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The current study focuses on four species from the primarily marine diatom genus Craspedostauros that were observed growing attached to numerous sea turtles and sea turtle‐associated barnacles from Croatia and South Africa. Three of the examined taxa, C. danayanus sp. nov., C. legouvelloanus sp. nov., and C. macewanii sp. nov., are described based on morphological and, whenever possible, molecular characteristics. The new taxa exhibit characters not previously observed in other members of the genus, such as the presence of more than two rows of cribrate areolae on the girdle bands, shallow perforated septa, and a complete reduction of the stauros. The fourth species, C. alatus, itself recently described from museum sea turtle specimens, is reported for the first time from loggerhead sea turtles rescued in Europe. A 3‐gene phylogenetic analysis including DNA sequence data for three sea turtle‐associated Craspedostauros species and other marine and epizoic diatom taxa indicated that Craspedostauros is monophyletic and sister to Achnanthes. This study, being based on a large number of samples and animal specimens analyzed and using different preservation and processing methods, provides new insights into the ecology and biogeography of the genus and sheds light on the level of intimacy and permanency in the host–epibiont interaction within the epizoic Craspedostauros species.

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With the advent of more comprehensive research into the microbiome and interactions between animals and their microbiota, new solutions can be applied to address conservation challenges such as husbandry and medical care of captive animals. Although studies on epizoic algae are relatively rare, and the function and role of those mainly photosynthetic organisms in the animal microbiome is not well understood, recent surveys on epizoic diatoms show that some of them exhibit traits of obligate epibionts. This study explores diatom communities on captive–born manatees from the Africarium in Wroclaw, Poland. Light and scanning electron microscopy analyses revealed that skin of all animals sampled was dominated by apochlorotic Tursiocola cf. ziemanii, an epizoic species described recently from Florida manatees, that reached 99,9% of the total diatom abundance. Despite using media with a wide range of salinity (0–34), the isolated Tursiocola cells did not grow, whereas the normally pigmented Planothidium sp., that was only occasionally found on the animal substratum, survived in all culture media tested. Our observations provide direct evidence that manatee–associated Tursiocola endure the dramatic salinity changes that occur regularly during their host life cycle, and can thrive in an artificial captive setting, if the manatee substratum is available. The impact of practices and routines used by the Africarium on manatee–associated diatoms, as well as ultrastructure of areolae in Tursiocola, are briefly discussed.

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