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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. [ABSTRACT FROM AUTHOR]

Several populations of a new Craspedostauros species were observed on museum specimens of juvenile green turtle and Kemp’s ridleys collected from Long Island Beach, New York, USA. The new taxon, Craspedostauros alatus Majewska & Ashworth sp. nov., exhibits a distinctive set of morphological features typical of the genus, including cribrate areolae, a stauros narrower than the fascia, multiple doubly perforated girdle bands, and two fore and aft chloroplasts, but clearly differs from all known Craspedostauros species in possessing partially fused proximal helictoglossae forming an internal thickening with a shallow central cavity, distinctive external wing-like silica flaps near the apices, and a combination of valve dimensions and stria density not observed in other taxa. The same taxon was identified in the NCMA culture collection (strain CCMP120), isolated in 1967 from a sample collected from the equatorial upwelling zone in the Pacific Ocean. Nine samples from three sea turtle species were collected from the same location (Long Island Beach). Statistical analyses suggested that the epizoic diatom flora composition was not affected by the collection season but differed among sea turtle species. The most significant difference was observed between the samples with and without C. alatus. Samples with C. alatus were always dominated by Achnanthes elongata and Berkeleya cf. rutilans, while those lacking the new taxon were characterized by remarkably higher contributions of epizoic Poulinea species. This observation suggests that the microhabitat provided by each sea turtle differs among specimens, which may be related to the different stages of biofilm development on the host sea turtle. Additionally, the value of zoological museum collections for epizoic diatom surveys is briefly discussed. [ABSTRACT FROM AUTHOR]