, as well as the crab's use of the non-native sponge Celtodoryx ciocalyptoides.