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

Abstract

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 non-manipulative method to determine these ecomorphotypes in the field. The present study uses the geometric morphometrics approach to investigate the 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 lamarcki (20.0 to 86.7 mm) and 67 from rouxi (25.3 to 95.9 mm) ecomorphotypes, were analyzed using three different 2D landmarks and semilandmarks 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 finding of the current study does not support that lamarcki, and rouxi corresponds to different ontogenetic states of P. ferruginea. We hypothesize that the change of allometric trajectories of rouxi, and lamarcki ecomorphotypes could be due to environmental constraints, but it might also be to currently undetected genetic ones. Since the allometry-free shape lateral view presented the most significant reliability in assigning ecomorphotypes, we evaluate an easy and inexpensive tool to assign field individuals 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.