On the Fallacy of Color Discrimination: The Rise and Fall of a Rare and Endemic Box Crab (Brachyura: Calappidae)

The Mediterranean Sea is one of the major reservoirs of marine biodiversity and harbors an exceptionally high number of endemic species. Nevertheless, the taxonomic status of many of these endemics still requires confirmation through integrative, modern approaches. We hereby first investigated the identity and status of a rare and endemic box crab, known either as Calappa rosea or Calappa rissoana, through a multidisciplinary approach that capitalized on the use of nomenclatural rules, museology, integrative taxonomic approaches (morphology, complete mitochondrial genomes, nuclear markers, and phylogenetic analyses), and passive citizen science. Almost all morphological characters failed to differentiate it from the related species Calappa granulata, whereas differences in coloration were mostly confirmed here. However, all molecular approaches supported the conspecificity of these two taxa. The combined use of passive citizen science and statistical analyses revealed that the formerly endemic species is an unrecognized ontogenetic stage of C. granulata, and, in particular, a rare transitional phase that connects early juveniles to fully developed adults. This renders the investigated taxon a new junior synonym of C. granulata and solves nomenclatural and taxonomic ambiguities related to native Mediterranean box crabs, which have remained unsettled for over 200 years. The present study, therefore, provides the first comprehensive reconstruction of the ontogenetic changes occurring in the common Mediterranean box crab throughout its various life stages and raises questions on whether these transitions occur worldwide across box crabs or are restricted to a few species. Finally, it presents new, conspicuous, and detailed morphological and molecular data on the type species of the genus Calappa, facilitating future phylogenetic reconstructions and taxonomic assignments within the entire family Calappidae, and discusses the putative occurrence of the other box crabs in the area, suggesting a critical re-evaluation of all historical data and records.