Detailed morphology as a step to solve taxonomic problems of Caribbean species of Elysia (Gastropoda: Sacoglossa)

The genus Elysia Risso, a diverse group of sea slugs within the order Sacoglossa, comprises highly specialised herbivores capable of photosynthesis. Despite their leaf-like appearance being relatively consistent across species, Elysia exhibits considerable morphological diversity in internal systems among clades. Ecological strategies and behaviours may also vary. Prior descriptions have often relied solely on external traits or radular morphology, a limitation that has contributed to persistent taxonomic misinterpretations. In this study, a high-resolution, comprehensive examination of external and internal morphological features was conducted on four key Caribbean species to elucidate taxonomic uncertainties and establish a detailed morphological baseline. The investigation involved detailed comparative analyses of these traits to assess morphological similarities among evolutionarily related species and identify potentially informative characters for accurate species descriptions. The analyses revealed high intraspecific variability in some external traits, limiting their reliability for species identification. However, specific features were identified, such as dorsal sinus patterns, renopericardial complex arrangement and parapodial shape, that are taxonomically informative. Internal systems also showed significant variation in the renopericardial complex, ganglion and buccal mass morphology, and reproductive structures, reflecting potential adaptive responses to ecological niches and providing valuable insights into species relationships. Overall, our study underscores the importance of integrating detailed morphological analysis of multiple internal systems, helping in resolving taxonomic uncertainties and enhancing our understanding of the evolutionary dynamics within Elysia. Integrating morphological analyses with molecular data holds promise for unravelling the intricate evolutionary history and ecological adaptations of these enigmatic marine organisms.