Abstract: In several European estuaries, the introduced Manila clam (Ruditapes philippinarum) has become a
widespread and predominating species supplanting the native carpet shell clam (Ruditapes decussatus)
whereas in other estuaries such as the Bay of Santander (Gulf of Biscay) this pattern has not been
detected. Using this estuary as a case study, the potential coexistence/predominance patterns between
these two species were explored with the objective of providing insight into the capacity of expansion of
R. philippinarum. Firstly, the Ecological Niche Factor Analysis (ENFA) was applied to determine the niches
of both species, using seven contemporary environmental variables, i.e. salinity, water depth, current
velocity, and sediment sand, gravel, silt and organic matter content. Secondly, ENFA-derived habitatsuitability
(HS) maps were simultaneously treated, using geospatial techniques and following HS indexbased
criteria, to determine the potential distribution patterns. Both species models performed well
according to the cross-validation evaluation method. The environmental variables that most determined
the presence of both clams were depth, current velocity and salinity. ENFA factors showed that
R. philippinarum habitat differs more from the mean environmental conditions over the estuary (i.e.
higher marginality) and has less narrow requirements (i.e. lower specialization). R. philippinarum
dominated areas, determined by relatively lower current velocities and percentages of sand, higher
organic matter contents and slightly shallower depths, were very reduced (i.e. 2.0% of the bay surface)
compared to coexistence (47%) and R. decussatus predominance areas (7.4%). These results suggest that
HS may regulate the expansion of R. philippinarum. ENFA, together with geospatial analysis of HS index,
seems to be a valuable approach to explore the expansion potential of estuarine invasive or introduced
species and thus support conservation decisions regarding native species.
