No abstract

1. Populations of the European flat oyster Ostrea edulis have experienced catastrophic declines across Europe and subsequent spread of the non-native species Crepidula fornicata has led to its occurrence in exceptionally high densities in some areas previously dominated by O. edulis. 2. Spatial and temporal concurrence of C. fornicata larvae within the zooplankton community occurs throughout the O. edulis spawning season. A C. fornicata larval peak density of 374.7 ± 96.5 larvae/ml (mean ± SD) was observed in Langston Harbour sympatrically with O. edulis density of 45.7 ± 20.1 larvae/ml in early August. Overall oyster larvae contribution to the zooplankton community was higher in Portsmouth Harbour (12%) than C. fornicata contribution (9.6%), whilst the opposite occurred in Langstone (oysters, 11.7%; C. fornicata, 12%). 3. Larval abundance is not reflected in recruitment on the seabed, owing to the conspecific substrate preference of O. edulis. Settlement of O. edulis spat was significantly greater on settlement discs covered with recently deceased oyster shells; 6.7 ± 1.2 (mean ± SE) spat/disc vs old smooth oyster shells, 2.7 ± 1.3, C. fornicata shell 1.7 ± 0.3, cemented discs 2 ± 1 or the plastic control disc 0.7 ± 0.7. 4. Settlement substrate type matters in the presence of high benthic and larval densities of C. fornicata. The Solent has become a substrate-limited system for O. edulis; substrate management or reef deployment is required to restore a selfrecruiting population. 5. Finally, although C. fornicata may provide functional equivalence in terms of filtering services, it supports a significantly different and less biodiverse faunal community from that of O. edulis. Therefore C. fornicata does not provide equivalence as an ecosystem engineer and mechanisms of ecological phase shift are occurring within areas dominated by this invasive species.

The European flat oyster, Ostrea edulis, is a habitat-forming bivalve which was historically widespread throughout Europe. Following its decline due to overfishing, pollution, sedimentation, invasive species, and disease, O. edulis and its beds are now listed as a threatened and/or declining species and habitat by OSPAR. Increasing recognition of the plight of the oyster, alongside rapidly developing restoration techniques and growing interest in marine restoration, has resulted in a recent and rapid growth in habitat restoration efforts. O. edulis seed supply is currently a major bottleneck in scaling up habitat restoration efforts in Europe. O. edulis has been cultured for centuries, however, research into its culture declined following the introduction of the Pacific oyster, Crassostrea gigas to Europe in the early 1970 s. Recent efforts to renew both hatchery and pond production of O. edulis seed for habitat restoration purposes are hampered by restoration project timelines and funding typically being short, or projects not planning appropriately for the timescales required for investment, research-and-development and delivery of oyster seed by commercial producers. Furthermore, funding for restoration is intermittent, making long-term commitments between producers and restoration practitioners difficult. Long-term, strategic investment in research and production are needed to overcome these bottlenecks and meet current ambitious restoration targets across Europe.