Exploring the distribution and underlying drivers of native and non-native mussel and oyster species in harbour environment

The increase of human population and their pressures in coastal areas is causing an exponential sprawl of artificial structures in marine areas, leading to the loss of natural habitats. Artificial structures are characterised by low species richness and a prevalence of non-native species compared to natural rocky reefs. Commercial and tourist ports are examples of artificial habitats. Little is known about the distribution and dynamic of the species inhabiting ports, and the factors leading to a prevalence of non-native species in these habitats are still not fully understood. Here, the distribution and abundances of two native (Mytilus galloprovincialis, Ostrea edulis) and two non-native (Xenostrobus securis, Crassostrea gigas) bivalve species that grow on the artificial seawalls of the canal-port of Ravenna were assessed to: 1) explore their distribution in different areas of the harbour, and 2) identify whether the observed patterns were related to variations in environmental parameters or to variable supply of larvae reaching different areas of the port and settling on the artificial seawalls. DNA extraction and amplification protocols were developed to barcode the bivalves settlers due to the impossibility to identify them microscopically. Results showed an increase of non-native species as the canal-port goes inland. Temperature, oxygen and nitrate seawater concentration explained most of the variation in species abundance among sites. The non-native mussel X. securis was associated to higher sea surface temperatures compared to the native M. galloprovincialis. Settler abundances were clearly correlated to the spawning window of the species, but not to adult abundances on the seawalls, suggesting a prevailing role of post-settlement processes. Future work should explore the potential role of other environmental variables, extend the duration of the observations, and use a metagenomics approach to characterise propagule pressure dynamics in the water column.