Interactions of larval dynamics and substrate preference have ecological significance for benthic biodiversity and <scp> Ostrea edulis </scp> Linnaeus, 1758 in the presence of <scp> Crepidula fornicata </scp>

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. 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 &#177; 96.5 larvae/ml (mean &#177; SD) was observed in Langston Harbour sympatrically with O. edulis density of 45.7 &#177; 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%). 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 &#177; 1.2 (mean &#177; SE) spat/disc vs old smooth oyster shells, 2.7 &#177; 1.3, C. fornicata shell 1.7 &#177; 0.3, cemented discs 2 &#177; 1 or the plastic control disc 0.7 &#177; 0.7. 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 self‐recruiting population. 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.