LEARNING POINTS/TAKE-HOME MESSAGE ∙ The common octopus (Octopus vulgaris) is of significant economic importance as a mariculture species and is widely maintained in aquaria and laboratory facilities for both educational and scientific purposes. Figure 1 depicts the characteristic gross lesions of coccidiosis caused by Aggregata sp., the most common infectious disease affecting octopuses ∙ Figure 2 presents the histological features of coccidial developmental stages and octopus nephridia containing endosymbiotic dicyemids, which facilitate ammonia excretion in the host. A reduced abundance of these symbionts may indicate compromised health in the octopus