Intraspecific allometric shift in an endemic and endangered marine mollusc: A new minimally intrusive tool to capture shell shape in the field

Patella ferruginea (Gastropoda, Patellidae) is an endemic and endangered marine invertebrate species from the Western Mediterranean that presents two ecomorphotypes. Currently, there is no objective and nonmanipulative method to determine these ecomorphotypes in the field. The present study uses the geometric morphometrics approach to investigate variation in the shell-shape allometric trajectories of both ecomorphotypes. In addition, we propose a new manipulation-free tool to assign ecomorphotype in the field. A total of 190 shells, 123 from the lamarcki (20.0 to 86.7 mm) and 67 from the rouxi (25.3 to 95.9 mm) ecomorphotypes, were analyzed using three different 2D landmark and semilandmark configurations capturing the lateral, internal, and dorsal shell views. We provide compelling graphical and analytical evidence that these ecomorphotypes differ in allometric growth patterns. However, the findings of the current study do not support that lamarcki and rouxi correspond to different ontogenetic states of P. ferruginea. We hypothesize that the difference in the allometric trajectories between the rouxi and the lamarcki ecomorphotypes could be due to environmental constraints, but it might also be due to currently undetected genetic factors. Since the allometry-free shape lateral view presented the most significant reliability in assigning ecomorphotypes, we evaluate an easy and inexpensive tool to assign individuals in the field to each ecomorphotype using a contour gauge and a simple landmark configuration (only three landmarks). Cross-validated classification indicated that ecomorphotype assignations for lateral contour present total reliability calculated at 93.15%. Therefore, we encourage it to be implemented in future research to avoid sacrificing or manipulating specimens of this endangered species.