<p>The study of the composition, size, and distribution of cnidae is fundamental for the identification and description of sea anemone species. However, intraspecific variability in cnidae length and types has already been documented for the group, leading some authors to question their usefulness as a taxonomic tool. Moreover, few studies have examined the intra-structural qualitative and quantitative variation of the cnidom within the tissues of individuals. In this study, we investigate patterns of variation in the composition and biometry of the cnidom in the tentacle crown of <i>Actinostola crassicornis</i>. To this end, 2014 cnidae were measured from three tentacle cycles—internal, middle, and external—and from three positions along each tentacle—apical, middle, and basal. Our results reveal a clear pattern in the cnidae of the tentacle crown of <i>A. crassicornis</i>: lengths tend to increase from the base toward the tip of the tentacle in all types analyzed, a pattern particularly pronounced in basitrichs and <i>b</i>-mastigophores, where differences are statistically significant. No consistent trend was observed among tentacle cycles. The random variable “individual”, included in the models, had a significant effect in all cases, indicating the commonly observed intraspecific variability. These findings underscore the importance of considering intra-structural variation within the group when the cnidom is used in descriptive studies, given its direct impact on the taxonomic and ecological interpretation of sea anemones.</p>