Interactions of larval dynamics and substrate preference have ecological significance for benthic biodiversity andOstrea edulisLinnaeus, 1758 in the presence ofCrepidula fornicata

Populations of the European flat oysterOstrea edulishave experienced catastrophic declines across Europe and subsequent spread of the non-native speciesCrepidula fornicatahas led to its occurrence in exceptionally high densities in some areas previously dominated byO. edulis. Spatial and temporal concurrence ofC. fornicatalarvae within the zooplankton community occurs throughout theO. edulisspawning season. AC. fornicatalarval peak density of 374.7 +/- 96.5 larvae/ml (mean +/- SD) was observed in Langston Harbour sympatrically withO. edulisdensity of 45.7 +/- 20.1 larvae/ml in early August. Overall oyster larvae contribution to the zooplankton community was higher in Portsmouth Harbour (12%) thanC. fornicatacontribution (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 ofO. edulis. Settlement ofO. edulisspat 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. fornicatashell 1.7 +/- 0.3, cemented discs 2 +/- 1 or the plastic control disc 0.7 +/- 0.7. Settlement substrate type matters in the presence of high benthic and larval densities ofC. fornicata. The Solent has become a substrate-limited system forO. edulis; substrate management or reef deployment is required to restore a self-recruiting population. Finally, althoughC. fornicatamay provide functional equivalence in terms of filtering services, it supports a significantly different and less biodiverse faunal community from that ofO. edulis. ThereforeC. fornicatadoes not provide equivalence as an ecosystem engineer and mechanisms of ecological phase shift are occurring within areas dominated by this invasive species.