On the morphological variations in a population of Cladonema radiatum (Hydrozoa: Cladonematidae): New evidence from the southern Gulf of Mexico

The lack of knowledge on the morphological variability in hydrozoan populations has resulted in numerous misidentifications over history. Cladonema radiatum Dujardin, 1843 is characterized by presenting different morphologies in different localities of the world, which has caused many taxonomic confusions within the group. In this study, the morphological variability in a population of C. radiatum is analyzed. The material came from a coastal lagoon in the southern Gulf of Mexico. The samplings were carried out for 3 months in 2017. Salinity, dissolved oxygen, and temperature were recorded at each sampling site. Cladonema radiatum was more abundant and bigger when the lagoon showed the lowest levels of salinity. Six different morphologies were recognized. The presence of gelatinous papillae on the abaxial surface of marginal bulbs and an abaxial arrangement of nematocysts in the shortest branches of the tentacles are features recorded for the first time in the species. These considerations enrich the taxonomic description of C. radiatum and contribute to the knowledge on the taxonomy of Hydrozoa in Mexico. The taxonomy of Cladonema continues to be confusing, and the present work sets a basis for further integrative studies clarifying the morphological boundaries of the genus. However, only integrative works based on the morphology and molecular analyses of the species would be able to decode its cryptic nature.