AbstractThe high morphological variability observed in the genus Ammonia, together with its global distribution, led to the description of a plethora of species, subspecies, and varieties. Until now, many researchers used a limited number of (morpho-)species, and considered the numerous varieties as ecophenotypes. Recently, molecular studies show that these putative ecophenotypes are, in reality, well-separated genetically and should rather be considered as separate species. This study aims to investigate the morphological characteristics of three phylotypes (T1, T2, and T6) belonging to the genus Ammonia, encountered along European coasts. For this purpose, Ammonia specimens were sampled at 22 locations between 2014 and 2016 and were imaged using an environmental SEM (Scanning Electron Microscope). For 96 specimens, images of the spiral, umbilical, and peripheral sides were obtained and pore features were investigated using 1000x magnified images of the penultimate chamber on the spiral side. Sixty-one morphometric parameters were measured for each individual. To assign specimens to their respective phylotypes, we employed molecular analyses using SSU (Small Sub-Unit) rDNA fragments. A multivariate approach (Factorial Analysis of Mixed Data, FAMD), allowing the joint analysis of quantitative and qualitative measurements, was used to determine the most reliable morphometric parameters to discriminate the three phylotypes. Our results show that the use of only two morphological characteristics is sufficient to differentiate the three pseudocryptic species: the raised or flush character of the sutures on the central part of the spiral side and the mean pore diameter. These two criteria, which can be observed with a standard stereomicroscope, provide an efficient method to discriminate T1, T2, and T6 with at least 90% accuracy. We consider that there is still insufficient information to reliably assign previously defined formal scientific names to the three phylotypes, and therefore we recommend the continued use of phylotype designations T1, T2, and T6. Our results make it possible to study the distribution of these three pseudocryptic species (T1, T2, and T6) on the basis of stereomicroscope examination alone, which means these species can also be more easily recognized in dead/fossil assemblages. Among other things, this will allow verification in sediment cores of the putative recent introduction in European coastal areas of T6, which is often considered an exotic species originating from East Asia.