Modelling benthic invasion by the colonial gastropod Crepidula fornicata and its competition with the bivalve Pecten maximus. 2. Coupling the 0D model of colony-forming species to a connectivity matrix for a realistic distributed simulation of benthic invasion

The anthropogenic introduction in U.K. waters of the north-American marine gastropod Crepidula fornicata (Linné, 1758), commonly called slipper limpet, and its consecutive spreading has led in less than a century to the invasion of a part of benthic grounds along the North-European coasts. Competition for space has hampered the maintenance of the native scallop Pecten maximus, whereas dredge clogging has drastically limited scallop fishing, especially in the Western English Channel. In order to assess the possible future distribution and abundance of both species (Crepidula and Pecten), an original model of slipper limpet chains joined to a simple year-class model of the scallop (Ménesguen and Grégoris, 2017) has been coupled to a connectivity matrix summarizing the annual dispersion of emitted larvae in a realistic marine domain. This distributed model can explore the dynamics of both populations at the century scale and has been applied at two different geographical scales: a local one (the bays of Brest and Douarnenez, Western Brittany, France) and a regional one (Bay of Biscay and English Channel). First eigenvectors of the connectivity matrix are used to delineate the main retention areas. For each species taken alone, simulations starting with a few animals in different spots do converge in less than a century towards the same geographic distribution, compatible with the actual field distribution. This suggests that hydrodynamic patterns of larval drift create a strong attractor for these populations; it is reached by different routes of colonization, depending on the initial inoculation. The non-linear interaction created by the lack of space when the populations have filled the benthic area seems to be stronger for scallop populations than for slipper limpets, because of the permanent capacity of Crepidula beds to fix some larvae on the top of existing colonies. Whereas the scallop abundance experiences a rather large limit-cycle, with a dominant 11 years period and a lot of harmonics, the slipper limpet has commonly a stable steady state abundance. Competition between both populations tends to lower the mean scallop abundance and to lengthen and damp its fundamental period of oscillations. Simulated distributions can be locally improved by linking the larval survival to a distributed environmental stressor, as the salinity for scallop or a metallic contamination for the slipper limpet in the case of the bay of Brest.