Comparing cnidocyst sizes among populations of Bunodosoma (Cnidaria: Actiniaria: Actiniidae) from the South Atlantic west coast

Abstract

The simplicity of a sea anemone polyp hinders the differences between species and populations, as very few external and internal morphological characters vary enough to distinguish closely related groups. Variations in size of different cnidocyst types could be an important way to characterize populations of sea anemones. We aim to identify which cnidocyst types could be useful to distinguish the variation between sea anemone populations, tested for two species of genus Bunodosoma from the South Atlantic west coast: Bunodosoma caissarum from the Brazilian coast, and Bunodosoma cangicum from the Brazilian and Uruguayan coasts. We measured cnidocysts in several parts of the sea anemones: acrorhagi, external and internal tentacles, column, actinopharynx and filaments, comprising a total of 35,593 measured capsules. Comparisons between cnidocysts lengths from each tissue type were performed by Generalized Linear Models. In B. caissarum, basitrichs from external and internal tentacles were found to be useful for detecting differences between populations and may indicate a latitudinal gradient. For B. cangicum, these aspects should be further analyzed in basitrichs from external tentacles, holotrichs from acrorhagi and microbasic b-mastigophores from the filaments. The basitrich from tentacles were found to be the most informative cnidae type in both species for population studies in a latitudinal sense.