Patterns of distribution, dynamics and genetic variation in the peppery furrow shell Scrobicularia plana

PhD ceremony: Ms. S. Araujo Santos, 14.30 uur Academiegebouw, Broerstraat 5, Groningen

Dissertation: Patterns of distribution, dynamics and genetic variation in the peppery furrow shell Scrobicularia plana

Promotor(s): prof. J.L. Olsen

Faculty: Mathematics and Natural Sciences

Scrobicularia plana (the peppery furrow shell) is among the most abundant bivalve species found along European sandy-muddy shores. It plays a key role in the soft-sediment ecosystem mainly as a deposit feeder within the food web as it feeds on benthic diatoms and marine phytoplankton, which will affect primary production; and as prey for many species of shore birds. In southern Europe it is also harvested for human consumption. In order to identify gaps in our general knowledge and to better evaluate S. plana’s potential vulnerability in response to, e.g., climate change, habitat fragmentation and commercial harvesting, Sílvia Araújo Santos started with a comprehensive review that examined numerous aspects of S. plana’s natural history. Her qualitative meta-analysis revealed no consistent combinations of factors that could explain, e.g., settlement patterns and the observed aggregations that are so characteristic of S. plana. Next Santos took an empirical, analytical approach with a focus on spatial and temporal patterns and their relation to population dynamics and population structure at three spatial scales: the European latitudinal gradient; within the Wadden Sea; and at the level of specific areas of <3 km 2 . Together these spatial-temporal patterns helped to assess at what level particular attributes inform adaptation potential, dispersal, effects of habitat fragmentation/isolation and the trajectory that management must adopt in order to preserve population sustainability.

S. plana ’s wide latitudinal distribution suggests both local adaptation and broad physiological plasticity. Temperature drives the species’ distribution and phenological cycles. The degree to which the biogeographic distribution of S. plana will be affected under climate change scenarios is probably minimal in light of the fact that trailing-edge, genetic diversity is high suggesting that local extinctions may not be significant as the potential for in situ rapid adaptation is present. Nevertheless, the species may shift its distribution northwards which would disrupt the species’ critical role in southern European countries, where S. plana is not only the main prey of birds but also economically valuable.

Growth in S. plana is strongly shaped by food availability. Determination of growth parameters was not possible since accurate age estimates in S. plana cannot be obtained using the traditional method of counting growth lines but only with isotope sclerochronology, a methodology that is prohibitively expensive when analyzing such large datasets.

At the local level, a patchy distribution is characteristic of S. plana. Hydrographic features and sediment quality shape patches and their size on a site-specific basis. The species’ spatial pattern is likely to be a factor leading to the self-recruitment of local populations and consequent low interpopulation connectivity.

Therefore, the overall conclusion of the PhD thesis of Santos is that S. plana is doing well at all of its spatial scales. However, low interpopulation connectivity will, most likely, lead to latitudinally disjunctive populations decreasing the ability for future populations to adapt to environmental changes.