Cassiopea xamachana Bigelow 1892

Cassiopea xamachana Bigelow, 1892 Distribution See Fig. 3, Mendoza-Becerril et al. (2025). Notes Cassiopea xamachana and C. andromeda have been described as morphologically similar and even indistinguishable species (Hummelinck 1968). Further evidence of sympatry comes from Florida, where C. xamachana and C. andromeda have been recorded co-occurring at the same localities across multiple sites (Muffett and Miglietta 2023), although C. andromeda was first described from the Red Sea. At those sites in Florida, individuals of both species show overlapping morphology and colouration, with no consistent external traits to distinguish them reliably (Fig. 2 e). Five specimens from the Mexican Caribbean were sequenced in this study for the 16 S marker and showed 99.63–100 % similarity with sequences of C. xamachana from Florida and Panama and 96.88 % similarity with C. andromeda from Florida (Suppl. material 1). These values fall within the interspecific divergence ranges reported for Cassiopea using 16 S (Gamero-Mora et al. 2022). Although based on a limited number of specimens and a single mitochondrial marker, these results confirm the presence and predominance of C. xamachana in the region and are consistent with previous morphological records indicating the species’ presence in the Mexican Caribbean. In citizen-science records from the Mexican Caribbean included in this checklist, individuals were originally reported as C. xamachana by contributors and community review and consistently show brightly-coloured oral appendages, a trait commonly associated with the species in the region. Unlike C. xamachana, for which colour variation has been documented at a single locality and whose consistency across populations remains unknown, C. andromeda usually lacks this pigmentation in the Mexican Caribbean (Gamero-Mora 2014). However, Muffett and Miglietta (2023) later showed that colouration in Cassiopea is highly variable and not species-specific. For this reason, colouration patterns should be interpreted with caution and should never be used alone for species identification without genetic or robust morphological verification. Accordingly, the presence of C. andromeda in the Mexican Caribbean may be underestimated in citizen-science records.